FAI Seminar on âManagement of coastal saline soils ...... âSyndromeâ in marine biota. ..... in Rice through Modified Prilled Urea in Coastal Saline Soils of Konkan.
At the mouth of the Ganga – Brahmaputra river system, Sundarbans represents one of the largest single chunk of mangrove forest within the latitude 21031/ and 22030/ North and longitude 88010/ and 89051/ East. The Indian Sundarbans falls under the jurisdiction of North 24-Parganas and South 24-Parganas districts in West Bengal comprising of 19 rural blocks (13 under South 24-Parganas and 6 under North 24-Parganas) (Fig – 1.1), covering a total area of 9630 sq. km. of which 5364.4 sq. km area is inhabited by human population and remaining 4226.6 sq. km is the present day Sundarbans mangals (Naskar & Guha Bakshi, 1987). Since 1973, about 2585.10 sq. km area of south-eastern part of Sundarbans mangals were declared as Sundarbans Tiger Reserve. This area of Sundarbans Tiger Reserve includes the land area of 1680 sq. km. while the estuarine rivers, creeks and canals cover the area of about 905 sq. km. This entire area under the Sundarbans Tiger Reserve has been further demarcated as “Core Area” (1330.12 sq. km.) and “Buffer Area” (1255 sq. km.). This core area has also been declared as the “Sundarbans National Park”. Considering the importance of this region, the National Park Area of Sundarbans Tiger Reserve is being conserved as “World Heritage Site” as designated by the IUCN in the year 1985. Since 1989, the entire Sundarbans with the total area of 9630 sq. km. has been declared as ‘Man & Biosphere Reserve’ and also declared as the ‘Sundarbans Biosphere Reserve’ Furthermore, within this core area of Sundarbans Tiger Reserve, an area of 124.09 sq. km. has also been preserved as primitive zone to act as “Gene pool” (Table – 1.1). There are three wildlife sanctuaries within and outside the Sundarbans Tiger Reserve viz. (i) Sajnekhali – with an area of 304 sq. km. within the Sundarbans Tiger Reserve; (ii) Lothian Island - with an area of 38 sq. km. outside the Sundarbans Tiger Reserve and (iii) Holiday
Island - with an area of 5.85 sq. km. also outside the Sundarbans Tiger Reserve. Sundarbans mangrove forest area in Bangladesh is estimated to be about 66%, while Indian Sundarbans covers an area of about 34% of the total (Naskar & Mandal, 1999) (Map – 1.2). In the Indian Sundarbans, six major estuarine rivers viz. Baratala or Muri Ganga, Saptamukhi, Thakuran or Jamira, Matla, Goashaba and Herobhanga or Harinbhanga meet the Bay of Bengal in their southern mouths and are inter connected with each other through numerous creeks, canals and small rivers creating 110 islands of which around 56 islands are still part of the Sundarbans mangrove ecosystem and the rest have been cleared, reclaimed and converted to human habitation areas.
1.2 About Mangroves
Mangrove ecosystems are the hot spots of biological diversity in the form of many varieties of flora and fauna. These are one of the important types of natural wetlands found in the inter-tidal zone of tropical and subtropical regions of the world (Tomlinson, 1986; Duke, 1992; Ricklets & Latham, 1993). They also exhibit self – generating and self – maintaining littoral plant formation often constituting a dynamic ecosystem inhabited by a complex assemblage of flora and fauna. Mangrove forests can be very productive (Clough, 1992) and their food chains and nutrient cycles are closely linked to the adjacent coastal waters (Alongi, 1996). Different areas of knowledge, such as plant – animal interactions and their effects on ecosystem, structure and the details of nutrient cycling and food webs have been revealed by recent researches in various geographical locations (Robertson and Alongi, 1992; Alongi, 1996). These biotic components consist of specialized plants
and animals well adapted to these unique ecosystems. The floral components consist of trees, shrubs, herbs, ferns, epiphytes, fungi, lichens, algae etc. Various kinds of fauna include mammals, birds, amphibians, shrimps, fishes, crabs, molluscs, insects and innumerable microorganisms. A total of 1434 numbers of animal species have been reported so far from the Indian parts of Sundarbans only (Das and Nandi, 1999). These animals consist of 989 species of invertebrates, one species of hemichordate and 445 species of vertebrates. According to their habitats, 486 species have been reported from the supra littoral zone, 499 species from tidal flats and 449 species from estuarine waters. The mangrove ecosystems depend heavily on various important environmental factors such as climate, tides, water current, salinity, dissolved oxygen, soil - water nutrient status etc. This ecosystem is basically an open ecosystem, which exchanges matter and energy with adjacent marine, freshwater and terrestrial ecosystems. The tides play an important role by controlling the interaction between the inshore and offshore systems. Moreover, it is evident that there are several environmental factors, viz., tropical temperature, fine grained alluvium, strong wave and tidal action, saline regimes and a large tidal range, which influence the occurrence and size of mangroves, species composition, species structural characteristics and overall ecosystem functioning. Understanding the ecology of mangroves, therefore, appears to be highly significant in managing and conserving the coastal resources for sustainable use. India has about 3.1 Mha of coastal soils including the area covered by mangrove forests (Yadav et al, 1983). Mangroves are abundant in broad sheltered and low lying coastal plains where topographic gradients are small and tidal amplitudes are large. Repeatedly flooded but well drained soils support good mangrove growth and high species diversity (Azariah et al, 1992). The growth and nutrition of mangrove vegetation are greatly influenced by soil
properties. The soil structure and soil salinity are the main agents controlling the distribution of mangroves in the Indian Sundarbans. Soils of the Sundarbans mangrove forest differ from other island soils in that they are subjected to the effects of salinity and water logging, which naturally influence the vegetation. In places soils are semi-solid and poorly consolidated. The conductivity, pH, cation exchange capacity etc are important abiotic parameters controlling the growth of mangrove species. Workers like Kusmana (1990); Rao et al (1992); Pezeshki et al (1997) stated that the deltaic alluvium has been formed by silt carried down by the rivers and the soils of the delta naturally show a good deal of variation. The Sundarbans mangrove and a stripe to the north, now reclaimed, are clays, with fresh sands along the sea face and highly saline. The most important factor, however, appears to be the nutrient concentrations of both soil and tidal waters. Mangals are finely balanced, highly effective nutrient sinks importing dissolved nitrogen, phosphorus and silicon. Nutrient fluxes in this environment are intricately related to plant assimilation and microbial mineralization (Alongi, 1996; Middelburg et al, 1996). Nutrient availability may limit growth and production in many mangals. Varying nutrient concentrations can also change competitive balances and affect species distributions (Chen and Twilley, 1998). As a result, nutrient pulses can create immediate and impressive changes in the vegetation. For example, on the south - east coast of India, high nutrient concentration and low salinity from monsoons produce rapid growth in the mangroves. It was reported by Kaly et al (1997) that the loss of nitrogen and phosphorus from the soil cause an adverse effect on mangrove vegetation. Salinity also has profound influence on the productivity and growth of mangroves. The distribution of plant species within the mangals in many cases is controlled by the salinity gradients (Ukpong, 1994; Ball, 1998). This type of salinity based zonation is
very prominent in the north - east coast of Bay of Bengal and the adjacent Hugli estuary. The Indian Sundarbans is bestowed with the highest floral diversity in the form of mangroves, coastal wetland flora, beach flora, marsh and swamp flora (Naskar and Guha Bakshi, 1987; Naskar, 1993; Naskar and Mondal, 1999). The recent published report of about 110 species includes about 25 species of true mangroves (Ghosh et al, 2003) (Table – 1.3.). Most of these plants are endemic in this inter-tidal high saline deltaic areas, for having their special adaptation in these physiologically dry soil. Besides these, about 40 numbers of mangrove associates and back mangrove species are also present in Sundarbans mangals (Naskar, 2004). The mangroves of this area thus have great significance both in terms of their direct role in resource utilization for forestry and fishery productions and also their indirect potentials in protecting coastlines and maintaining estuarine ecological balance (Ghosh et al, 2002). Although some studies have been carried out to asses the characters of these mangrove soils (Sahoo et al, 1985) ,yet information pertaining to specific variations in different properties of such soils due to occurrence of the mangrove vegetation and, again, the effects of various soil properties on establishment of mangrove forest of Sundarbans are meager. The major form of vegetation, which imparts richness to the bio-diversity of Sundarbans, is the mangroves. Although there exists relatively few species, which can be designated as true mangroves or major and minor components of the mangroves. These mangrove ecosystems are unique because they include structural niches and refuge for numerous non-mangrove species. The uniqueness of the Sundarbans mangroves lies not only in terms of numerical diversity but also in the kind of distribution of these floristic components into different tidal niches and saline regimes. On the survey of different areas in the Indian
Sundarbans both inside and outside the Sundarbans Tiger Reserve, patches of diverse plant groups were revealed (Naskar et al, 2002). The mangroves of this area thus have great significance both in terms of their direct role in resource utilization for forestry and fishery production and also their indirect potentials in protecting coastlines and maintaining estuarine ecological balance (Ghosh et al, 2002). Although some studies have been carried out to asses the characters of these mangrove soils (Sahoo et al, 1985), yet information pertaining to specific variations in different properties of such soils due to occurrence of the mangrove vegetation and, again, the effects of various soil properties on establishment of mangrove forest of Sundarbans are meager. In the present study, therefore, an attempt has been made to study some relevant chemical aspects of mangrove habitats of Sundarbans. It is hoped that this study will help to provide the basic data for adoption of soil specific conservation practices for these mangrove forests.
1.3 Objectives of the study
To study the nature and properties of the mangrove soils of Sundarbans.
To assess the importance of these soil properties on occurrence of different mangrove species in this island.
To find out the effects of the mangrove species on availability of different nutrients in the soils of Sundarbans.
To develop, thereby, a comprehensive idea about the nature of habitats of mangroves in the Indian Sundarbans.
To generate information for the development of more eco-friendly conservation of the mangrove species in this island.
1.4 Practical Utility
As has been discussed earlier, good amount of studies have so far been carried out on mangrove ecosystem of Sundarbans island. However, detailed information on relationships of these mangroves with their habitats are meager. It is hoped that the present study will be able to provide some useful information on this aspect throwing light on preferred habitats of the common mangrove species occurring in Sundarbans forest. Such information is likely to help in developing more effective conservation measures for these mangroves, keeping in view the importance of more desirable kind of habitats for growth and survival of different mangrove species.
1.5 Work Programme The study was carried out in different phases. During the first phase of the study, soil samples were collected from a mangrove dominated soil and an adjacent without mangrove soil in ten locations in Sundarbans island. The collected samples were analysed for different properties of mangrove and non-mangrove soils for assessing the major differences between these two kinds of soils.. During the second phase, collection of soil samples were done from the Indian part of Sundarbans covering 15 blocks of Sundarbans Tiger Reserve and their adjoining areas (Map – 1.3). These blocks were divided into five major components viz. i) Eastern Zone ii) Western Zone iii) Central Zone iv) Northern Zone and v) Southern Zone according to their occurrence. Soil samples were collected from each of the blocks and analysed for some general properties in order to develop a gross idea about the general properties of
mangrove soils of Indian
Sundarbans and also to understand their
interrelationships. Twenty three mangrove species commonly occurring in this soil zone were identified. In the third phase of study, soil samples were collected from their root zones in different areas and analysed for assessing the preferred habitat and nature of soil required for the specific mangrove species for their growth and survival. Since some important properties of the mangrove soils are likely to be influenced largely by the quantum and qualities of the mangrove leaves which tend to accumulate in the bottoms of such vegetation, during the next phase of study, some biochemical properties of the commonly occurring mangrove leaves were analysed and attempts were made to correlate these properties with the qualities of the bottom soils which receives these leaves as a sink. During the last phase of the work programme, soil maps of Sundarbans Tiger Reserve Forest pertaining to some relevant physico-chemical properties of mangrove soils were prepared using Geographic Information System (GIS). It has been hoped that these GIS mapping will be highly useful in identifying the general soil characters of the mangrove forests at different zones for undertaking any mangrove conservation measures in future. It is hoped that the study will help to develop a better understanding about the chemical aspects of the mangrove habitats of Sundarbans. This will, in turn, be useful in identifying the habitat preference of the common mangroves occurring in this forest leading to development of eco-friendly conservations programmes for such mangrove species in Sundarbans.
Review of Literature Chapter
2.1. General Information
Mangroves are the sea-land inter-phase, inter-tidal or tidal habitat, tropical and subtropical woody plants. They grow generally within the sheltered inter-tidal flat deltaic lands, funnel-shaped bays, broad estuarine mouths, shallow or frequently tidal inundated coast lines (Thom, 1982). The plants best grow in the newly silted up clay loam or sandy beaches and salt marshes. The atmospheric temperature between 20oC and 35o C, are the ideal for the better growth and development of the mangrove ecosystem. These plants prefer humid atmosphere (60% - 90%) with moderate to high annual rainfall ranging between 1000 mm – 3000 mm (Anonymous, 1978). Tomlinson (1986) reported that mangroves can not tolerate the frost. Mac Nae (1968) said that the dominant mangrove ecosystems are generally restricted within the 30o00/ N – 30o00/ S latitude. In the sub-tropical regions the distribution of these mangals are frequent and scattered (Tomlinson, 1986). The entire coastal strip of Indian Sundarbans delta ranges from 50-70 km wide from Bay of Bengal and is a low lying silted up deltaic land (Yadav et al, 1979). The salt contents are of mostly chloride and sulphate of Na, Mg and Ca though bicarbonates are also present in traces (Yadav et al, 1981). In submerged condition and with higher salinity, the decomposition rate of the organic matter is less as the microbial population in those area are generally poor due to high salinity (Gupta and Bajpai, 1974).
2.2. Mangrove Ecosystem of the World
The mangrove ecosystem or the mangals are generally restricted in the tropical and sub-tropical zones of the world mainly within the latitude
32o00/ N (Japan) and 33o00/ S (South East Australia) (Macintosh, 1984). However, in Australia, mangroves have been reported from Victoria Coast at Corner Inlet extended over 38o00/ S to 45o00/ S latitudes (Avicennia marina Vierh) also, which forms the southern limit of the world distribution of mangroves (Tomlinson, 1986). The dominant mangrove zones in the world are found to be distributed in South-East Asia, North-East Australia and South-East Africa between the longitudes 30o00/ E and 165o00/ E (Macintosh, 1984), these form the mangroves of the Old World. In the New World, mangroves are distributed in North America at Louisiana and Pacific Coasts of South America (Macintosh, 1984). Some sporadic mangals have also been reported by different workers from the West Coast of Africa and East Coast of South America, i.e. on Atlantic Ocean. The actual area under the world mangrove ecosystem have not been well estimated or are not unanimously accepted by experts all over the world. The main reason for such a disparity being the fact that the definitions of mangroves and mangals are both highly variable among different experts in this field. The Indo-West Pacific mangroves or the Old World mangroves are estimated to be spread over an area of 1,02,631 sq. km. and the New World mangroves over an area of 71,029 sq. km. (Macintosh, 1984). Naskar and Mandal (1999) have given an elaborate account of the world distribution of mangroves. Since the term mangroves have been defined differently by different experts, the total number of mangrove species is also different among the different published works.
2.3. Distribution of Mangroves in India
In the Indian sub-continent, the mangrove ecosystems are found within the inter tidal, tidal and supra-tidal deltaic zones, estuaries mouths, sheltered
coasts and edges of the islands of both East Coast facing the Bay of Bengal and West Coast facing the Arabian Sea; besides the coastal inter tidal zones, the mangals are also spread over the Bay Islands of Andaman and Nicobar and in the Lakshadweep and Minicoy Coral Atolls (Naskar and Mandal. 1999). In the nine different maritime states of India the mangals are distributed in the deltaic regions, viz., in West Bengal, they are restricted in the estuarine regions of Sundarbans between the river Hugli (Ganga) and the river Harinbhanga; in Orissa at Bhitarkanika between the river Devi and Dharma at the mouth of the river Mahanadi; the Coringa mangrove area between the Krishna and the Godavari rivers in Andhra Pradesh; in the Pennar delta of Tamil Nadu at the mouth of the river Cauvery, all these accounting for the major mangrove areas of the East Coast facing Bay of Bengal. In the West Coast, the Mangals are distributed in the State of Gujrat in the Indus delta in five different regions, out of which, the two Gulfs, viz., Gulf of Kachchh and Gulf of Cambay form the major mangrove zones and other three, viz., Rann of Kachchh, Saurashtra Coast and South Gujrat Coast constitute the minor mangrove zones. In Maharashtra, the Mangals are distributed along the coastal belt of the districts Sindhudurg, Ratnagiri, Raigad, Mumbai and Thane; in Goa the mangrove areas are distributed between the Mandovi and Zuari estuaries and other five minor estuaries and Canbarjooa canal; in Karnataka, the major mangrove areas are in Coondapur and Malpe estuaries complex and in Kerala, a very negligible mangrove area is reported from the Cochin estuary (Naskar and Mandal, 1999). Out of the total mangrove area in the Indian sub-continent, Sundarbans accounts for 65% followed by Andaman and Nicobar Islands accounting for 18% of the total distribution (Naskar, 1999). The eminent mangrove workers of the Indian territory are Navalkar (1940, 1951), Waheed Khan (1959), Mathauda (1959), Sidhu (1963), Untawale (1984, 1986, 1987, 1992), Dagar et al (1991) and many others. Besides them, a good number of
workers have attempted to study the mangrove flora in different aspects. The eminent foreign botanists or mangrove ecologists and their publications are Mac Nae (1968), Chapman (1975, 1976), Blasco (1975, 1977, 1982, 1984), Tomlinson (1986), Snedekar and Snedekar (1984), Teas (1983), Spalding (1997), Siddiqi (1994) and many other world reputed publications have come out during the last few decades many of whom have focused their attentions to Indian subcontinent also.
2.4. Mangroves of Sundarbans
Sundarbans mangrove ecosystem has attracted ecologists, scientists and researchers since long. But it’s hostile environment and perilous wildlife has contravened much fruitful research of a holistic nature. Since eighteenth century, the world reputed botanists became interested mostly about the mangrove vegetation of the Sundarbans. Roxburgh, the great and pioneer botanist of the Royal Botanic Garden obtained a number of interesting plants from these mangrove regions with the help of Buchanan Hamilton and William Carey. After that, the publications of Roxburgh – Hortus bengalensis (1814), Voights (1845) – Hortus sundarbanensis calcuttensis, Prain’s – Bengal Plants (1903a), Flora of Sundribun (1903b) and the Vegetation of the Districts of Hugli – Howrah and 24 Pergunnahs (1905) have been authentic and original works. These publications are the basis of the systematic attempts of the subsequent workers to study the Floristic composition of West Bengal in general and the Sundarbans mangals in particular, viz. Majumdar (1956, 1962 and 1963), Patil (1961), Chanda (1977), Mukherjee and Mukherjee (1978), Naskar (1981, 1983, 1985, 1988, 1993), Naskar and Guha Bakshi (1982, 1983, 1984, 1987, 1989), Mandal (1996), Naskar and Mandal (1999), Guha Bakshi and Pal
(1999) and others. A comprehensive working plan of Sundarbans dealing with ecology and resources with emphasis on forest resources was done by Curtis (1933). The map of Sundarbans prepared by Rennel (1779) is well used as a bench mark, when next changes were documented by the surveyors Dampier and Hodges, who marked in 1830 the northern boundary of Sundarbans mangrove plants to demarcate the exact inter-tidal zone. The working plan prepared by Curtis (1933) was based on a survey of various ecological and physiological characteristics and spanned the floristic composition of the entire Sundarbans delta. Based on this, subsequent classifications of the forest, particularly by Champion and Seth (1968), were based on more parameters than just inundation levels. Prain (1903) divided the entire Sundarbans into three zones viz. (i) Southern coastal strip and south - western part consisting of mangrove species, (ii) Central zones of Heritiera fomes and (iii) North - eastern part of Savannah type vegetation. The Indian Sundarbans falls in the first category. Tensley and Chipp (1926) found that certain conditions of soil make the development of the climatic climax permanently impossible and vegetation ultimately developed on such soil is best considered as an edaphic climax. Troupe (1926) shared this view and stated that edaphic forest formations are well represented in India and among the most obvious examples, which may be mentioned, are the mangrove and tidal forests of the littoral region. Curtis (1933) also divided the Sundarbans into three mangrove forests types, these being i) Freshwater forest ii) Moderately salt-water forest, and iii) Salt- water forest. Champion (1936) classified the tidal forests under primary seral type or moist tropical seral formations and did not regard the mangrove as a climax or pre-climax forest types. He divided the forests of the Sundarbans region into mangrove forests consisting of i) Low mangrove forest; ii) Salt water Heritiera fomes
and iii) Freshwater Heritiera fomes forest. The Indian
Sundarbans falls under categories (i) and (ii) while the Sundarbans forest in Bangladesh are at large considered to be representative of category (iii). Champion and Seth (1968) later made one of the most comprehensive assessments of the vegetation communities of the Indian Sundarbans. They divided the forest into categories based on broad characteristics of physiognomy and structure. The communities were defined irrespective of physiographic, edaphic or biotic factors. Champion and Seth (1968) were of the opinion that some communities were clearly associated with a definite site factor, which differed appreciably from the surrounding area. Chaudhury and Chakraborty (1973) divided the forests into four types based on tidal level as i) High tide; ii) Above general tide level; iii) Frequently inundated by salt water and iv) Below tide level. Blasco (1975) identified the following species compositions in his classification: i) Back mangroves (euryhaline zone), found on the river bank; ii) Dense mangrove consisting of many species of plants; iii) Tall, dense trees of Heritiera fomes with primary associate Excoecaria agallocha; iv) Brackish water of mixed Heritiera fomes forests with Rhizophora species over a very limited area; v) Palm swamps consisting of pure Phoenix paludosa. According to Blasco (1975), Champion and Seth (1968) had not considered the anthropogenic and biotic factors for spatial zonation. Sidhu (1963) pointed out that as a result of constant biotic interference, species of Acanthus and Avicennia may be generally pioneers. Naskar and Guha Bakshi (1982) grouped this forest into five major zones as i) Sea face of beach forest, ii) Formative island flora, iii) Flora of reclaimed land and low lying area, iv) Flora of river banks and v) Swamp forest. The first category is dominated by xerophytic plants due to the dryness of the soil and numerous sand dunes. The flora of the formative islands consists mainly of Porteresia coarctata, Salicornia brachiata, Suaeda maritima, Suaeda nudiflora, Phragmites karka, Acanthus ilicifolius and a few tree species such as Avicennia,
Sonneratia and Excoecaria. The reclaimed areas are dominated by mesophytic flora while the last two zones are dominated by halophytic mangrove species. Ninety five percentage of the river bed vegetation in Sundarban mangrove forest is dominated by Porteresia sp. The level of nutrients is higher in leaf litter of Avicennia marina than that in other components of litter in a tidal creek of Lothian Island of Sundarbans (Ghosh et al., 1990). Morgan and McIntire (1959) have expressed that in these Ganga - Brahmaputra estuarine mouths, the delta formation is very rapid and dynamic, as well. Deb (1956) said that the morphological nature of the entire Sundarbans is more or less uniform and these deltaic lands are raised above 5-8 meters from mean sea level. Naskar and Guha Bakshi (1987) has mentioned that the western and south-eastern region of these Sundarbans delta are in full “orogenic phase”. Ellison et al. (2000) have pointed out that the association between abiotic variables and patterns of species distribution and abundance are a major preoccupation of community ecologists. In many habitats, this association is manifested in discrete zones of vegetation. They used statistical methods to examine tree species distribution patterns in relatively undisturbed regions of the Sundarbans of Bangladesh. Duke (1992) said that distribution of mangrove is limited by temperature and they prefer moist atmosphere and freshwater inflow, which brings in abundant nutrients and silt from terrestrial sources. Repeatedly flooded but well-drained soils support good growth of mangroves, but impeded drainage is detrimental (Gopal and Krishnamurthy, 1993).
Ghosh et al (1987) studied the ecology and faunistic association of the inter-tidal mangrove habitats in the western fringe areas of Sundarbans and some of the salient observations on these aspects are presented so as to serve a basin conservation strategy. Mukherjee (1984) carried out the environmental impact analysis for three mangroves of Indian Sundarbans.
The paper dealt with the three mangrove species of Indian Sundarbans namely Heritiera fomes, Nypa fruticans and Acrostichum aureum, which have become endangered due to different environmental impacts on them. The main factors causing rapid decrease in the populations of these three species are (i) lack of fresh water supply (ii) human population pressure (iii) over exploitation.
Sarkar et al (1988) in their observations confined to lower gangetic delta under Sundarbans, West Bengal, studied the morphological and physicochemical properties of some extensive wet soils of the area and revealed that these soils belong to Haplaquepts and Fluvaquents with saline phases. Soils are deep, fine in texture, neutral to slightly alkaline with high base saturation and fertile. Das et al (1991) made a characterization of organic matter in some soils of coastal Sundarbans, West Bengal. Adhikari et al (1991) stated the seasonal effects on soil properties of coastal saline soil reveal that the salinity is highest in summer and lowest in rainy season. pH is low in summer and increased slowly in rainy season. Soil conservation measures like land leveling, bounding, followed by impounding rain water are required for restriction of ingress of salt water as well as to reduce the salt concentration at the root zone. Adhikary et al (1987) also studied the physico-chemical properties of coastal saline soils of Sundarbans, West Bengal and also found out the mineralogical make up. They reported the status of the organic matter content of Sundarbans soils range from 0.28 – 1.15%. They also observed the soil pH of Sundarbans to range between 7.3 – 7.9. However, Nath and Dey (1999) reported the pH of Sundarbans soil to be slightly alkaline and to range between 7.6 – 8.58. Maji and
Bandyopadhyay (1995) also
reported similar variations in pH values in such soils. Bandyopadhyay and Sarkar (1987) observed the occurrence of acid saline soils in coastal area of Sundarbans of West Bengal which behaviour has largely been attributed to
excavation of mangrove soils.. Similar studies have been carried out by Bandyopadhyay and Sen (1987) also. . Bandyopadhyay et al (1997) further stated that the soils show very irregular distribution of organic matter in the soil profile with highest amount is usually found in the surface horizon and also the spectral properties of different fractions of soil organic matter indicates that the soil organic matter is of recent origin. Bandyopadhyay and Bandyopadhyay (1983) said that the high organic matter content in the soils of Sundarban is due to the formation of soil under mangrove forest vegetation, poor decomposability of mangrove plants due to high lignin and wax content and also slower decomposition rate of organic matter at higher soil salinity. Sahoo et al (1985) also stated that percentage of organic carbon and humus C in surface muds was higher than those in subsurface soil. The relatively higher values of organic and humus C in the surface mud was due to the confinement of the organic residue in these layers and also the nutrient status of some mangrove soils as has been reported by Jadav and Chowdhury (1985) while working on litter production of mangrove plants in Lothian island of Sundarbans. Maji et al (2004) showed the ecology and the soil health of coastal ecosystem. Bandyopadhyay and Dutt (1985) observed the mangrove soils to be generally rich in K and also the soils have also been reported to be rich in micronutrients (Bandyopadhyay et al, 1983; Maji and Bandyopadhyay, 1989, 1991; Dasgupta et al, 2003).
Chanda (1977) carried out an eco-floristic survey of mangroves of Sundarbans. Similar studies have been done by Ghosh et al (2003) also. Naskar and Mandal (1999) have worked extensively on the ecology and biodiversity of the mangrove ecosystem and highlighted the morphology and anatomy of the mangrove species. Ghosh, et al. (2002) has inventoried 110 species of mangroves, mangrove associates, back mangroves, parasites,
epiphytes and beach flora for the entire Sundarbans. Ghosh, et al (2003) also studied on floral diversity of mangroves and mangrove associated species in the Indian Sundarbans. Banerjee (1985) made comparative studies on the mangrove of the Sundarbans and that of the Mahanadi delta in Eastern India. The different major mangroves, minor mangroves and mangrove associates are known to be specifically present in specific zone and their abundance also varies in different zones accordingly in the Indian Sundarbans (Anonymous, 1999). Guha Bakshi and Naskar (1991) stated that the Indian species of Sonneratia generally grow and are very specific in the diverse saline zones of Sundarbans. Sonneratia caseolaris prefers the inner estuary with slightly polluted zone, while Sonneratia apetala and Sonneratia griffithii prefer the mid and outer or true estuary respectively. Dutta and Bal (1987) observed the salt tolerance mechanisms of some plant species. Sarkar et al (1999) studied the nutrient status of different mangrove species and different mangrove zones of the Indian Sundarbans working mainly in the area of Sagar, Bakkhali, Canning, Ramgopalpur area of the Island.
Physico-chemical parameters of different mangrove soils and waters have been studied (Chakrabarti, 1993; Bava and Seralathan, 1999 and others)..As observed by many of them, the soils are basically similar on both the sides, except in conductivity, soil texture and NPK ratio. Distribution of the mangrove species in the two sides is, however, different. The western side islands are dominated by Avicennia spp. and Acanthus ilicifolius, and the eastern side by Aegiceras majus. Ceriops-Phoenix association occurs in elevated land areas and Excoecaria species and Ceriops decandra exist over the entire forest of the Sundarbans. The distribution of some trace metals in the mangrove flora and fauna of Sundarbans has been studied. Among metals, Zn showed high values in all species of plants and animals followed by Cu and
Pb (Chakrabarti et al., 1993). Diurnal changes in temperature, salinity, dissolved oxygen, CO2, and ionic product of calcium carbonate have been studied in virgin and reclaimed mangrove waters of Sundarbans during monsoon run off. Surface water of both the places are under saturated with respect to oxygen and partial pressure of carbon dioxide remained high. Lower calcium / chlorinity values than those in the open ocean are obtained (Ghosh et al. 1987).
Limited work on the biochemical properties of the mangroves in the Indian Sundarbans has been done till today. Misra et al (1985) showed the changes of leaf hydrocarbons and wax esters of the three species of mangroves grown under tidal stress. She observed that the hydrocarbons and wax esters in the submerged plants were about three times greater than those in normal plants. Dutta et al (1985) have studied the variations in leaf fatty acids of three species of mangroves i.e. Avicennia officinalis, Acanthus ilicifolius and Bruguiera gymnorrhiza. The changes of the sterol compositions of those mangrove species grown under tidal water stress was observed by Ghosh et al (1985) also. From that study it was showed that the fatty acid de-saturating enzyme activity has been increased in the submerged plant leaves. This is probably necessary for the leaves to maintain proper rigidity to perform physiological functions under water at a relatively lower temperature, compared to that of the surrounding atmosphere. Misra et al (1986) studied the effects on the leaf lipids of three species of mangrove i.e. Avicennia officinalis, Acanthus ilicifolius and Bruguiera gymnorrhiza of periodic submergence in tidal water The seagrass Porteresia coarctata grown in Prentice and Chuksar Islands of Sundarban mangroves has been studied for biochemical components. Major sterols are sitosterol and stigmasterol, and other components are campesterol and cholesterol. Major pentacyclic triterpenoids are lupeol and oleanolic acid; other components are i)-amyrin, ii)-amyrin and ursolic acid. Considerable
quantitative differences are observed in the sterol and triterpenoid compositions. Among the triterpenoids, betulin occurs only in the sample of Chuksar Island (Misra et al. 1987). Bagchi et al. (1988) observed that the lipids and waxes in leaves of some mangroves of Sundarbans show a similarity. Mangrove leaves have low triglycerides and simple fatty acids. Fatty acids with C16 and C18 chains are common. Presence of C18 chain in wax esters is an important character. This similarity in fatty acid composition maintains the water economy and helps in the adaptation of mangroves in physiologically dry soil. Basak et al (1996, 1998) showed the seasonal variations in chlorophylls, carotenoids, proteins and the tannin concentrations in the leaves of 14 species of mangrove which include nine mangrove species of Rhizophoraceae and also the seasonal changes in organic constituents in leaves of nine mangrove species. They showed that the concentrations of these organic constituents in leaf tissues are highest during the rainy season. Protein concentration varied from 0.108g/g dry wt in Bruguiera cylindrica to 0.231g/g dry wt in Bruguiera sexangula. The polyphenol concentration ranged from 0.111g/g dry wt in Bruguiera sexangula to 0.488g/g dry wt in Ceriops decandra and the tannin concentration from 0.088g/g dry wt in Bruguiera parviflora to 0.408g/g dry wt in Bruguiera sexangula. In Aegiceras majus leaves, major triterpenoids are lupeol, amyrin, elenolic acid and ursolic acid. In Sesuvium portulacastrum, major fatty acids are myristic, palmitic, stearic, oleic, linoleic and linolenic acids. Major sterols found in leaves of this plant are campesterol, stigmasterol and sitosterol. These typical components are biotransformed by benthic animals like mudskippers, bivalves and gastropods and thus entering into the food chain of this ecosystem (Chattopadhyay et al., 1990). Another observation by Ghosh et al (1985) showed the occurrence of pentacyclic triterpenses in mangrove plants at Sundarbans.
The discussion reveals the fact that in spite of the seeming substantial work done with regards to mangrove flora and also the physico-chemical properties of the soils of the Indian Sundarbans, very little work have so far been carried out on habitat ecology of mangroves in Sundarbans, especially in Tiger Reserve area of the forest.
3.1. Collection and Processing of Samples
The soil and plant samples were collected from the Indian Sundarbans mainly from Sundarbans Tiger Reserve and their adjoining areas. The collections of soils were carried out in different phases while mangroves leaves were collected once only for biochemical studies. The soil samples were collected using standard methodology (Jackson, 1973). The sampling of soil always being a challenging problem, mainly at Sundarbans Tiger Reserve, requires detailed considerations. The entire Sundarbans Tiger Reserve is spread over an area of 2585 sq. km. This area has 15 forest blocks under its jurisdiction. The collections of soil samples and mangrove leaves were carried out on small boats/ mechanical boats/ launches and on foot wherever possible. Keeping in mind the extensive variability in the soil and composition of the Sundarbans Tiger Reserve Area, it was pertinent to device an entirely satisfactory method of sampling. The method included the possibility of taking into account the variability of soil, handling and processing the sample, and final sub-sampling for actual analytical determination. In the Sundarbans, the rooting pattern is found to reach, on an average, a little higher up than 90cm. During the present study, therefore, profile pits were excavated till 90cm depth, which was considered to be deep enough to reveal the principal features effecting plant growth. However, for the studying the changes in soil properties due to occurrence of mangrove vegetation, only surface soils of 15 cm depth were considered. The soil samples were collected from different point of a particular area. At least 3 replicates were sampled so as to ensure more or less uniform and representative soil sampling. After collection, all the sub-samples were put in a clean polythene sheet and mixed the soil samples thoroughly. Then the soil samples were divided into four parts, from these, two of them were removed and remaining two portions were mixed again. After that some amounts of soil were discarded and about 1 kg of soil for each sample was retained for the further analysis. Along with the variation of soil, the profile depth and landscape areas were also kept in mind. The entire work was carried out in five phases. During the first phase of the
study, soil samples were collected from one mangrove dominated and one adjacent without mangrove soils in ten locations under Sundarbans. The collected soil samples were analysed for different soil properties for assessing the specific variations in such properties between mangrove and non-mangrove soils. During the second phase, regular collections of soil samples were done from the Indian part of Sundarbans covering 15 blocks of Sundarbans Tiger Reserve and adjoining areas (Fig. - 3). These blocks were divided into five major zones viz. (i) Eastern Zone (ii) Western Zone (iii) Central Zone (iv) Northern Zone and (v) Southern Zone according to their occurrence. Soil samples were collected extensively from these belts following the procedures mentioned above. Analysis of the collected soils were carried out by following standard procedures, as stated in sec. 3.2. During the third phase of the study, the nature and properties of rhizosphere soils under different mangrove vegetations were assessed. Twenty three mangrove species commonly occurring in these soil zones were identified and soil samples were collected from their root zones to assess the preferred habitat and nature of soil required for the specific mangroves for their growth and survival. The methods of the collection of the samples have been described earlier and the analytical procedures have been discussed in section 3.2. During the next phase of the study, some common bio-chemical properties of different mangrove leaves were studied for assessing the relationships of these properties with the natures of mangrove habitat soils. For this purpose, leaves of 23 numbers of commonly occurring mangrove plants of Sundarbans were collected from different mangrove forests and their bio-chemical compositions were studied following standard methods. Details of this study have been presented in sec. 3.3.
During the last phase of the study, mangrove soil maps for Sundarbans Tiger Reserve Forest were prepared with respect to some important
soil properties viz. pH, organic carbon, available nitrogen, phosphorus and potassium and texture using Geographic Information System (GIS) with the major objectives of providing gross information on the variations in these properties in different mangrove habitats in Sundarbans. The steps taken for preparation of GIS maps have been presented in sec. 3.4.
3.2. Analysis of Soil Samples: The collected soil samples were air dried at a temperature of about 25º-30º C and relative humidity of about 20%-60% in the laboratory. This was ground with the help of wooden mortar to pass initially through 2 mm size sieve and finally through 80 mesh size sieve. The standard methods adopted for these analyses are as follows: (i) pH - pH was analysed by potentiometric method using a pH Meter, which directly gives readings after proper calibration. The soils were subjected to shaking with distilled water at 1:2 ratio for about half an hour after which pH was determined with the help of an electrically operated pH meter (Jackson, 1973). (ii) Electrical Conductivity (EC) - EC values indicate the total concentration of ionised constituents of a system. It is closely related to the sum of cations and anions, as determined chemically, and usually correlates with the amount of total soluble solids. EC was determined with the help of an EC bridge known as Conductivity Cell. The cell constant of the cell was first determined by measuring the conductance of standard potassium chloride (KCl) solution, specific conductance of which is known. Cell constant is equal to the known specific conductance of standard KCl solution/conductance shown by the cell. After calibration, the soil samples were used for determining conductivity as before. Electrical conductivity was measured from the same 1:2 soil: water extracts of the soil samples, as was used for pH estimation. EC = Measured conductivity (mmhos cm-1) X cell constant.
(iii) Salinity - Cl- ions can be conveniently estimated by titration with silver nitrate (AgNO3) in the presence of chromate ions. AgNO3 forms silver chloride (AgCl2) by reacting with the (CrO—4) ions present in the water. After the Cl- in water becomes exhausted, AgNO3 then reacts with the CrO— 4 ions to show a red color of silver chromate (AgCrO4) indicating that the titration has been completed. For this purpose, the AgNO3 solution was first standardised by using 10 ml of standard NaCl solution in a porcelain basin. To this distilled water was added for making the volume to100ml. Then added 0.5ml of K2CrO4 to the solution and titrated with AgNO3 till the appearance of a permanent red precipitation. Standardised the factor of the strength of AgNO3 solution with the help of titrate values. For the Cl- estimation, took 5g of soil in 20 ml of DW in a 50 ml conical flask then added 0.5ml of K2CrO4 and titrated with AgNO3 to get the end point on formation of permanent red precipitation. By this method the chloride ions was estimated and for conversion of this value to salinity of the soil sample, the following formula was used. Salinity (ppt) = Chlorinity (ppt) X 1.805 + 0.03. (iv) Organic Carbon (OC) - The rapid titrimetric method of Walkley and Black (1934) using heat of dilution was used for determination of organic carbon in soils. It influences various physico-chemical properties of the soil including release of different nutrients to more available form. This method has an advantage that it excludes less active elementary carbon of soil (e.g. graphaite) and includes only that part of OC which may be biologically available. Under this method the organic matter in soil is oxidized with excess standard potassium dichromate (K2Cr2O7) using heat of dilution of added concentrated H2SO4. The unutilized K2Cr2O7 is then titrated with standard ferrous ammonium sulphate (Fe(NH4)2(SO4)2) and the amount of organic carbon is determined from the amount of standard K2Cr2O7 used for oxidation. In a 500ml conical flask 1g of soil was taken and moistened with a few drops of water and kept for 10miniutes. Then added exactly 10ml of 1N K2Cr2O7 and then 20ml of
concentrated H2SO4. The contents were mixed thoroughly and kept in a dark place for 30 minutes. Then 200ml of DW and 10ml of Orthophosphoric Acid (H3PO4) were added to this conical flask. Then 1ml of diphenyl amine (DPA) indicator was added to develop a deep blue color. The solution was titrated with standard (Fe(NH4)2(SO4)2) solution and at the end point the blue color suddenly changed into a bright green.
A blank with all the reagents but
without the soil was also carried out. Organic Carbon (%) = (B - A) x 0.3 Where, B = titration value (ml) of Fe(NH4)2(SO4)2 in blank set. A = titration value (ml) of Fe(NH4)2(SO4)2 with soil. (v) Nitrogen - (a) Available Nitrogen – Fractions of organic nitrogen like amino acid, peptides or easily decomposable proteins constitutes what is known as easily mineralisable nitrogen. The amount of N under these states are easily transformed into readily available forms and may thus provide an index of available nitrogen status of the soils. The amount of N under easily mineralisable form was determined by oxidizing the soil organic matter with mild oxidizing agents so that only the easily mineralisable forms of organic nitrogen are oxidized. 0.32 percent potassium permanganate (KMO4) is commonly used for this purpose (Subbiah & Asija, 1956). The released NH3 is absorbed in H3BO3. In a Kjeldhal flask 20g of soil samples was taken and 20ml of DW was added and mixed them thoroughly. To this flask about 1ml of liquid paraffin and few glass beads were added to prevent bumping and frothing. Then 100ml of 0.32 KMO4 and 100ml of 2.5 percent NaOH were added to this flask. The mouth of the condenser was immediately connected and the contents were distilled and nearly 100ml of the distillate was collected in a H3BO3 solution (20ml) of a receiving conical flask. Then the distillate was titrated with 0.02N H2SO4 till a blue color was developed. If the volume of 0.02 N H2SO4 required for titration is X ml, then amount of easily mineralisable N in soil will be X x 1.4 mg per 100g soil (Chattopadhyay, 1999).
(b) Total Nitrogen – It was determined by Kjeldahl digestion and distillation method (Jackson, 1973). Most of the nitrogen (N) in soil occurs in organic forms from which state this nutrient element is gradually released to available form. 5g of soil sample was taken in a Kjeldhal flask, then 20ml of DW was added to this, the samples were kept for about half an hour. Then 35ml of conc. H2SO4 was added and mixed thoroughly. After this, 10g of digestion mixture was added and the sample was heated at 100o C for 20 mins. and then the digestion was raised temperature between 360-410oC. The boiling process continued until the color of the mixture becomes more or less transparent. The completion of this digestion process took about one and half an hour. After the completion of the digestion process, the mixture was cooled and about 250ml of DW was added to it. The Kjeldhal flask containing the digested sample was fitted to the distillation set. Then a few glass beads and about 1ml of liquid paraffin was added into it. 25ml of 4 percent H3BO3 – indicator was taken in a 150ml conical flask and the flask was fitted with the receiving end of a distillation set, the delivery tube of the distillation set dipping into the boric acid. 100ml of 40 percent NaOH was added to the solution and the mouth of the flask was closed immediately with the connector of the distillation set and the distillation was carried out as stated earlier. If the volume of 0.1 N H2SO4 required for titration of the sample = x, Then, concentration of total N in soil (%) = x X 1.4 / W Where, W = Wt. of the soil (g) taken. (Chattopadhyay, 1999) (vi) Cation Exchange Capacity (CEC) – The total quantity of cations which a soil can adsorb by cation exchange is termed as Cation Exchange Capacity (CEC) of the soil and is expressed as milliequivalent per 100g of soils (USDA, 1961). The amount of easily exchangeable cations being higher in soils with higher CEC values, such soils are likely to provide more amount of
positively charged nutrient elements in easily available form. For the determination of CEC, the soil exchange phase is first saturated with sodium (Na + ) ions. These Na + ions are again replaced by any other suitable cation and the amount of Na + ions released from the exchange site are measured as index of CEC. In a 250ml bottle 5g soil sample was taken and into this 100ml of 1 N CH3COONa (pH – 8.2) was added and shaken for half an hour. Then the bottle was kept horizontally overnight and on the next day the supernatant liquid was decanted very carefully. After that 35ml of ethyl alcohol was added to that bottle, shaken well, kept for sometime and then the supernatant portion was filtered carefully without disturbing the soil. This procedure was repeated for three times and for every time the same filter paper was used for all the filtration processes. The filter paper was put inside the bottle with soil sample and 100ml of 1N CH3COONH4 (pH – 8.0) was added and the bottle was shaken for half an hour. Then the total contents in the bottle was filtered in a clean 1 l volumetric flask and the volume of the filtrate was make up to 1l with DW and the concentration of Na + ions in this solution was determined flame photometrically. If the concentration of Na + ions in extracted solution is X ppm then, CEC of the soil sample will be 20X/23 meq per 100 g soil. (vii) Exchangeable Sodium and Potassium – Exchangeable Na + and K+ can be estimated by using standard procedures after replacing these cations from the exchange complex by NH4+ ions using neutral normal ammonium acetate (CH3COONa). In a 250ml bottle, 5g soil sample was taken and into it, 100ml of 80% alcohol was added and the solution was shaken for five minutes. and the sample was kept lying for 2 hrs without disturbing the bottle so that the suspended colloids settle down and the supernatant portion was filtered with Whatman No. 42 filter paper. After the filtration, the wet filter paper was opened and placed on a funnel. Then the funnel was put on the mouth of the bottle. 100ml of neutral ammonium acetate was taken into a 100ml cylinder.
The ammonium acetate was pipette out and the filter paper was washed in such a way that the residual soils in the filter paper were washed down to the soil sample in the bottle. Then the bottle was closed with a stopper and shaken for half an hour in a mechanical shaker. The entire suspension was filtered through the same filter paper used earlier. Then the required amount of aliquots were taken from this filtrate and determined the exchangeable Na + and K+ ion concentration in the sample were determined flame photometrically by following Jackson (1973). (viii) Available Phosphorus – The amount of available phosphorus (P) in soils depends largely on the magnitude of distribution of it’s major inorganic forms which are influenced largely by some soil properties, especially the pH of the soils. . The available phosphorous was estimated by using Olsen’s sodium bicarbonate extractable method due to the alkaline nature of the soils in the Indian Sundarbans. This method (Olsen et al, 1954) involves the extraction of soil P by sodium bicarbonate solution (NaHCO3) adjusted to pH 8.5. This extraction procedure decreases the activity of Ca in soils and thus helps some P to be extracted from calcium phosphate form. In addition, this reagent also extracts some P from the surface of aluminium and iron phosphate (Jackson, 1971). This form of P is widely estimated by the spectrophotometric method after the development of phosphomolybdic blue colour. In this method 5gm of soil samples were taken in a 250ml bottle, and 100ml of NaHCO3 solution and one tea spoon of activated charcoal were added in this bottle. The suspension was shaken for half an hour and then filtered through a P free filter paper. Then 10ml of aliquot was taken in a 25ml volumetric flask from the filtrate and diluted the solution by adding of 15 ml of DW. A few drops of 2 - 4 DNP indicators were added and the color solutions turned yellow. Dilute HCl was added drop by drop in this way that the color of the solution suddenly changed to colorless. Then 5ml of ascorbic acid molybdate mixture were added to this solution and then the volume was made upto 25ml by adding DW. The optical density (OD) of the solution was measured spectrophotometrically at 660nm.wave length. A standard curve
of blank solution using all the reagents and different concentrations of standard phosphate solutions was also prepared. If OD reading for the 25ml volume with 10ml aliquot corresponds to X ppm of P, in standard solutions then the concentration of Olsen’s extractable (available) P in the soil will be X x 50. (ix) Soil Texture – The objective of determining the textural composition of soils is to know the percentage of soil materials contained in different grain size fractions viz. sand, silt and clay and to classify the soils to act particular textural group so that the dominant grain sizes present in the soil can be identified easily. Soil Texture was determined by International pipette method. In a 1000ml of conical flask 50gm soil sample was taken and into this soil 100ml of calgon was added and then 400ml of DW was added. The sample was kept for 10 minutes without disturbing the flask. Then after the resting period, some amount of DW was added to fill the conical flask to 2/3rd level and the content was stirred for 6 minutes, for sand, 10 minutes for light sandy loam and 15 minutes for all other soils. Then the content was poured and washed to a measuring cylinder and diluted the content to the lower mark 1130 ml, while the hydrometer was in the suspension. The reading of the hydrometer was taken and the ambient temperature was also recorded. 0.2 scale units on the hydrometer is made for each degree above 670 F. Corrected hydrometer reading = R + (t -67) X 0.2 Percent material still in the suspension = x / 50 X 100 = Y Similarly, with the hydrometer reading at the end of 2 hours, the % of clay calculated. Both the percentage of sand and clay fractions were added together and subtracted from 100 to give the percentage of silt.
3.3. Collection and analysis of the mangrove leaves The mangrove leaves were collected from the Indian Sundarbans specially from the Sundarbans Tiger Reserve and their adjoining areas. The
collection was conducted with the help of forest guard on a mechanized boat Since some important properties of the mangrove soils are likely to be influenced by the qualities of the mangrove leaves which tend to accumulate in the bottom of such vegetation, during the fourth phase of the study, some biochemical properties of the commonly occurring mangrove leaves were analysed. These were as follows: (i) Carbohydrate (mg g-1) - The carbohydrate content of leaves was determined by hydrolysing the polysaccharides into simple sugars by acid hydrolysis (5ml of 2.5 N. HCl) in boiling water bath for about 3 hours. After cooling at room temperature, the solution was neutralized with sodium carbonate. The volume was made into 100 ml and then centrifuged. The supernatant solution was used for analysis. To the aliquots of samples taken, anthrone reagent was added and again heated in a boiling water bath for 8 minutes. The sample was cooled rapidly and the green to dark green color was read at 630 nm in the spectrophotometer. Amount of carbohydrate present was determined by plotting on a standard curve. The amount of carbohydrate was expressed as mg/g. (ii) Protein (mg g-1 ) - The water soluble protein content was determined by Lowry’s method. 0.1 g of leaf sample was taken then added 2ml of DW and the sample was homogenization with mortar and pastle. After homogenized the 1.5ml of the sample was taken from the total solution and centrifuged at 14000 rpm at 40 C for 20 minutes. Then 1.2 ml of the supernatant was taken and estimated the total protein content by Lowry’s Method using Folin reagent. (iii) Lipid (%) - The lipid content was analysed by acid hydrolyzing the samples to limit out the other esterifiable compounds. Estimation of total lipids that are extractable in chloroform – methanol solution was done by gravimetric method. The moisture content of this air-dried samples was first determined. The samples were then extracted with chloroform: methanol: water (2:1:0.8, volumetrically), according to Bligh & Dyer (1959). The
chloroform layer containing the lipids was then collected. This chloroform was completely evaporated to obtain the total lipid of the sample. The total content was determined gravimetrically and expressed as a percentage on dry weight basis.
3.4. Preparation of GIS Maps Geographic Information System (GIS) is a recent technology which offers an appropriate method for integrating the land and water resources information and for identifying agro-climatically coherent zones for suggesting locality specific prescriptions and treatment packages. During the last phase of study, GIS maps for some important physico-chemical properties of mangrove soils of Sundarbans Tiger Reserve Forests were prepared. Six soil parameters viz. pH, organic carbon, available nitrogen, available phosphorus, exchangeable potassium and textural composition were used for these maps. Data on relevant soil properties collected in the present work programme for different blocks under Sundarbans Tiger Reserve Forest were put to different locations on a satellite map of the said reserve forest and the maps for different soil properties were prepared by using the instrumental facilities and expertise of the Regional Centre of National Bureau of Soil Survey and Land Use Planning of Indian Council of Agricultural Research situated in Kolkata.
Results and Discussion Chapter
4.1. Introduction to mangrove habitats of Sundarbans
As have been discussed earlier (sec.1.2), mangrove forests constitute an important component of the deltaic Sundarbans in India. The significance of this forest in providing sustenance to different aquatic and terrestrial lives is also well documented (Naskar and Mandal, 1999). The distribution and growth of these mangroves, on the other hand, depend largely on the geographical properties of the delta. Before going into the results of the present work programme, therefore, an attempt has been made in this section to provide some basic geographical information about Sundarbans, which are required to develop an idea about the nature of mangrove habitats in this island.
As has been discussed in section 1.1, Sundarbans forms the largest single tropical deltaic mangrove forest within the latitude 21o31/ and 22o30/ North and longitude 88o10/ and 89o51/ East, situated in the Ganga-Brahmaputra estuary covering parts of both India and Bangladesh. The Indian Sundarbans falls under the jurisdiction of North 24 Parganas district and South 24 Parganas district in West Bengal.
The land mass of Sundarbans is of comparatively recent origin (6000 - 7000 year BC) and has been built up through the gradual deposition of silt and
clay particles, carried down by the river Ganga and Bramhaputra from the Great Himalayas and Chhota Nagpur Hills (Morgan and McIntire, 1959).
Similar to the evolutionary processes in the biological community, the coastal evolution is a continuous process through millions of years. Geographical locations of the coastal zone keeps changing through geological time due to the effect of the evolutionary processes. The Bengal Basin is an interesting basin from the geological point of view resulting from a number of interrelated global processes viz., plate tectonics, Gondwana break, closure of Tethys Ocean and effects of glacial, deglacial phases together with the tectonic event of upliftment of the great Himalayas as a result of Alpine - Himalayan Orogenesis (Sanyal, 2002).
The entire Bengal Basin was formed mainly under the influence of marine environment. Still fluctuations between marine facies to continental facies have occurred many times by the effect of tectonic and glacial cycles. Environment of deposition in the western geo-province has gradually changed from deep marine Abyssal, Bathyal facies to continental slope (Banerjee, 1998). During the 16th - 18th century the Bengal basin was affected by a neo-tectonic movement by way of which an eastward tilt came along a hinge zone, i.e, from Sagar to north of the district Malda of West Bengal and then gradually curving towards Dhaka, Bangladesh. As a result of the trend of surface elevation contours ENE-WSW, the present course of Ganges started flowing along the river Padma within Bangladesh leaving Hooghly as a mere tidal channel. Even till the early eighties the tidal effect of Hooghly could be felt upto 281 km upstream upto Nabadwip in the district of Nadia, West Bengal (Sanyal, 2002).
Morgan and Mc Intire (1959) reported that with gradual eastward tilting of the
Bengal basin, the entire delta is characterised by significant
subsidence. The river Ganges used to flow along the course of Tamralipta (one within the district of Midnapore East, West Bengal) till 12th Century A.D. and between 12th and 16th Century there was an eastward tilt of the Bengal basin which resulted in a shift of the Ganges towards east and river Padma became active. During this period, the Matla and Bidyadhari river systems formed innumerable network of creeks between the Ganges and the Padma.
4.1.3. Soil Properties
The lower Ganga delta is covered solely by the quaternary sediments carried and deposited by the river Ganga, Matla and Bidyadhari river courses. According to a Report of the Geological Survey of India (Anonymous, 1974), there are two major groups of deposits in this area - Recent to sub-recent Newer alluvium; Pleistocene-Older alluvium. The pleistocene deposits comprise of clay, silt, kankar and boulders (assorted), which are locally cemented. These are characteristically coloured as reddish brown on the exposed surface. Recent sediments consist of sand, silt, clay and pebbles. The whole sediment is composed mainly of montmorillonitic, which is very sticky. They are derived from the basic and semi-acidic rocks like dolerite, gneiss and mica schist lying within the course of the Ganges flow (Management Plan, 2000-2010). Salinity raises with the age of the sediment, older the sediment, higher is the salinity within the forest area.
However, extensive studies on the specific soil characteristics of the
Sundarbans mangrove habitats are very meager. Only limited numbers of studies on general properties of Sundarbans soils have been carried out (Bandyopadhyay, 1987; Yadav et al, 1979) which also included mangrove dominated areas (Sahoo et al, 1985 and other workers).
The climate of the Sundarbans Tiger Reserve is equable to the tropics mainly due to its close proximity to the sea, having heavy annual rainfall and overall humid climate. The summer extends from the middle of March to the middle of June and the winter extends from December to February. The climate is more equable in the areas covered by forest than in the neighboring cleared areas. The monsoon starts usually between the middle of June and lasts up to the middle of September. Rough weather usually persists mid March to September and the fair weather generally prevails between middle of September to middle of March.
Rainfall Rainfall is the main source of increasing the dilution factor in and around the mangrove ecosystem. The dilution factor practically increases both due to precipitation and subsequent run off from the adjacent land masses. However low salinity (as witnessed due to heavy rainfall) along with prolonged periods of flooding may create an adverse impact on the vegetation through reduced cell turgor and decreased respiration (Triwilaida and Intari, 1990). The average annual rainfall of the Sundarbans Tiger Reserve is reported to be 1920.30 mm of which about 80% occurs during June to September only.
Temperature Mangroves grow mostly in lower latitudes where solar radiation and temperature of air and water are commonly high. In Indian Sundarbans, air and water temperature is very congenial to the rate of assimilation
times of the year.
Tidal Amplitude The tidal range is an important parameter for controlling the growth of mangroves. The mangrove forests are universally situated in deltaic or estuarine areas, which are regularly under the influence of tidal regimes. The mangroves are well adapted to cope with the waves and currents of the marine and estuarine environments. However, the extent of current and tidal amplitude influence the floristic distribution of mangroves, mangrove associates and back mangroves. It also plays a decisive role in erosion and sedimentation prediction. Tidal amplitude and cycles has its effect on animal behavior and also governs anthropological activities in the forest areas like fishing, honey collection, timber collection, crab collection, catching of fish and prawn seeds, etc. While generating information about the mangrove habitats of Sundarbans, therefore, it is necessary to have a gross idea about nature of tidal impact, its cycles, rate of progress of tidal ingress and related factors including wave dynamics, littoral environment, near shore bathymetry etc.
The deltaic Sundarbans experience semi-diurnal tides. The tidal amplitude and current has profound influence on the survival and growth of mangrove vegetation. It also influences the sedimentation and erosion processes. This tidal amplitude is governed by a number of factors, including waves, wind direction, wind energy, season and temperature.
With the change in seasons, tidal interactions in the estuarine system in and around the Indian Sundarbans also change (Pillay, 1958). During the monsoon month, the effect of flood tide is more or less countered and nullified by freshets and there is a strong predominance of ebb tide. The strength of flood tide over ebb tide is at a minimum during the post-monsoon season. Conversely, during the pre-monsoon season, the effect of flood tide is considerably stronger than that of the ebb tide.
This estuarine area is much fragile and prone to severe natural calamities. Frequent cyclones and unfavourable tectonic and geodynamic problems prevail in this area. Every year, several cyclonic storms are of common occurrence in the lower Ganga delta during mid March - mid June and occasionally during October - November. During cyclones and storms, the sea or river water rises up far beyond its normal reach and the wind force hits the surrounding and neighboring areas. Fosberg (1971) emphasized that this delta at head of the funnel shaped Bay of Bengal possesses perhaps the most serious threats from the surges driven by storm waves. While occurrence of Sundarbans forest vegetation helps in reducing and breaking the violence of the cyclonic weather (Rao, 1959), such vagaries of nature tend to exert profound influence on mangrove vegetation also. In view of this importance of mangrove vegetation on the ecology of Sundarbans, it appears to be pertinent to develop a thorough understanding about the habitats of such vegetations.
4.2. A comparative study on the properties of mangrove vis-à-vis non-mangrove soils of Sundarbans.
Substrate characteristics are important determining factors that control the community structure and growth of mangrove ecosystem (Koch and Snedakar, 1997). On the other hand, occurrence of mangrove vegetation may also exert considerable influence on nature and properties of coastal saline soils (Sarkar et al, 1999). In view of this interdependence of mangrove vegetation and their habitats, a comparative study was undertaken to assess the difference between mangrove soils and their adjacent ones which were devoid of mangrove vegetation. It has been hoped that the study would be helpful in generating a gross idea about the nature and properties of mangrove soils with relation to the non-mangrove coastal saline soils of Sundarbans.
The methodologies followed in this investigation covering collection of soil samples, processing of the samples and the methods followed for different physico-chemical analyses have been presented in section 3.
General properties of the mangrove soils and their nearby non-mangrove ones in Sundarbans have been presented in Tables 4.1 – 4.9. Although electrical conductivity values of coastal saline soils vary widely in different seasons depending on precipitation and evaporation (Yadav et al, 1979), comparison of two kinds of soils at a particular location on a point of time was expected to provide usable information on this property. As observed from table 4.1, all the soils under both the systems exhibited high electrical conductivity (EC) values, obviously owing to being situated in an estuarine area. However, it was interesting to observe that in spite of being situated under inter tidal
zones receiving highly saline estuarine water at frequent intervals, the average EC values of the mangrove soils were marginally lower than the non-mangrove soils. This behavior was attributed to the fact that the non-mangrove soils occurred mostly in slightly higher elevations and under more dry conditions. This dryness of non-mangrove soils was due to regular evaporation of the soil-water from the surface area leading to increased capillary movement of the highly saline ground water from lower levels to surface soils (Yadav et al, 1979). This upward movement of saline water and consequent evaporation of water from the soil surface resulted in increased salt content of the nonmangrove soils. On the other hand, the soils under mangrove zones were subjected to regular inundation by tidal water and maintained equilibrium with the salinity of the estuarine water. However, when analyzed statistically, the variations did not appear to be significant (Table – 4.10).
Similar observations could be found for total salt content also (Table – 4.2). Since EC values of soils reflect the total salt content of soils, such behaviour was expected to occur. Jackson (1973), while discussing the relationship between electrical conductivity and salinity of soils, mentioned a correlation between the two properties and it was observed that the direct relationship between these two properties can be grossly established by dividing the total salt content value (ppt) by 0.64. However, this relationship may vary under different soil conditions and with nature and properties of soils as well as salts. Almost similar relationships between EC and salinity values of soils were observed in the present study also. As stated for EC earlier, the non-mangrove soils showed slightly higher levels of salinity as compared to the mangrove soils. Reasons of such behaviour have been discussed earlier. As in case of EC, salinity values also did not show statistically significant variations for mangrove and non-mangrove soils (Table – 4.10).
pH values of both the kinds of soils varied within narrow ranges, between 7.4 to 8.2 for mangrove soils and 7.5 - 8.4 for non-mangrove soils with respective average of 7.8 and 8.0 (Table – 4.3). Such alkaline pH values in coastal saline soils of Sundarbans is well documented (Bandyopadhyay et al, 1987; Mandal and Chattopadhyay, 1987) and the behaviour may be attributed to high salt content of these soils. Positive correlation between pH and electrical conductivity in brackishwater fish pond soils of Sundarbans was reported by Chattopadhyay and Mandal (1982). In the present study, the variations in pH values in both the kinds of soils were found to be limited. This behaviour was probably due to buffering action of estuarine water preventing large scale variations in pH values, as has been described by Reid (1961). As a result, the variations in pH in these two soil groups did not appear to be statistically significant (Table – 4.10).
Organic carbon content of the mangrove soils were comparatively higher than the non-mangrove soils (Table – 4.4). Such increased organic C value in mangrove soils was reported by Sahoo et al (1985) and many other workers also. This behaviour was attributed to accumulation of leaf litters and other plant residues in the mangrove soils. However, such accumulation of organic materials was likely to be reduced to some extent by the tidal water which tended to wash out a large share of the organic residues from the mangrove area thus narrowing the difference between the organic carbon contents of mangrove and non-mangrove soils. In spite of this behaviour, the differences in organic carbon status of mangrove and non-mangrove soils were found to be statistically significant (Table – 4.10). Since organic matter influences numbers of physical, chemical and biological properties of soils (Brady, 1980), such significant increment in organic carbon content of mangrove soils is likely to play important roles in governing productivity levels of these soils.
Cation exchange capacity (CEC) of the soils indicate the capacity of the soils to hold easily exchangeable cations in the exchange complex of the soils and this value has been widely correlated with the fertility status of the soils (Brady, 1980; Russel, 1975). In the present study, the mangrove soils appeared to exhibit considerably higher CEC values as compared to the nonmangrove soils (Table – 4.5). This behaviour could be largely attributed to occurrence of higher amount of organic matter in the mangrove soils. High EC values of organic matter is well known (Kononova, 1966). However, as compared to the variations in organic carbon values, differences in CEC in mangrove soils were comparatively larger. Probably, occurrence of the slightly higher percentage of finer particles in mangrove soils might have increased the CEC values of mangrove soils in addition to the effects of organic matter in such soils. The variation in cation exchange capacity in mangrove and non-mangrove soils also appeared to be statistically significant (Table – 4.10), as was found in case of organic carbon also. Such increased cation exchange capacity in mangrove soils is likely to benefit such soils in holding different cationic nutrients in easily available forms and to benefit the mangrove vegetations growing in these soils.
Available nitrogen status was marginally higher in mangrove soils (Table – 4.6). This behaviour was also attributed to higher organic C content of these soils. Since the availability of nitrogen in the studied soils was determined from easily mineralisable form, which form is directly related with organic matter status of the soils (Subbiah & Asija, 1954), a positive dependence of easily mineralisable nitrogen on organic matter content of soils was expected. However, as compared to the difference in organic matter content of mangrove and non-mangrove soils, the variations in available nitrogen status between these two kinds of soils were found to be smaller and was not found to be
statistically significant (Table – 4.10). While washing away of a good part of mineralized nitrogen by tidal water may be a major cause of such lowering in differences, slower rate of mineralization of nitrogen from organic to mineral form under saline environment (Chattopadhyay and Mandal, 1980b) may be another reason for such behaviour.
Almost similar was the situation for available phosphorus also. Mangrove dominated soils exhibited higher availability of this nutrient, as compared to the soils without mangroves (Table – 4.7). The difference was however, more prominent than nitrogen. Beneficial effects of organic matter in increasing availability of phosphorus in coastal saline soils have been discussed by Chattopadhyay and Mandal (1980a). Hence the higher rate of accumulation of organic matter in mangrove soils probably resulted in increased availability of phosphorus in such soils. In addition, phosphorus being more strongly bound with soil particles than nitrogen (Brady, 1980), possibility of washing of the available phosphorus from the mangrove soils was not so high as was in case of mineralized nitrogen. This resulted in significantly higher occurrence of phophorus in available form in mangrove soils (Table – 4.10), which may be considered to be beneficial for maintaining their productivity.
Status of the exchangeable potassium was high in both the soil groups (Table – 4.8). Among them, however, mangrove soils showed comparatively higher concentrations than the non-mangrove soils. Increased cation exchange capacity in mangrove soils has been discussed earlier. This behaviour probably helped to hold more amount of potassium in soil exchange complex and tended to increase the amount of potassium in exchangeable form in the mangrove soils. However, the variations in exchangeable potassium content in mangrove and non-mangrove soils was not statistically significant (Table – 4.10). High
potassium content of highly saline estuarine water (Chattopadhyay and Mandal, 1980c) probably masked the difference in this easily available form of potassium for these two kinds of coastal saline soils.
Textural composition of these soils showed the mangrove soils to exhibit slightly higher occurrence of fine textured soils than non-mangrove soils (Table – 4.9). Mangroves being situated in intertidal zones are subjected to regular tidal inundation carrying suspended finer soil particles. Since the flow of this tidal water is restricted by the mangrove vegetation, these finer particles get deposited on mangrove soils and tend to increase their concentrations in such soils. On the other hand, the bare non-mangrove soils are subjected to high wind action of the coastal areas leading to large scale loss of finer sizes particles through wind erosion. These two opposite actions are likely to increase the concentrations of the finer particles in mangrove dominated soils than the areas where mangrove vegetations are absent. However, the variations in any component of textural analysis were not statistically significant.
Mean values of different parameters in the two soil groups have been indicates that occurrence of mangrove vegetation tends to exert considerable influence on nature and properties of coastal saline soils as compared to their counter parts without mangroves. Of the studied properties, variations in organic carbon content, cation exchange capacity and availability of phosphorus were statistically significant in mangrove and non-mangrove soils. Most of the variations in different physical and chemical properties in mangrove soils appeared to be beneficial for soil quality as well as plant growth and are likely to result in favourable effects of mangrove vegetation on coastal saline soils.
4.3 Studies on some physico-chemical properties of mangrove soils of Sundarbans.
Results of the studies presented in previous section (sec. 4.2) have shown that the soils under mangrove vegetation exhibit several soil properties which differ considerably from those of nearby non-mangrove soils. These variations may be attributed to the occurrence of mangrove vegetation in these soils which, on one hand, influence the nature of the mangrove vegetation growing in such soils and, on the other hand, are influenced by these vegetation due to some chemical and biochemical manifestations. Considering these relationships between mangrove soils and the nature of the vegetations on such soils, therefore, a reconnaissance survey was carried out to study nature and magnitude of some physico-chemical properties of mangrove soils under Sundarbans Tiger Reserve Forest.
As discussed in the Materials and Method section, soil samples were collected from the Indian part of Sundarbans covering 15 blocks of Sundarbans Tiger Reserve and adjoining areas covering five major zones. The soils were analysed for different parameters by following standard methods, as have been described earlier (sec. 3.0).
Texture: The textural compositions of the mangrove soils in the Indian Sundarbans Tiger Reserve have been presented in Table 4.11. As observed from the table, the sand particles dominated in the textural compositions in all the soils. However, the direct effects of sands were masked by moderately good occurrence of comparatively finer particles like clay which rendered the soils to be largely loamy in texture. Since mangrove soils occur in intertidal
zones and finer soil particles are generally transported to these zones by tidal water, good accumulation of finer particles in such alluviated soils may be expected. Such occurrence has been observed in the previous study (sec. 4.2) and by different workers (Adhikari et al, 1987 and others) also. The minor difference in soil clay content between the previous study (sec. 4.2) and the present one may be due to variations in locations as well as soil depths under these two studies. While in the previous study, only surface soil samples were collected, samples from root zone depths of mangrove soils were collected in the present study. Clay particles exhibited statistically significant negative correlations with occurrence of silt and clay (Table – 4.14) indicating that the values increased with decrease in the amount of other two groups of soil particles. On the other hand, sand and silt particles maintained a significant positive relationship between these two, showing that these properties behaved in compatible manner in terms of their relative occurrence in mangrove soils.
Occurrence of such loamy texture may be
helpful to maintain a loose soil condition in mangrove soils encouraging good rooting of mangrove vegetation in such soils. In addition, these soils are likely to provide better plant nutrition to the mangrove than the soils which are coarser in texture. In the present study, only two soil zones viz. Pirkhali in northern zone and Mayadwip in southern zone showed clayey soil texture. Although these two zones are situated in almost opposite situations but they have one similarity that both of them are characterized by dense mangrove vegetation. This might have trapped larger amount of finer soil properties from the tidal water and increased the clay content of the soils. Since variations in textural compositions are associated with differences in nutrient status, chemical and physical properties and also the biological properties of soils (Brady, 1980), such changes in textural compositions are likely to influence the nature of mangrove vegetations under different textural zones.
pH: pH values of these soils appeared to be alkaline in nature, the mean values ranging between 7.7 and 8.44 (Table – 4.12 ). Apart from Pirkhali and Panchmukhani in the northern blocks, all the soils showed pH values above 8.0 and remained within a narrow range of 8.0 to 8.44 only. Such limited variations in pH values of mangrove soils have been reported in previous study (sec. 4.2) also. Reid (1961), while discussing various properties of estuarine water, stated such water to have high buffering capacity which prevents wide variations in pH values in these water body. This property of estuarine water probably tended to maintain a narrow pH range in the mangrove soils which receive tidal estuarine water at frequent intervals. pH values were correlated significantly with electrical conductivity values of the soils (Table– 4.14), indicating, once again that the highly saline condition of the mangrove soils was largely responsible for the alkaline pH values. Such slightly alkaline pH range may be congenial to the mangrove vegetations growing in such soils since most of the transformation of different nutrients and biological activities are likely to be optimum under this neutral to slightly alkaline pH range. Statistically significant negative correlations between pH and organic carbon and also total nitrogen content of soils (Table – 4.14) also indicate that biological degradations of organic matters in mangrove soils were influenced by the alkaline pH values.
Salinity: Salinity values of such soils were high, the mean values for different centres varying between 11.1 ppt and 25.2 ppt (Table – 4.13), obviously owing to remaining under action of highly saline tidal water. Such high salinity in mangrove soils has been reported by Satyanarayana et al (2002) and many others. Since salinity of estuarine water is a major function of precipitation
and evaporation, such water bodies, in general, show a wide seasonal variation recording high values during summer and low values during monsoon. However, as stated earlier, recording of soil salinity values from different places at a particular time will help in making a gross comparative assessment of the salinity status of these regions. In the present study, the mangrove soils under the southern block showed comparatively higher salinity values and this behaviour may be attributed to it’s being situated nearer to the sea. Similar distribution of soil salinity in Bangladesh part of Sundarbans has been reported by Hossain et al (2001) also. Importance of salt content on various biochemical activities of mangrove soils has been discussed earlier. In the present investigation also, salinity status was found to exert profound influences on different properties of mangrove soils. This property showed significant negative correlation with available phosphorus and total nitrogen values of mangrove soils (Table – 4.14). Decreased availability of phosphorus in coastal saline soils due to large scale fixation as calcium phosphate has been reported by Chattopadhyay and Mandal, (1980a). On the other hand, dominance more resistant components of mangrove leaf litters in the soils with wide C:N ratios may be responsible for the observed negative relationship between soil salinity and total nitrogen status of such soils. Availability of nitrogen was, on the contrary, observed to be positively correlated with soil salinity. Since high salinity levels tend to reduce the nitrification rates, thereby, restricting escape of more labile nitrate form of nitrogen from the soil phase (Chattopadhyay and Mandal, 1980b), such a relationship could be possible.
Electrical Conductivity: Electrical conductivity (EC) values are the functions of total soluble salt content of the soils which controls the conductance of the system. The
EC values (Table – 4.12), therefore, exhibited close relationship with total salt content of the soils (Table – 4.14), as has been discussed earlier in sec. 4.2. In the present study also, the soils of southern block, which exhibited high amount of total salt content, resulted in highest EC values. Other soils also maintained the same kind of relationship. As have been observed for soil salinity, electrical conductivity values also showed significant negative correlations with total nitrogen and available phosphorus values and positive relationship with available nitrogen status of the mangrove soils, as have been discussed earlier. The hypothesis that the high saline conditions were dominated by soil organic matter with wide C:N ratio has been substantiated by a positive correlation between electrical conductivity and C:N ratio of the soils (Table – 4.14).
Organic Carbon: Organic carbon content of the mangrove soils varied considerably in different zones. While Eastern, Western and Northern zones showed higher values of soil organic carbon, the Central and Southern zones exhibited comparatively lesser occurrence (Table - 4.13). The values of organic carbon in soils appeared to be associated with occurrence of mangrove vegetations and also the tidal flow in different zones. Aksornkoae (1986) stated that high rate of leaf production is a characteristics of mangrove vegetation. When these leaves fall in the stream, there is initially a rapid leaching of dissolved organic matter (Fell et al, 1975). The remaining leaf biomass, particulate organic matter, is decomposed more slowly (Robertson et al, 1992) and thus contribute to the organic carbon content of mangrove soils. Although the rates of leaf litter fall and exchange with the coastal water may vary under different situations, studies carried out on this aspect have shown about 50% of leaf litter carbon to be transported through coastal water, 25% recycled
by the mangroves themselves and the remaining 25% to be accumulated in mangrove sediments (Robertson and Daniel, 1989; Lacerda, 1992). Considering that the Indian Sundarbans have an approximate leaf litter fall of 721 g C m2
yr-1 (Jennerjahn and Ittekkot, 2002), considerable amount of organic carbon
is likely to be accumulated in mangrove soils every year. In the present study, the soils situated in the core areas exhibiting comparatively denser vegetations due to lack of human interference resulted in higher accumulation of organic matter in soils through increased leaf fall. In addition, movement of tidal water also played an important role due to washing effect of engrossing water on soil organic matter. Decrease in mangrove derived organic carbon accumulation in sediments with increase in distance from the main shore has been discussed by Torgersen and Chivas (1985) also. Thus mangrove soils situated in the Southern zone, which experience higher magnitude of tidal washing, showed lowest occurrence of organic carbon in soils. Organic carbon is known to influence many important physico-chemical properties of soils (Brady, 1980). In the present study also, this character was found to be significantly correlated with several properties of mangrove soils of Sundarbans. Exchangeable sodium values of the soils showed a significant negative correlation with organic carbon. Such beneficial effects of organic matter on reduction of exchangeable sodium values has been reported by Chattopadhyay and Mandal (1980c) for coastal saline soils. This property also showed a negative correlation with pH values, as has been discussed earlier. It was interesting to observe that organic carbon values also showed negative correlation with available nitrogen status of the mangrove soils, which was contrary to general observations. It has been discussed earlier that with the mineralisation of easily decomposable components of mangrove litters in the soil, most of the released nitrogen get washed away. This results in accumulation of degradation resistant organic materials with low mineralisable
nitrogen values in the soils thus widening the ratio between organic carbon and available nitrogen in the soils.
Total Nitrogen: About 98-99% of total nitrogen in soils occurs under organic form and the nutrient is gradually mineralized into mineral forms which are readily available to the plants (Brady, 1980). As compared to the organic carbon contents of these soils, total amount of nitrogen appeared to be comparatively low (Table – 4.13). This behaviour may be attributed to moderately wide C:N ratios of these soils indicating that after primary mineralisation of the easily degradable components of accumulated organic matter, the more resistant constituents remained left in the soils. These fractions are not degraded easily by the limited populations of microorganisms prevalent in the highly saline aquatic environment (Gupta and Bajpai, 1974). This behaviour resulted in slow but gradual accumulation of organic nitrogen in the soils. In the present study, therefore, significant positive correlation between total nitrogen and organic carbon values could be observed, showing interdependence of these two properties. As discussed earlier, in mangrove soils, easily mineralisable components of total nitrogen are washed away through tidal water. This behaviour is accelerated under alkaline pH levels and the pH values in mangrove soils are also influenced by soil salinity. Hence in the present study, available nitrogen, pH and electrical conductivity showed significant negative correlations with total nitrogen content of the soils. Since increased concentration of organic matter tends to improve the availability of phosphorus in coastal soils (Chattopadhyay and Mandal, 1980a) and total nitrogen values showed positive correlation with organic carbon content of these soils, total nitrogen content in mangrove soils was also found to be positively correlated with the availability of phosphorus in such soils.
Available Nitrogen: Available soil nitrogen, as extracted by alkaline permanganate method in this study (Subbiah and Asija, 1954), includes easily mineralisable components of nitrogen from organic forms. Hence organic matter content of any soil generally maintains a direct bearing with the available nitrogen status of that soil. However, in spite of large scale occurrence of moderate to high concentrations of organic carbon in the studied mangrove soils (Table – 4.13), the availability of nitrogen in the present study appeared to be predominantly low showing a negative relationship with organic carbon contents of the soils (Table – 4.14). This may be due to lower rate of mineralisation of nitrogen under the high saline conditions existing in mangrove soils and also washing away of the released nitrogen by tidal water, as stated earlier. While working on transformation of organic matter in brackishwater wetlands soils, Chattopadhyay and Mandal (1980b) observed release of mineralized nitrogen from organic forms to decline gradually with increase in salinity levels. This behaviour might have restricted further the mineralization of the slowly degradable components of organic matter in highly saline conditions of mangrove soils to release nitrogen into mineralized forms and thus prevented them from being washed away by tidal water. This behaviour was reflected in significant positive correlations of available nitrogen with electrical conductivity, salt content and also exchangeable sodium values which are known to reduce the rate of nitrogen mineralisation from organic matter.
C:N ratio C:N ratios of the soils were moderately wide in most of the soils (Table – 4.13). Accumulation of some slowly degradable components of mangrove leaves having wide C:N ratios have been discussed earlier. It was possible that accumulation of such leaf litters in the soils resulted in wide C:N ratios of
the organic carbon values of the soils. Such wide C:N ratios of mangrove soils of Sundarbans indicate slow release of nitrogen into available form from the soil organic matter thus substantiating the low availability of nitrogen in such soils.
Available Phosphorus: In soils, phosphorus generally remains fixed largely as Fe and Al-P in acidic condition and as Ca-P in alkaline conditions bearing only a small fraction of total P content of soils in available form (Brady, 1980; Russel, 1975). In the present study, the available phosphorus status of mangrove soils of Sundarbans appeared to range between moderate to high values (Table – 4.13). Chattopadhyay and Mandal (1980a), while working on the nature and properties of brackishwater wetlands of West Bengal, reported P to remain fixed largely as insoluble calcium phosphate under highly saline conditions. In the present study also, negative correlations between availability of phosphorus and electrical conductivity as well as salinity of mangrove soils have been observed. However, dominance of near neutral to slightly alkaline pH values probably restricted such large scale fixation of soil Pin the studied soils. In addition, higher organic carbon status of the mangrove soils also helped to reduce the magnitude of transformation of P into insoluble forms and thus helped to maintain comparatively higher status of P in available form.
occurrence of phosphorus in available form in most of the mangrove soils of Sundarbans is likely to be beneficial for such vegetation.
Exchangeable Potassium Exchangeable potassium values of the studied mangrove soils, as determined from estimation of exchangeable forms appeared to remain in moderate to high levels. High K content in estuarine water (Reid, 1961) may
be an important factor contributing to such occurrence of available K in the studied mangrove soils. In addition, dominance of potassium bearing illitic clay minerals in coastal saline soils of Sundarbans may be another reason for such behaviour. Since this nutrient occurred in moderate to high concentrations in all the studied mangrove soils (Table – 4.12), the availability of this nutrient did not appear to be critical to influence occurrence and relative distribution of mangrove species in these soils.
Exchangeable Sodium Occurrence of sodium in exchangeable form constitutes an important property for saline and alkaline soils. Significant effects of exchangeable sodium percentage on various physical, chemical and biological properties of soils are well documented (USDA, 1961). In the present study, mean occurrence of exchangeable sodium varied between 3.8 and 7.1 meq 100 g–1 soil (Table – 4.12). While studying the relative occurrence of different cations in estuarine water, Chattopadhyay and Karmakar (1984) also reported high concentrations of sodium in such water, which might have influenced the occurrence of this element in soil exchange phase also. High concentrations of exchangeable sodium are likely affect various physiological activities of many common plants (USDA, 1961). However, the mangroves being highly adaptable to such salinity based stressed conditions, it is to be seen to what extent mangrove vegetations are likely to tolerate
the occurrence of exchangeable sodium
ions in these coastal soils. In addition, effects of this ion on various relevant properties of the soils e.g. occurrence of exchangeable potassium, calcium and magnesium, microbiological activity, mineralisation of organic matter etc may be indirectly related with the growth of mangrove plants in these soils.
4.4. Studies on some physico-chemical properties of mangrove rhizosphere soils with relation to nature and magnitude of vegetation in Sundarbans Tiger Reserve. General nature and properties of mangrove soils under Sundarbans Tiger Reserve Forest have been discussed in sec. 4.2 and 4.3. The studies have shown that mangrove soils vary considerably as compared to the nonmangrove soils even in same locations and also with other mangrove soils under different locations with regard to some properties. While studying mangrove zonation pattern, several workers have shown that physiological adaptations to such variations may appear to be useful for explaining the observed zonations of mangroves (Smith, 1992, Satyanarayana et al, 2002). Under this context, it was thought that studies on the rhizosphere soils of different mangroves are likely to provide some useful information on habitat preference of these mangrove species. In the present study, therefore, some important physico-chemical properties of mangrove rhizosphere soils of Sundarbans have been studied with relation to intensity of mangrove vegetations in the area.
For the purpose of the study, soil samples were collected from the rhizospheres of different mangrove species in the Indian Sundarbans. Details of the procedures of collection have been presented in sec. 3.0. Twenty three mangrove species commonly occurring in Sundarbans, were identified for this purpose. The soils were collected from 15 cm level of the upper soil zone for each of these mangrove rhizosphere under different locations. The methods of different analyses have been presented in sec. 3.0. Relative occurrence of these mangroves in different locations were also studied and were graded into three categories based on the intensity of their occurrence.
Efforts were made to correlate the recorded intensity levels of mangrove species with the studied properties of the soils in order to develop a gross idea about the possible habitat preference of different mangrove species.
Texture Textural compositions of rhizosphere soils for different mangrove species commonly found in Sundarbans have been presented in Tables 4.15A to 4.15E. The results are similar to those reported in the previous study (sec. 4.3). In the present study, an effort was made to correlate the occurrence of different particle size fractions with the intensity of mangrove vegetations. This effort did not appear to be statistically significant indicating that gross occurrence of the mangrove species did not depend on the textural composition of their habitat soils. However, species wise occurrence of different mangrove vegetation appeared to be more influenced by the soil textural groups. In table 4.15F, an effort has been made to identify the mangrove species which were found to be associated with different textural sub-groups of the soils. As observed from the table, loamy textures under different subclasses viz. loam, clay loam, sandy loam and sandy clay loam harboured more diverse occurrence of mangrove vegetation while in clay, sandy clay and silty clay compositions, only a few species of mangrove survived. Such beneficial effects of loamy soil texture on maintenance rich diversity of mangrove species has been discussed in sec. 4.3. Since loamy soils constitute a large share of the textural compositions of mangrove soils of Sundarbans, these soils may be, in general, considered to be conducive for good occurrence of diverse mangrove species with regard to their textures.
pH: pH values of different rhizosphere soil samples of mangrove forests of
Sundarbans have been presented in Table - 4.16. The values varied in between near neutral to slightly alkaline regime with a range of 7.3 and 8.5. Reasons for occurrence of such a narrow range of pH in such estuarine soils have been discussed in sec. 4.3. It has also been stated earlier that this slightly alkaline range of pH may be considered to be congenial to the mangrove vegetations. However, it was not possible to find out any habitat preference for different mangrove species with regard to pH values of the soils probably owing to such a narrow range of this property.
Salinity and Electrical Conductivity As has been discussed earlier (sec. 4.3), salt contents of coastal saline soils tend to vary widely depending on rainfall and evaporation. However, studies on this property with a number of soils at a particular point of time is likely to provide a guideline about the relative variations of this property in different soils. Salinity values of rhizosphere soils under different mangrove species have been reported in Table 4.17. Electrical conductivity (EC) values also refer to the total salt contents of the soils (sec. 4.2), and has direct relationship with salinity of the soils (Table – 4.18). Considering that the EC values maintain similar trend as are observed for soil salinity, influence of both the properties have been discussed together in the present study.
Although mangroves are, in general, salt tolerant plants, yet their levels of tolerance vary considerably among different species (Siddiqi, 2001). Variations in occurrence of mangrove species due to changes in salinity levels have been reported by Wells (1982), Smith (1992) and other workers. Kathiresan et al (1996) reported mangal vegetations to be more luxuriant under lower salinity than in higher salinity ranges. In the present study also, such variations in adaptability to saline conditions have been observed by
different mangrove species. Among the commonly occurring mangroves, plants like Avicennia were found largely under comparatively lower saline stretches. Naidoo and Von-Willert (1995) reported that low saline conditions reduce carbon losses in Avicennia sp. and lead to greater CO2 assimilation resulting in better growth of the plants. On the other hand, species like Exoecaria and Phoenix were observed in comparatively higher saline zones with a few exceptions. That such plants can accumulate excess salts in the leaf vacuoles has been reported by Azocar et al (1992). Sonneratia plants, on the other hand, showed wide adaptations and could be found under a long range of salinity. The results thus indicate that although mangrove plants are essentially habituated to saline condition, yet they differ in their adaptability to levels of salinity. The occurrence of particular mangrove species in estuarine regions will, therefore, depend largely on the capacity of the mangroves to adapt the specific saline condition presented by the habitat.
Organic Carbon: Importance of organic carbon in formation of soil organic matter and, thereby, in influencing various physico-chemical and biological properties of soils have been discussed by Brady (1980), Russel (1975) and many other workers. Behaviour of soil organic matter in mangrove soils have been dealt in the previous section (sec. 4.3) also. Status of soil organic carbon in the root zones of various mangrove plants under different locations have been presented in Table- 4.19. Distribution of some of the mangrove species were found to be related with the organic carbon status of the soils. While different species of Avicennia and Nypa showed rhizosphere soils with comparatively high organic carbon content, species like Exoecaria and Phoenix were found mostly in soils with relatively low organic carbon status. This behaviour may be due to differences in leaf production, fall and transportation by tidal water. Exoecaria
and Phoenix being found mostly under highly saline zones were probably subjected to comparatively slower vegetative growth and thus contributed to lesser accumulation of organic matter in the soils. That mangrove vegetation is less luxuriant under higher salinities have been discussed by Kathiresan et al (1996). A large part of these organic matters was also washed away by the more intense tidal flow under the high saline zones thus resulting in further lowering of organic carbon content of these soils. In general, intensity of mangrove vegetation in different locations was observed to show a positive relationship with organic carbon status of the soils. However, the correlation was not observed to be statistically significant (Table –4.24).
Available Nitrogen Status of available nitrogen in the rhizosphere soils of different mangrove species, as estimated for easily mineralisable form, have been presented in Table – 4.20.The values appeared to be low, in general, and ranged between 15.8 and 99.6 mg kg -1 soil only. The reasons of such limited occurrence of soil nitrogen in available form have been discussed in sec. 4.3. Although nitrogen forms an essential component of plant nutrition, the availability of nitrogen in the studied mangrove soils may be influenced by several factors, especially the washing of the mineralized nitrogen by tidal water. This mobility of nitrogen in these inter-tidal soils makes the assessment of nitrogen nutrition to mangrove vegetation very difficult and it was not possible to find out the independent effect of available nitrogen of mangrove soils on different species from the present investigation. However, since adequate availability of nitrogen is essential for vegetative growth of any plant, low availability of this nutrient element appeared to be a critical soil factor in influencing the intensity of mangrove vegetation, in general, and the relationship was found to be statistically significant Table-4.24).
Available Phosphorus Variations in available soil phosphorous status in the root zones of different mangrove species have been presented in Table - 4.21. As has been discussed in sec. 4.2, mangrove soils tended to exhibit moderate to high concentrations of phosphorus in available form. Such easy availability of soil phosphorus to the mangroves made the assessment of the relationship between these soil property and occurrence of different species of mangroves difficult. However, the mangrove plants like Nypa and Phoenix generally occurred in the soils with comparatively higher available phosphorus status. On the other hand, species like Exoecaria, Sonneratia, Avicennia were found under entire ranges of available soil phosphorus. However, in general, this property was not found to exert profound influences on occurrence of different mangrove species and also on gross intensity of mangrove vegetation (Table – 4.24).
Exchangeable Potassium Status of exchangeable potassium in rhizosphere soils under different mangrove species has been shown in Table - 4.22. As observed from the table, availability of K in these soils were under moderate to high ranges. This resulted in easy access of different mangrove plant species to available potassium in these soils. Since this nutrient did not occur in sub-optimal ranges in any of the soils, most of the mangrove species growing in different zones did not suffer from deficiency of the soil potassium and relative distribution of different mangrove species was not found to be influenced by the availability of potassium. However, a few mangroves like Phoenix, Rhizophora, Avicennia etc. were found to exhibit preference for higher amount of potassium and occurred mostly in the soils with comparatively higher potassium status.
Exchangeable Sodium Exchangeable sodium values of the studied soils showed a wide range varying between 2.0 and 8.4 meq 100g-1 soil. Importance of this soil property on different physico-chemical and biological properties of soils have been discussed earlier (sec. 4.3). This study on rhizosphere soils under different mangrove species showed wide adaptability of most of the species to variations in exchangeable sodium status of soils. The mangrove species are mostly well tolerant to variations in soil salinity and exchangeable sodium status is a major component of the saline conditions of coastal soils. Hence some adaptability of mangrove species to exchangeable sodium values in their rhizosphere soils were expected. Since exchangeable sodium status in a soil depends on several other properties like electrical conductivity, organic matter, texture, exchangeable calcium, sodium, and potassium etc., it was very difficult to find out direct relations of different mangrove species with exchangeable sodium values of soils. However, among all the mangrove species, some showed moderate trends of preference with regard to this property. Nypa fruticans generally occurred under the moderately high exchangeable sodium values while species like Phoenix paludosa and Exoecaria agallocha were found mostly under comparatively lesser concentrations. On the other hand, Bruguiera species, which occurred in scattered manner in different locations, showed preference to low to medium exchangeable sodium values.
This study indicates that several properties of the rhizosphere soils tend to influence and are also influenced by the nature of mangrove vegetation in these soils. However, these relationships are mostly interdependent and hence getting clear pictures of such relationships are often difficult. Among the studied properties, availability of nitrogen in rhizosphere soils appeared to be significantly correlated with the intensity of mangrove vegetation. On
the other hand, textural composition of the soils tended to influence the species diversity. However, some of the soil properties of mangrove forests may vary widely in different seasons under the fragile environment of Sundarbans and more detailed investigations is necessary to develop a clear idea in this regard.
In view of the observed trends of associations of some of the studied properties with occurrences of different mangrove species in Sundarbans soils, an effort was made to assess the habitat preferences of different mangrove species with regard to a few properties of rhizosphere soils viz. salinity, organic carbon and availability of nitrogen and phosphorus. These properties were primarily observed to show good amount of variations in the studied soils and were considered to influence, to some extent, occurrences of different mangrove species. For this purpose, the mean soil properties of the rhizospheres of 23 numbers of common mangrove species for each block were calculated and have been presented separately in figures
figures indicate that there exist variable preferences for salinity by some mangrove species in Sundarbans soils. While species like S. caseolaris .was observed to grow mostly in low saline zones, species like B. sexangula showed wide adaptation with regard to this property. On the other hand, B. parviflora was found to occur in comparatively higher salinity values with very narrow range in all the five zones. With regard to organic carbon status, K. candel, C.tagal etc were found to occur mostly in soils with higher organic carbon values while B. cylindrica and S. caseolaris were observed under lower organic carbon ranges. Available nitrogen status were predominantly low in the studied soils. However, some of the species like S. caseolaris was found to grow even in very low available nitrogen status. On the other hand, species like A. alba, C. tagal etc. were found in soils with comparatively higher available nitrogen
values. Availability of phosphorus was moderate to high in the soils under study. However, among them, S. caseolaris was found in comparatively lower ranges while B. sexangula was observed in wide ranges of the nutrient. Many other habitat preferences for different mangrove species have been observed in the figures. All these observations indicate that
the varying natures of
soils of Sundarbans can sustain wide ranges of mangrove species. However, the preference for habitat for these species needs to be borne in mind when preparing any mangrove conservation programme.
4.5. Bio-chemical compositions of some mangrove species of the Indian Sundarbans.
That mangrove system is very productive and supports a high abundance of diverse variety of wild life (Ong, 1995) has been discussed earlier. Aksornkoae (1986) suggested this behavior to be the result of high leaf production, leaf fall and breaking down of the detritus. The significance of mangrove leaf litter in the maintenance of detritus based food webs in the estuarine environment has been emphasized by workers like Odum and Herld (1975), Lee (1995) and others. The magnitude of such breakdown of leaf litters depends largely on the biochemical compositions of the mangrove leaves through their decomposability and, in turn, capacity to release different nutrients. In the present investigation, therefore, an effort was made to study the biochemical compositions of some commonly occurring mangrove species in Sundarbans and to assess the effects of such compositions on some properties of their habitat soils.
For the purpose of this study, water soluble protein, carbohydrate and lipid contents in leaves of twenty four numbers of mangrove species, which are commonly found in Sundarbans, were estimated (Table – 4.25). The methodologies for these estimations have been presented in sec. 3.0.
Carbohydrates are the major structural and basic storage unit in all autotroph organisms. The basic unit of carbohydrates is the monosaccharides, which cannot be further split into simple sugars. On analysis of the carbohydrate contents of the major mangrove species, the values exhibited wide variations and the said value was found to be the highest in Rhizophora apiculata (90.0 mg/g) followed by Avicennia alba (77.0 mg/g) and the lowest was observed in
Aglaia cucullata (10.0 mg/g). The carbohydrate content of the other mangroves were Rhizophora mucronata (70.0 mg/g), Bruguiera gymnorhiza (66.0 mg/ g), Bruguiera sexangula (70.0 mg/g), Bruguiera cylindrica (69.5 mg/g), Bruguiera parviflora (58.5 mg/g), Ceriops decandra (39.0 mg/g), Ceriops tagal (41.0 mg/g), Kandelia candel (46.5 mg/g), Avicennia officinalis (64.0 mg/g), Avicennia marina (64.6 mg/g), Sonneratia apetala (29.3 mg/g), Sonneratia caseolaris (26.0 mg/g), Sonneratia griffithii (36.0 mg/g), Aegiceras corniculatum (26.6 mg/g), Aegialitis rotundifolia (32.0 mg/g), Heritiera fomes (27.3 mg/g), Xylocarpus mekongensis (36.0 mg/g), Xylocarpus granatum (20.0 mg/g), Nypa fruticans (20.0 mg/g), Phoenix paludosa (67.0 mg/g) and Exoecaria agallocha (75.0 mg/g).
Carbohydrates being very easily decomposable components of organic matter did not exert much influence on the studied soil properties except available potassium status (Table – 4.26). Rapid rate of decomposition of organic matter is likely to release good amount of the major and micro plant nutrients held in organic forms in the leaf litters. While mineralized nitrogen was highly soluble in water, this nutrient became dissolved in water and escaped from the soil phase, as described earlier. Similar was the behavior of mineralized phosphorus which remained largely in labile form under the near neutral pH and organic matter rich environment of the soils. On the other hand, increased cation exchange capacity of the mangrove soils, as has been discussed in sec. 4.3, tended to retain some of the released potassium in the soil exchange phase. This behavior resulted in statistically significant correlation of available soil potassium to carbohydrate contents of mangrove leaves in the present study. This behaviour probably resulted in removal of some of the exchangeable sodium from the soil exchange complex resulting in a significant negative correlation between carbohydrates contents of mangrove
leaves and exchangeable sodium status in soils. However the relationships between other soil properties and carbohydrate contents of mangrove leaves were not statistically significant in any of the cases. The results indicate that the nutrients released through rapid rates of decomposition of carbohydrates were largely released to waters phase resulting in nutrient enrichment of the water rather than the soil. The study indicates that the areas harbouring mangrove species with higher carbohydrate contents are likely to benefit more the aquatic productivity than the soil phase. However, no study could be undertaken on this aspect under the present work programme.
Protein forms the structural and functional basis of the cells of all the living organisms. Proteins are the basic units of amino acids joint together by peptide bonds, and, therefore, rich in nitrogen. The highest protein content was observed in Sonneratia griffithii (7.0 mg/g), while the lowest protein content was observed in Bruguiera sexangula (0.7 mg/g). The protein content of other mangroves were Rhizophora mucronata (1.1 mg/g), Rhizophora apiculata (1.35 mg/g), Bruguiera gymnorhiza (2.5 mg/g), Bruguiera cylindrica (2.9 mg/g), Bruguiera parviflora (1.34 mg/g), Ceriops decandra (3.0 mg/g), Ceriops tagal (3.0 mg/g), Kandelia candel (0.9 mg/g), Avicennia officinalis (2.75 mg/g), Avicennia alba (4.45 mg/g), Avicennia marina (5.15 mg/g), Sonneratia apetala (5.3 mg/g), Sonneratia caseolaris (5.9 mg/g), Aegiceras corniculatum (0.78 mg/g), Aegialitis rotundifolia (0.75 mg/g), Heritiera fomes (2.9 mg/g), Xylocarpus granatum (4.6 mg/g), Xylocarpus mekongensis (5.5 mg/g), Aglaia cucullata (4.1 mg/g), Nypa fruticans (1.10 mg/g), Phoenix paludosa (5.1 mg/g) and Exoecaria agallocha (5.2 mg/g).
In terms of degradability, protein occupies intermediate position in
between carbohydrate and lipid (Brady, 1980). In the present study the concentration of water soluble protein in different mangrove leaves were observed to be comparatively lower in most of the cases. As a result, the impacts of protein content of the mangrove leaves on the soil properties were not practically visible. However, the mangrove species having comparatively higher leaf protein values showed relatively higher available nitrogen status in the soils. As for example, Sonneratia plants showed comparatively higher protein content in the leaves ranging between 5.3 and 7.0 mg/gm. When accumulated to bottom soils these leaves tended to release higher amount of nitrogen to available form during the course of their decomposition. Thus, the mangrove rhizosphere soils with Sonneratia vegetation showed relatively higher occurrence of nitrogen in soils. Similar was the case for Phoenix and Exoecaria also. However, comparatively lower occurrence of this component as organic matter in mangrove leaves and also the water solubility of released nitrogen, as discussed earlier, did not permit this impact of leaf protein on availability of nitrogen to be significant in mangrove rhizosphere soils. In this case also, the nitrogen, mineralized from protein component, was released largely to water phase and was likely to benefit more the aquatic phase than the soil phase.
In this study the highest lipid content was found in Aegiceras corniculatum (6.2%) and the lowest lipid content was found in Ceripos tagal (0.28%). Both the species of Xylocarpus and three species of Sonneratia and Kandelia candel also showed high lipid contents in their leaves. It may be said that this high percentage may be due to the presence of some essential oils in these mangroves.
Since lipid is more resistant to decomposition, the mangrove leaves
with high lipid content did not degrade easily and contributed more to organic carbon content of mangrove habitats when accumulated in the soils. Therefore, different species of Avicennia and Nypa which showed higher lipid content in leaves were found to exhibit higher organic carbon content in their adjacent soils. On the other hand, species like Exoecaria and Phoenix which showed lower lipid content in their leaves had lower amount of organic carbon in their surrounding soil. This behaviour was reflected in positive correlation between lipid content of mangrove leaves and the organic carbon content of the respective rhizosphere soils. However, the relationship was not statistically significant. Availability of phosphorus in mangrove soils also showed similar trend of behaviour and showed a positive correlation with lipid content of the leaves. Such beneficial effects of soil organic matter on availability of phosphorus in mangrove soils have been discussed earlier. However, in this case also, the relationship did not appear to be statistically significant. On the other hand, lipid contents of mangrove leaves resulted in a significant negative correlation with exchangeable potassium in the soils.
This primary study on some biochemical properties of mangrove leaves and their effects on rhizosphere soils show that various organic components of mangrove leaves are likely to exert variable effects on the soil and water properties in the mangrove zones. For getting comprehensive effects of nutrient cycling, therefore, mixed populations of mangrove species are likely to be more important.
4.6. GIS mapping of some relevant properties of mangrove soils of Sundarbans
Previous studies have shown a mutual association to exist between the mangrove vegetation and the nature and properties of their habitat soils in Sundarbans. While the magnitude and nature of mangrove vegetation are controlled largely by some properties of the soils on which they grow (sec. 4.2, 4.3), the mangrove may also, in turn, exert considerable influence on some properties of these soils (sec. 4.4, 4.5). In view of these relationships between the mangroves and their habitat soils, a gross idea about some relevant soil properties in different mangrove zones of Sundarbans appeared to be imperative. Since pH, organic carbon, textural composition and availability of nitrogen, phosphorus and potassium values are generally considered to be the major soil factors determining productivity of any soil, these general properties were considered for preparation of the GIS maps. Some of these parameters like pH and available potassium status did not appear to be very crucial for mangrove vegetations in the soils of Sundarbans. However, they were included in the study for providing comprehensive pictures of the soils at different locations.
In recent years, Geographic Information System (GIS) linked with simulation models has appeared as one of the most powerful tools in natural management. Patil et al (2006) stated the spatial visualization technology for displaying scientific results using GIS to improve significantly the interpretation of the analyses. The results, once linked with GIS, helps in extending the site specific technologies to other similar locations. This linkage makes it possible to transform the results in the form of geo-referred management maps. These management maps can be used by the extension workers or direct users for
making decisions on the sites. Various aspects of GIS mapping have been discussed by Polive and Aubert (1998) and other workers. Considering the importance of generating soil maps for the relevant soil properties of mangrove habitats in Sundarbans, efforts were made to utilize GIS technology for this purpose. It has been hoped that such detailed mapping will be helpful in developing gross ideas about the spatial occurrence of the relevant soil properties in the mangrove habitats of Sundarbans for the benefits of the future workers in this field.
For the purpose of preparation of the soil maps, help was taken from the Kolkata Regional Centre of National Bureau of Soil Survey and Land Use Planning of Indian Council of Agricultural Research and the data generated under this work programme was utilized for the soil maps. For the sake of convenience, however, the maps were restricted to Sundarbans Tiger Reserve Forest area only.
As observed from Figure–, pH values of the studied mangrove habitats were dominated by slightly alkaline (7.5 – 8.0) values while a few zones like Pirkhali, Pancmukhani, Harinbhange and Chandkhali showed alkaline pH values. Importance of near neutral to slightly alkaline pH values in the sustenance of mangrove vegetation has been discussed earlier. It appeared from the map that the Sundarbans soils are by and large conducive to mangrove vegetation with regard to their pH values.
Textural composition of the soils appeared to be predominantly loamy (Fig –) with only exception of Mayadwip soils, which have been observed to the clayey in nature. For textural sub-classes, the soils are mostly dominated by clay loam texture followed by sandy clay loam and sandy loam. Importance
of loamy texture in maintaining favourable physico-chemical properties of soils and also diverse occurrence of mangrove species has been discussed earlier. In view of large scale occurrence of loamy texture, the soil conditions of Sundarbans may be considered to be favourable for sustaining diverse mangrove species.
Mangrove soils are, in general, rich in organic carbon content owing to accumulation of leaf litters in the soils. The GIS map (Fig. - ) also showed most of the mangrove soils to be medium to high with regard to organic carbon content of soils. Soils of Pirkhali, Jhilla, Panchmukhani, Netidhopani, Harinbhanga and Chottohardi exhibited high organic carbon content. That some mangrove species of Avicennia and Nypa showed preference to comparatively high organic carbon content of the soils has been discussed earlier (sec. 4.4). Hence these areas with high organic carbon content on soils may be considered to be preferred natural habitat for those species of mangroves. On the other hand, soils of Arbesi. Khatuajhuri, Matla, Chamta and Bagmara appeared to be medium in this respect. Interdependence of mangrove vegetations and soil organic matter in mangrove habitats has been discussed earlier. Occurrence of medium to high organic matter content in large parts of the mangrove habitat soils is likely to be beneficial for sustenance of such vegetation in Sundarbans.
Available Nitrogen status of the mangrove soils of Sundarbans are predominantly very low. Low values of this nutrient element are available in Panchmukhani and Mayadwip areas while all the rest areas exhibit very low occurrence of nitrogen in available form (Fig. - ). As discussed earlier, high solubility of nitrogen tends to release most of the mineralized nitrogen to water phase to increase the aquatic productivity while the soil phase becomes
depleted with regard to availability of nitrogen, rendering this nutrient a significant critical plant nutrient for growth of mangroves. With this behaviour, nitrogen nutrition from soils by mangroves may appear to be a major problem and this aspect needs to be looked into through future studies.
Available Phosphorus status of mangrove soils of Sundarbans, are largely medium in different zones excepting Mayadwip, Goasaba, Gona, Bagmara and Pirkhali where the levels of availability of phosphorus are moderately low (Fig - ). This behaviour has been dealt elaborately in sec 4.3. In general, availability of phosphorus in different soil zones of Sundarbans may not appear as a major productivity limiting factor for sustenance of mangrove vegetation.
Available Potassium status ranged between very high to high values in all the mangrove soils of Sundarbans (Fig. - ). The values were mostly very high while soils of Panchmukhani and Bagmara showed mixed occurrence. In any case, this nutrient do not appear to be a limiting factor for growth and nutrition of mangroves in Sundarbans.
The GIS maps indicate soils of Sundarbans to be moderately favourable for mangrove vegetation with respect to most of the soil properties rendering the island as an ideal habitat for mangroves. However, low availability of nitrogen and variations in some other properties in different locations may be taken into consideration for developing any mangrove conservation strategy in this forest.
Sundarbans is the largest mangrove delta in the world formed at the estuarine phase of Ganga-Brahmaputra river system covering both Bangladesh and India. The mangrove ecosystem in Indian counterpart of Sundarbans covers a total area of 4,266,6 sq. km. Of this, about 1,952.87 sq. km. is covered by dense mangrove forests and 226.18 sq. km is sparsely covered by mangroves as estimated by Department of Environment, Government of West Bengal and Space Application Centre, Ahmedabad. The importance of these mangrove vegetations in providing sustenance to different aquatic as well as terrestrial biotic population in the estuarine area and also in conservation of the deltaic soils are well documented. Although good amount of studies have so far been carried out on distribution of different mangrove species in different parts of Sundarbans and their influences on productivity levels of this ecosystem, yet very little work have so far been done on the chemical aspect of mangrove habitats which forms an important component for developing any mangrove conservation programme for this zone. In the proposed work programme, therefore, efforts were made to undertake chemical investigations on mangroves with emphasis on their habitats for developing a better understanding about this ecosystem.
5.2 Work Done For the purpose of the study, the work plan was divided into different phases. During the first phase of the programme, relevant geographical and ecological information on mangrove habitats of Sundarbans were collected for developing a gross idea about the general natures of mangrove habitats in Sundarbans.
During the second phase, a comparative study between mangrove dominated and their adjacent non-mangrove soils was undertaken with regard to some common soil properties. The study revealed mangrove soils to exhibit comparatively higher amount of organic carbon, available phosphorus and cation exchange capacity, as compared to their nearby non-mangrove soils.
In the next phase of the work programme, a reconnaissance survey was carried our to study the general nature and properties of mangrove soils of Sundarbans covering all the 15 blocks under Sundarbans Tiger Reserve Forest. The study revealed these saline soils to exhibit moderate to high exchangeable sodium values along with good to moderate organic carbon and total nitrogen status, moderate concentrations of available phosphorus and high occurrence of available potassium. However, contrary to general expectations, availability of nitrogen was observed to be comparatively lower. This behaviour was attributed to washing away of mineralised nitrogen from the soils due to tidal action.
With this knowledge on various chemical properties of mangrove soils and considering that the mangrove vegetations and some properties of their rhizosphere soils are interdependent to each other, another study was undertaken to assess the interrelations of mangrove vegetations and their rhizosphere soils under different zones of Sundarbans. This study generated some interesting results showing reciprocating relationship of intensity of mangrove vegetations and occurrence of some soil properties.
Considering that variations in bio-chemical compositions of mangrove leaves are likely to exert differential influences on mangrove soils through the differences in leaf litter decomposition, another study was undertaken to
assess some bio-chemical compositions of the leaves of the commonly occurring mangrove plants of Sundarbans. Studies on statistical correlations of different bio-chemical properties of the leaves with various properties of rhizosphere soils showed the composition of the mangrove leaves to exert significant contribution on several soil properties. This behaviour was largely attributed to relative decomposability of different organic components of mangrove leaves.
In view of the importance some properties of mangrove habitat soils in influencing the nature and dynamics of mangrove vegetations, it was proposed to prepare mangrove soil maps of Indian Sundarbans pertaining to relevant soil properties using Geographic Information System (GIS). GIS maps of the reported mangrove soil properties for Sundarbans Tiger Reserve Forest area have presented variations in those properties under different mangrove zones in the forest. Since such variations are directly and indirectly related with intensity and distribution of mangrove species in the forest, it has been hoped that the developed GIS maps on mangrove habitats
will be highly
useful in developing any mangrove conservation programme in this area.
Future research needs
The study had attempted to elucidate some important aspects of chemical properties of mangrove habitats in Sundarbans. However, with the completion of the work programme, needs of some future studies in this particular field have appeared to be necessary for furthering the research and development programme. Mangrove forests of Sundarbans experience wide variations in tidal action in different seasons as well as in different locations. Such variations
are likely to exert profound influences on transportation of leaf litter and, thereby, the composition of mangrove rhizosphere soils. It will be an important contribution if the effects of the varying tidal waves on nature and properties of mangrove soils can be studied. Seasonal variations in salinity of the estuarine ecosystem are also likely to exert profound influence on various soil properties in mangrove habitats. Such effects may also result in variations in some properties of mangroves in different seasons. It is necessary that this aspect is also studied in depth for developing a better idea about the nutrition of mangrove plants from their habitats. In the present study, only primary investigations have been carried out on inter-relations of mangrove habitat soils and the biochemical compositions of mangrove leaves. Since this primary study has generated some interesting results, it is suggested that more detailed study may be carried out in this field.
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