Management Practices and Soil Biota - CSIRO Publishing

Aust. 3. Soil Res., 1995, 33, 321-39

Soil Biology and Biochemistry

Management Practices and Soil Biota

M. M. RoperA

and V. V. S. R. GuptaB

A Division of Plant Industry, CSIRO, Private Bag, Wembley, P.O., W.A. 6014. CRC for Soil and Land Management, Private Bag No. 2, Glen Osmond, S.A. 5064.

Abstract The soil biota consist of a large number and range of micro- and macro-organisms and are the living part of soils. They interact with each other and with plants, directly providing nutrition and other benefits. They regulate their own populations as well as those of incoming microorganisms by biological control mechanisms. Microorganisms are responsible for organic matter decomposition and for the transformations of organically bound nitrogen and minerals to forms that are available to plants. Their physical structure and products contribute significantly to soil structure. Management practices have a significant impact on micro- and macro-organism populations and activities. Stubble retention, an increasing trend in Australia, provides an energy source for growth and activity. Significant increases in the sizes and activities of microbial biomass, including heterotrophic microorganisms, cellulolytic microorganisms, nitrogen-fixing bacteria and nitrifying and denitrifying bacteria have been observed. In addition, increases in protozoa and meso- and macro-fauna have been seen. Stubble retention provides a means of maintaining or increasing organic matter levels in soils. The way in which stubbles are managed may impact further on the activities of the soil biota and may alter the population balance, e.g. bacterial: fungal ratios. In general, no-tillage results in a concentration of microorganisms closer to the soil surface and causes least disruption of soil structure compared with conventionally tilled soils. Some plant diseases increase with stubble retention and with no-tillage, partictriarly where the next crop is suscept~bieto the same disease as the previous crop. However, the general increase in microbial populations resulting from stubble retention can exclude pathogens through competitive inhibition and predatory and parasitic activity. Cropping sequences may be used to break disease cycles. Crop rotations that include legumes may provide additional nitrogen and stimulate mineralization processes. Coupled with no-tillage in stubble retention systems is an increased usage of herbicides to control weeds. Continued herbicide use has been shown to significantly depress some groups of microorganisms and some of their activities but, in Australia, little information is available about the effects of herbicides on microbial populations. Although we know that micro- and macro-organisms are vital in maintaining ecosystem function, our knowledge about them is still very limited. New techniques in molecular microbial ecology promise further advances. Much more detailed information about the effects of specific managements on the size and activities of populations is needed. Soils and their processes are extremely complex and, in order to develop appropriate management practices, integration of new and existing information is necessary. This is now being made possible through computer simulation modelling. Keywords: microorganisms, macroorganisms, microbial biomass, stubble, tillage, nutrient transformations, soil structure, pesticides. 0004-9573/95/020321$10.00

M. M. Roper and V. V. S. R. Gupta

Introduction Soils contain a wide variety of micro- and macro-organisms with cross-sectional measurements ranging from 1 pm to 20 mm (Fig. 1). Microorganisms include bacteria, fungi, algae, protozoa and some nematodes. Macroorganisms include a range of invertebrates such as micro- and macro-arthropods, earthworms and termites. It is estimated there are millions of species of microorganisms in the terrestrial ecosystems but, because of the limitations of conventional techniques, only about 40000 (-1%) have been cultured or identified so far (Microbial Resources Panel 1981; Hawksworth and Mound 1991). One estimate (Jong 1989) is that microorganisms constitute about one quarter of the total biomass on the earth. Micro- and macro-organisms are responsible for a wide range of functions which affect nutrient availability and soil structure. However, the way we manage soils, in both agricultural and forest ecosystems, has a significant effect on the survival and activity of these organisms. In this paper we concentrate primarily on the role of microorganisms in soils in agricultural systems. ) Microflora and Microfauna

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~rI5 million tonnes year-1) is produced in Australia and can stimulate microbial activity. In addition, the management of these residues, in particular the form of tillage or no tillage, plays an integrd part in the deve!opment of microsites which support the conditions for microbial activity and the maintenance or improvement of organic matter levels. Retention of stubble significantly decreases bulk density and improves infiltration of water into the soil (Carter and Steed 1992), and this further encourages the development of microbial populations. Biederbeck et al. (1984) and Doran (1980a, 1980b) observed an increase in the populations of total heterotrophic microorganisms in the soil in response to the retention of crop residues (wheat and maize respectively). An increase in the populations of total heterotrophic bacteria, fungi and actinomycetes was observed in Australian soils by Gupta and Roper (1992a) in response to wheat and barley stubbles. Treatment of the retained stubble may also affect the distribution of microbial populations within the soil profile. For example, Doran (1980b) and Gupta et al. (1994b) observed a concentration of microorganisms near the surface (0-7.5 cm) of the soil in no-till/stubble-retained systems, whereas in stubble incorporated systems there was a more even distribution throughout the soil profile (0-30 cm). In addition, in the no-till systems, fungal hyphae development was greater in the surface soil near the standing stubble compared with incorporated stubble systems (Holland and Coleman 1987; Gupta and Roper 1992a).


Changes in populations of specific functional groups of microorganisms have been observed in soils where stubble treatment has been changed from burning to stubble retention and the nature of this change is dependent on the quality and treatment of the stubble (Cochran et al. 1994). Perhaps one of the most significant changes to occur is in populations of microorganisms involved in the deconlposition of stubble. For example, Eitminaviciute e t al. (1976) and Gupta and Roper ( 1 9 9 2 ~ observed ) that stubble retention significantly increased populations of cellulolytic bacteria and fungi. In addition, the development of populations of cellulolytic bacteria and fungi was influenced by the quality of the stubble and in particular the nitrogen and the lignin content. Legume stubbles resulted in higher numbers of cellulolytic bacteria whereas cereal stubbles supported greater numbers of cellulolytic fungi (Eitminaviciute et al. 1976; Broder and Wagner 1988; Cochran et al. 1994). The reason for these differences is related to the ability of fungi to decompose residues with lower nitrogen contents (Burns 1982). These differences in microbial development may account in part for the more rapid rates of decomposition observed in legume stubbles compared with cereal stubbles (Ladd and Foster 1987; Cochran et al. 1994). Stubbles with a higher lignin content tend to decompose more slowly due to the recalcitrance of lignin (Lynch 1983; Paul and Clark 1989). Another example of the stimulation of a specific functional group by retention of crop residues is the response of populations of free-living, nitrogen-fixing bacteria to the retention of wheat stubbles (Roper 1983; Roper e t al. 1989; Gupta and Roper 1992b), sugar cane litter (Patriquin 1982), maize stubbles (Hegazi e t al. 1986) and rice stubbles (Rajaramamohan Rao 1978; Reddy and Patrick 1979; Ladha et al. 1987). Nitrogenase activities measured in situ in a vertisol and in a red earth increased significantly with increasing amounts of cereal stubble remaining in/on the soil (Roper 1983). Nitrogenase activity was best under moist and warm conditions (Roper 1983, 1985) over the warmer months of the year (December-March), although lower levels of activity (5-10% of the average summer rate) were observed in the cooler months (April-September) provided moisture was present (Roper 1985). The stimulation of nitrogenase activity was greater in the Vertisol which contained 51% clay compared with the red earth which contained 17% clay (Roper 1983) partly because the Vertisol supported 10 times the number of nitrogen-fixing bacteria compared with the red earth. Because of its high clay content, the Vertisol also provided microsites of lower oxygen availability required by the nitrogenase enzyme, resulting in a lower moisture requirement for activity in this soil than in the red earth (Roper 1985). Tillage associated with stubble retention had a significant effect on the levels of nitrogenase activity (Roper et al. 1989) and stubble decomposition and associated nitrogenase activity was significantly better in treatments where stubble was lightly mixed at the surface (scarified), compared with treatments where stubble was left standing (no-tillage) or incorporated (with discs). The reasons for these observations are likely to be related to the degree of soil-straw (and hence microorganism-straw) contact required for microbial decomposition, as well as the availability of oxygen to decomposer microorganisms (Roper et al. 1989). Nitrifying and denitrifying bacteria respond to the retention and specific treatment of stubble. For example, retention of stubble has been shown to significantly increase the populations of ammonifying, nitrifying and denitrifying


bacteria in Australian and North American soils (Doran 1980a, 1980b; Gupta and Roper 1993), although the increases in populations of nitrifying bacteria are dependent on soil moisture regimes (Doran 1 9 8 0 ~ ) .Increases in the size of populations and activities of denitrifying bacteria occur because of increased soil moisture and carbon resulting from stubble retention (Firestone 1982; Cochran e t al. 1994). Tillage distributes populations of nitrifying and denitrifying bacteria throughout the plough layer whereas no-tillage results in a concentration of the bacteria near the soil surface (Doran 1980b; Biederbeck e t al. 1984). The effects of stubble retention on rates of nitrification are variable and rates sometimes increase (Groffman 1984) or remain unchanged (Cochran e t al. 1975). Our knowledge of the effects of stubble retention and management on other functional groups, such as microorganisms involved in phosphorus and sulfur transformations, is limited, particularly in Australia. In North America, however, amendment of soils with bermuda grass (Cynodon dactylon (L.) Pers. var. dactylon) has resulted in stimulation of oxidation of elemental sulfur (Cifuentes and Lindemann 1993). Biomass/microfEora and fauna Measurements of microbial biomass carbon and microbial biomass nitrogen offer a means of assessing the response of total microbial populations to changes in agricultural management. Stubble retention has been shown to increase microbial biomass carbon significantly (Dalal 1989; Anderson and Domsch 1990; Haines and Uren 1990; Carter 1991; Chan e t al. 1992; Van Gestel et al. 1992; ~ in the Thompson 1992; Gupta and Roper 1992b; Gupta e t al. 1 9 9 4 ~ )even short term (1-2 years) (Powlson et al. 1987; Gupta e t al. 1994b). Microbial biomass nitrogen also increases with stubble retention but the magnitude of this increase is limited by the availability of nitrogen in the soil (Jenkinson and Ladd 1981; Haines and Uren 1990; Campbell e t al. 1991; Carter and Mele 1992). Microbial biomass represents only a small portion of soil organic matter, but it is living and dynamic and therefore is more sensitive to changes in management practices than the total organic matter (Powlson e t al. 1987). Although microbial biomass carbon has been shown to be concentrated in the top 5 cm of soil under all management systems, tillage practices modify the distribution of microbial biomass with depth, i.e. there is a concentration of biomass near the surface in no-tillage/stubble retained systems and a more even distribution with depth in incorporated stubble systems (Haines and Uren 1990; Van Gestel e t al. 1992). The quality of stubble, i.e. its nitrogen content, influences the amount of biomass derived from the stubble. Ladd and Foster (1987) showed that biomass 14C from decomposing wheat straw accounted for less of the input of 14C than did biomass 14C from legume stubble. They also found that wheat straw 15N, in contrast t o legume 15N, was completely immobilized during the first year of decomposition. Although we have considerable information about microbial biomass carbon and nitrogen, little is known about the effect of management practices on microbial biomass phosphorus and sulfur, especially under Australian conditions. Bacterial : fungal ratios reflect changes in the composition of microbial populations in response to management. The ratio of bacteria to fungi responds to retention of stubble, to the quality of stubble retained and to stubble management (Hendrix e t al. 1986; Allison and Killham 1988; Gupta and Roper 1 9 9 2 ~ ) .For example,


bacterial :fungal ratios are wider (1:3) under stubble retainedlno-till systems than in cultivated soils (1: 1) where stubble has been burnt (Gupta and Roper 1992a) because of higher levels of C (plant residues) and less damage to fungal hyphae from cultivation. In addition, stubble management, e.g. standing v. incorporated, has a significant effect on the bacterial :fungal ratios. Holland and Coleman (1987) and Gupta and Roper ( 1 9 9 2 ~ )observed that fungal biomass was significantly larger in the proximity of surface stubble than in incorporated stubble systems. The reason for this is related to the ability of fungal hyphae t o bridge surface stubble t o the soil below more readily than bacteria. Mycorrhizae (fungal-root associations) are important symbioses which facilitate the uptake by plants of nutrients, especially phosphorus (Hayman 1980). In Australia, there is little information about the effect of stubble retention and management on the inoculum size of mycorrhizae t o a subsequent crop. In North America, Dhillion et al. (1988) found that fire in a prairie significantly reduced subsequent vesicular-arbuscular mycorrhizal fungal colonization and sporulation. In addition, in Europe, Kruckelmann (1975) demonstrated that, compared with no-tillage, spore frequency was increased by shallow ploughing, whereas tilling by a rotary hoe decreased it. Chopped straw increased the differences between treatments and particularly the spore numbers in the shallow ploughed soil. O'Halloran et al. (1986) reported that disturbance of soil reduced the intensity of mycorrhizal infection in corn roots compared with no-till. This could be attributed to the breakage (or killing) of mycorrhizal hyphae due to the soil disturbance (Evans and Miller 1990). Bacterial-feeding and fungal-feeding protozoa are important in soils for their ability to participate in the turnover of organic matter and in maintaining the balance of microbial populations (Alexander 19773; Old and Chakraborty 1986). Management practices may influence protozoan populations indirectly by altering the availability of their food source (bacteria and fungi) and by modifying the soil environment (Foissner 1987). For example, Gupta and Roper (1992a) showed that stubble-retained, no-till treatments contained higher populations (10- to 100-fold) of fungal-feeding protozoa and higher populations (5- to 10-fold during the summer season) of bacteriefeeding protozoa than stubble-burnt systems. Information on the impact of management practices on protozoa is still being gathered. Meso- and macro-fauna (nematodes, microarthropods and earthworms) feed on microorganisms and organic material in the soil (Lee and Pankhurst 1992; Cochran et al. 1994), and therefore management practices involving stubble retention and modified tillage increase their numbers and activity. A conceptual model of detritus food webs involved in plant residue decomposition is shown in Fig. 2. Hendrix et al. (1986) and Parmelee and Alston (1986) observed increases in fungivorous nematodes in response to stubble retention and no-till whereas, in incorporated stubble systems, bacterivorous nematodes predominated. With microarthropods, stubble retention and no-till increased numbers of collembolans that feed on fungi (Hendrix et al. 1986; Reddy and Venkataiah 1989) and significantly influenced the numbers and composition of populations of mites (House and Parmelee 1985; Parmelee et al. 1989). Numbers and activities of earthworms are encouraged by stubble retention and reduced cultivation (Buckerfield et al. 1992). Stubble removal and cultivation can reduce numbers of earthworms to less than one sixth of those in stubble retained with no-till (Thompson 1992; Doube et al. 1994).


The impact of plant diseases may be significantly influenced by stubble retention and tillage through their effect on soil moisture and by transferring disease, particularly above ground (Rovira et al. 1990). Stem and leaf diseases are most likely to be transferred with stubble but some transfer of root pathogens may also occur, e.g. Pythium spp (Cook et al. 1990). Stubble management has a significant role in the development of root diseases. Rhizoctonia, Gaeumannomyces graminis var. tritici, Fusarium spp, Pythium spp and Bipolaris sorokiniana are all encouraged by no-tillage, especially under stubble retention (Chan et al. 1987; Rovira et al. 1990). Soil disturbance through cultivation breaks up fungal hyphae and reduces the incidence of disease (Chan and Mead 1990; Rovira et al. 1990). In contrast, carry-over of cereal cyst nematodes (Heterodera avenae) is reduced by no-till (Roget and Rovira 1985). Some increases of deleterious rhizobacteria have been observed with stubble retention and no-till (Elliott and Lynch 1984; Chan et al. 1987). The increase in general microbial population sizes resulting from stubble retention may reduce the impact of plant pathogens because of exclusion through competitive inhibition and the development of predatory and parasitic microorganisms (Kundu and Nandi 1985; Old and Chakraborty 1986;

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Fig. 2. A conceptual model of detritus food webs involved in plant residue decomposition in no-tillage (boxes with dots) and conventional-tillage (plain boxes) systems. Boxes indicate nutrient storages and arrows indicate nutrient transfer pathways. The model is from Hendrix et al. (1986) and is modified using information (i) on fungal-feeding protozoa (Gupta and Roper 1994), and (ii) on mycorrhizae (Evans and Miller 1990; Thompson 1994).

Cropping Sequences Cropping sequences offer a number of advantages in agricultural systems such as disease breaks and improved plant nutrition, organic matter and soil structure. Crop rotations include cereal-legume (forage and grain), cereal-oil seed and


pasture (grasses and grass/legume mixtures)-crop (cereal, legume, oil seed). Crop rotations that include legumes and/or pastures are reported to increase populations of total heterotrophic microorganisms, particularly in the rhizosphere (Biederbeck et al. 1984; Collins et al. 1992) and of nitrifying and denitrifying bacteria (Collins e t al. 1992). Crop rotations that include legumes may improve free-living nitrogen fixation associated with the stubbles and the rhizosphere, and oil seeds may alter populations and activities of sulfur oxidizing microorganisms (Grayston and Germida 1990). All these changes result in improved mineralization and nutrient supplying potential of the soil. On the negative side, some rotations which include non-host plants result in a reduction in the size of mycorrhizal populations, e.g. lupin and cruciferous-oil seed rotations (Thompson 1991). The size and activity of the microbial biomass (carbon and nitrogen) is favoured by rotations which include legumes because of the additional input of nitrogen through symbiotic nitrogen fixation (Biederbeck et al. 1984; Campbell et al. 1992; Ladd et al. 1994). In pastures, apart from the additional nitrogen from the legume components, the large input to the soil of dry matter from the pasture itself contributes t o the microbial biomass and organic matter levels in the soil (Campbell et al. 1991, 1992; Collins et al. 1992). Crop rotations affect the size and diversity of bacterial-feeding protozoan populations (Stout 1960; Tomescu 1978). Rotations including pastures result in greater numbers and diversity of collembolan populations compared with annual crop rotations (Anderson 1987). Disease cycles may be broken even in stubble retained systems if the next crop is not a host, e.g. the host range of take-all (Gaeumannomyces graminis var. tritici) is confined to grasses, wheat, barley and triticale (Wilkinson et al. 1985; Rovira et al. 1990), the host range of various species of Pythium includes pea, wheat and barley (Rovira et al. 1990) and cereal cyst nematodes are confined to cereals (Meagher and Rooney 1966; Rovira and Simon 1982). Therefore, a break with a non-susceptible crop between two susceptible crops can reduce the incidence of the disease in the later crop (Cook 1981; Rovira and Simon 1982; Wilkinson et al. 1985; Pankhurst and McDonald 1988). Despite a wide host range of RMzoctonia, root rot, crop rotations have been reported to reduce disease incidence (Rovira et al. 1990). Crop rotations may also interrupt the growth and persistence of host specific inhibitory microoganisms in the rhizosphere (Schippers et al. 1987).

Chemical Use Fertilizers Fertilizers affect the soil biota in various ways. For example, nitrogen fertilizers increase decompositon of plant residues (Sain and Broadbent 1977; Bhardwaj and Novak 1978) but decrease both symbiotic and asymbiotic nitrogen fixation (Knowles and Denike 1974; Cejudo and Paneque 1986). The size and activity of heterotrophic microorganisms and microbial biomass are reduced if fertilizer application causes a change from the normal pH (Ladd et al. 1994), whereas, if the environmental conditions remain unchanged, there may be increases in microbial biomass and populations of heterotrophic microorganisms (Jenkinson and Powlson 1976; Sparling et al. 1983; Biederbeck e t al. 1984; Collins et al. 1992). For example, application of nitrogen results in an increase in the


numbers of nitrifying and denitrifying bacteria (Focht and Verstraete 1977), and similarly, sulfur application results in an increase in the population and activities of sulfur-oxidizing microorganisms (Lawrence et al. 1988). Frequently, the bacterial: fungal ratios under surface-straw treatments are increased by nitrogen application (Holland and Coleman 1987). However, repeated application of elemental sulfur fertilizer, for 5 years, was shown to reduce fungal biomass, thereby narrowing the bacterial: fungal ratios (Gupta et al. 1988). Infection of roots with mycorrhizae may be reduced by the input of inorganic phosphorus (Singh et al. 1986) or unaffected (Porter et al. 1978). Following the application of both mineral and organic fertilizers, Kruckelmann (1975) reported an increase in spore numbers of mycorrhizae in sandy soil but a reduction in silty clay loam. Spore numbers are more closely related to soil pH (Kruckelmann 1975). Inorganic nitrogen fertilizers significantly increase populations of protozoa, in particular amoebae (Singh 1949; Elliott and Coleman 1977), whereas application of elemental sulfur reduces the populations of bacterial-feeding and fungal-feeding protozoa (Gupta and Germida 1988b). These responses to fertilizers are linked to the availability of food (Elliott and Coleman 1977; Gupta and Germida 1988b). For example, Gupta and Germida (1988b) observed a significant relationship (R = 0.82; P 5 0.01) between fungal biomass (food protozoa) and populations of fungal-feeding protozoa. Fertilizers may influence root pathogens either directly or indirectly. Cook (1962) and Garrett (1976) reported that the survival and growth of Fusarium solani f. sp. phaseolz and take-all fungus were enhanced by the availability of nitrogen. The application of chloride fertilizers suppresses take-all (Gaeumannomyces graminis var. tritici) by reducing nitrification and maintaining a favorable NH$-N : NOS-N ratio (Christensen and Brett 1984; Brennan 1993). The changes in pH resulting from nitrogen application may reduce take-all (Smiley and Cook 1973). By improving plant growth and vigour, fertilizers indirectly may reduce the severity of pathogen attack. Pesticides/herbicides There is ample evidence to show that the continued use of herbicides in agroecosystems has major ecological effects (Greaves et al. 1976; Wardrop 1986; Edwards 1989; Hicks et al. 1990), but the effects of different herbicides on various microbial groups and biological processes depend upon the nature of the chemical, the dose and the method of application, soil type, temperature and moisture regimes, crop residue and soil management practices. Herbicides such as paraquat, simazine and atrazine have a negative impact on fungi (Camper et al. 1973; Edwards 1989), whilst information about their effect on total bacterial populations is conflicting (Wardrop 1986; Edwards 1989). Herbicides such as triazines, MCPA, MCPB, Diuron, Terbutryn and trifluralin have been reported to depress algal populations (Wardrop 1986; Edwards 1989). Much of the research on the effect of pesticides on microorganisms has focused on nitrogen transformations in soils. In a review, Goring and Laskowski (1982) showed a range of effects of different pesticides on nitrogen mineralization, immobilization, nitrification, denitrification and fixation. Nitrifying bacteria are most sensitive to herbicide application (Edwards 1989). Hegazi et al. (1979) observed that


nitrogenase activity as well as numbers of asymbiotic nitrogen-fixing bacteria declined in soils dosed with a group of pesticides. The effect of herbicides on organic matter breakdown also depends on the method of residue treatment. Herbicides are reported to have a variable effect on micro-, meso- and macro-fauna (Eijsackers and Van de Bund 1980; Edwards 1989). The effect of herbicides on soil fauna is either a direct toxic effect or through the influence on predator-prey interactions. Hendrix and Parmelee (1985) reported that paraquat and glyphosate treatment reduced the decomposition of crop residues but increased the rate of nutrient loss from litter by altering microbial and microarthropod activity. They hypothesized that herbicides promote the microbial utilization of herbicide as a C source, increase the importance of microarthropod grazing (microflora) and reduce the importance of predatory microarthropods resulting in an accelerated soluble nutrient loss and a slower decay of C from the litter. Herbicides are reported to increase the damage from root diseases (Katan and Eshel 1973; Greaves et al. 1976; Altman and Rovira 1989). In Australia, application of chlorosulfuron increased root disease caused by Rhizoctonia solani (Rovira and McDonald 1986). Similarly, treatment with glyphosate increased the incidence of take-all fungus (Mekwatanakarn and Sivasithamparam 1987) and seedling root rot disease (Blowes 1987). Mekwatanakarn and Sivasithamparam (1987) hypothesized that herbicides help the survival and pathogenicity of take-all fungus indirectly through their effect on soil microflora. Much of the work reported on herbicide effects on soil biology is from the U.S.A. and Europe, but little work has been reported from Australia.

Conclusions Micro- and macro-organisms in soils are extensive and vital in maintaining ecosystem function (Roper 1993). Through their activities they make available nutrients for other microbial functions and plant growth. They regulate organic matter turnover and are responsible for biological control. They contribute to soil structure and hence reduce erosion and soil loss. The functions of soil biota are summarized in Fig. 3. Our knowledge of micro- and macro-organisms in soil is still very limited, but new molecular techniques, where 16s ribosomal RNA sequences are used, promise to expand our knowledge of the identity and function of microorganisms in the soil. New advances have been made already, e.g. Liesack and Stackebrandt (1992) have discovered new bacteria in soils near Brisbane. Combinations of traditional microbiological methodologies and new molecular techniques have been used to describe functions and activities of microbial groups in soil, and Smith and Tiedje (1992) have used this approach to explain changes in denitrifying populations resulting from altered management practices. Similarly, Holben et al. (1992) have combined traditional Most Probable Number methods with hybridization analyses, using probes for genes that encode for 2,4-D degradation, to monitor changes in populations of bacteria that degrade 2,4-D in soils with different histories of herbicide application. To understand soil biota and their activities in the soil is only part of our need for knowledge. Management practices in agriculture significantly affect the survival and activity of the soil biota and we need to develop managements which nurture organisms and provide the benefits of their activities. With the increasing swing towards no-tillage practices in Australia, there is a greater use of


herbicides to control weeds. Much more information is needed about the effect of pesticides in general on the overall balance and functions of microorganisms and macroorganisms. Developments in biological control are needed as alternative pest controls where appropriate.

Management Practices and Soil Biota I

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Fig. 3. Interactions between management practices and soil biota and their role in nutrient transformations and soil structure (Gupta, V.V.S.R., unpublished).

Because of the complexity and range of soils and soil processes, it is essential to integrate existing and new information to develop appropriate management strategies. This is now being made possible through computer simulation modelling. Further developments in this field will help us to predict the most beneficial managements for a range of climatic conditions and soil types. In developing management strategies it is essential that scientists and practitioners work together so that not only are new managements appropriate for the biological health of the soil, but they are cost-effective. Understanding the biology of our soils and integrating this knowledge with developments in management practices so as to sustain productivity is probably one of the biggest challenges facing soil scientists and practitioners in Australia. References Abbott, L. K., and Robson, A. D. (1982). Field management of VA mycorrhizal fungi. In 'The Rhizosphere and Plant Growth'. (Eds D. L. Keisier and P. B. Cregan.) pp. 355-62. (Kluwer Academic Press: Netherlands.) Alexander, M. (1977a). 'Introduction to Soil Microbiology.' (John Wiley: NY.) Alexander, M. (1977b). 'Microbial Ecology.' (John Wiley: NY.)


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Manuscript received 23 March 1994, accepted 14 December 1994