Soil and Fertilizer Nitrogen

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we don't know exactly how much nitrogen is in the soil and how much will be ..... one mole of N2 gas, at the expense of 16 moles of ATP and a supply of ...
Chapter 18

Soil and Fertilizer Nitrogen YADVINDER-SINGH AND BIJAY-SINGH One reason is that the system assumes the soil is a blank medium and devoid of natural nitrogen. We know that’s not true. And the problem we run into is that we don’t know exactly how much nitrogen is in the soil and how much will be available to the crop. The release of nitrogen is dependent on the weather, so there’s always a possibility of adding more or less nitrogen to the soil than is needed – Robert Mullen

18.1. Introduction Nitrogen (N) is the most important nutrient for plant growth, yield, quality and the environment. It is a key component of amino acids, auxin, cytokinins, alkaloids, glucosinolates and proteins, as well as other major food components. It is also a part of the chlorophyll molecule, which controls photosynthesis, the solar energy capturing reaction of green plants. Adequate supplies of N are needed to support photosynthesis and to produce proteins in harvested crops. It exists in nature primarily as N 2 gas, which makes up about 78% of the Earth’s atmosphere. But, it must be converted to ammonium or nitrate to be utilized by plants. Low soil N availability is often the major nutrient factor limiting the growth and yield of crops. Synthetic N fertilizers have been used extensively to increase crop yields since the end of the Second World War. The bulk of this N is derived from ammonia synthesized by the Haber–Bosch process, which can be considered as one of the milestones in the history of humanity. Over the past few decades, India has achieved spectacular increase in food grain production mainly through using fertilizers, irrigation, and high yielding variety seeds. Of all the nutrients, N additions have had the single largest effect on crop yields. Fertilizer N consumption in India was 1.5 million tonnes (Mt) in 1970, which increased to 16.5 Mt in 2010 and is expected to increase to 27.5 Mt by 2020. Only a portion of the fertilizer N applied to the soil is taken up by plants; a part is lost to the environment and the remainder is incorporated into soil organic matter (SOM) and fixed as NH 4+ in clay minerals.

18.2. Forms of N in the Soil There are three major forms of N in soil: organic N, ammonium N (NH 4+-N), and nitrate N (NO3--N). Organic forms of N make up for the highest percentage (>90%) of the total N

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in the soil; only a small part is in the inorganic or mineral N forms (NH 4+ and NO3-). Plants can only use NH4+ and NO3- forms of N. Organic N is not directly available to plants, but can be converted to mineral N forms by microorganisms. Organic N is present in the relatively stable SOM, and in living soil organisms such as bacteria. Organic forms of N in soils are: amino acids, amino sugars, nucleic acids, glycerol-phosphatides, amines, vitamins, and DNA as well as more complex organic molecules. Ammonium N exists in exchangeable and non-exchangeable forms.

18.3. Fertilizer N: Sources and Formulations Nitrogenous fertilizers are manufactured in different formulations. They all essentially begin with anhydrous ammonia which is manufactured from air and natural gas by the Haber-Bosch process (named after the German scientist Fritz Haber, and industrial chemist Carl Bosch) through the chemical reaction (3H 2+N22NH3) under high heat and pressure. This process, has allowed the production of sufficient food for the growing world population by annually producing about 453 billion kg (Mt or Tg) of anhydrous ammonia, which helps feed 50% of the world’s population. Anhydrous ammonia is then reformulated into several other N fertilizer sources to best meet their crop needs and meeting logistical requirements. Some of the more common N formulations are described in Table 18.1. In India, commonly used N fertilizers are urea, calcium ammonium nitrate and ammonium sulphate. Other fertilizers that contain significant amounts of N, but are used primarily as sources of other nutrients are: mono ammonium phosphate (NH 4H2PO4; 11% N, 52% P2O5), diammonium phosphate [(NH4)2HPO4; 18% N, 46% P2O5)] and different grades of nitrophosphates. All N fertilizers are equally effective when properly applied. Anhydrous ammonia is historically the least expensive N fertilizer, but it requires injection into the soil, which is a more expensive application method than broadcasting or surface banding. The leading position of urea in the world N fertilizer market is mainly due to its low production cost, high N content and relatively less costs associated with its transport and storage. It is suitable for the production of compound fertilizers and also for application in the form of granules, foliar solution or coated.

18.3.1. Acidity or Basicity of Different N Fertilizer Materials The hydrogen ions (H+) released during nitrification of the NH 4+-based fertilizers are the major cause of acidity in soils. Ammonium sulphate causes acidification due to release of H+ during nitrification and the conversion of SO42- to H2SO4 in the soil. Both Ca(NO3)2) and KNO3 are base-forming fertilizers. The acidification due to NH 4-N can easily be controlled through application of liming materials such as CaCO 3 and CaO. Acidification induced by N fertilizers ranges from 0.6 kg for CAN, 1.0 kg for urea to 3 kg of CaO equivalent kg-1 of N supplied through ammonium sulphate (on the basis of 50% utilization rate).

18.4. The Nitrogen Cycle Abundant supply of N in the earth’s atmosphere in the form of N 2 gas is unavailable for use by most of the organisms because there is a triple bond between the two N atoms which makes the molecule almost inert. In order for N to be used for plants it must be converted (or fixed) to the forms of NH 4+ or NO3-. The weathering of rocks releases these ions so slowly that it has a negligible effect on the availability of fixed N. Thus N is often the limiting factor for growth and biomass production in all environments where there is suitable climate and availability of water to support life. The N cycle (Figure 18.1) shows

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Table 18.1. Characteristics of N fertilizers Name of N fertilizer

Characteristics (N content refers to total N)

Anhydrous ammonia (NH3)

It is the most concentrated commercial fertilizer N source (82% N) and is usually applied to the soil by injection at a depth of 10 to 20 cm as a pressurized liquid that immediately vaporizes, and reacts with soil water to convert to NH4+.

Aqua ammonia

It is produced by mixing ammonia with water and contains 20% N. This form can be added to irrigation water as an alternate means of application.

Ammonium sulphate

It is produced as an industrial byproduct and is one of the oldest

[(NH4)2SO4]

manufactured N fertilizer. It comes from manufacturing of steel, nylon, and other processes which use sulphuric acid. It contains 21% N and 24% S, making it a useful choice where S is needed.

Ammonium sulphate nitrate

It contains 26% N and 13% S. It is manufactured by reacting anhydrous

(ASN) [(NH4SO)2.NH4 NO3]

ammonia with a mixture of nitric acid and sulphuric acid. The N content of the product will vary depending on the proportion of nitric to sulphuric acid. It has good storage and handling properties. It is very satisfactory for direct application, use in blended fertilizers, and is a good replacement for ammonium nitrate.

Ammonium nitrate (NH4NO3)

It contains 33 to 34% N and is produced as a concentrated solution by reacting ammonia gas with nitric acid. The solution (95 to 99% NH4NO3) when dropped from a tower solidifies to form prills, which can be used as fertilizer. It can also be made granular by spraying concentrated solution onto small granules in a rotating drum. Its high solubility makes it well-suited for fertigation and foliar application.

Urea ammonium nitrate

It contains 28-30% N and is commonly used as a liquid fertilizer N source.

(UAN) [NH4NO3+ (NH2)2CO]

It is a non-pressure solution of ammonium nitrate, urea, and water. Its two most common grades are: 28-0-0 and 32-0-0. Individual solutions can be made from urea or ammonium nitrate; however, higher analysis solutions are possible when urea and ammonium nitrate are combined. Commonly available N solutions will contain about one-half of the N from urea and the other half from ammonium nitrate.

Urea (amide N) [CO(NH2)2]

It contains 45-46% N. It can be applied to soil or as a foliar spray and provides the quickest possible N supply, but for this purpose the biuret content must be below 0.3%. It is hygroscopic and has to be stored in moisture-proof bags.

Ammonium bicarbonate

It is commonly used as an inexpensive N fertilizer in China. Ammonium

(17% N) (NH4HCO3)

bicarbonate (NH4HCO3) is produced by combining carbon dioxide and ammonia as per equation: CO2 + NH3 + H2O  NH4HCO3.

Calcium nitrate [(Ca(NO3)2 ]

It contains 13-14% N, quick-acting and increases the soil pH.

Potassium nitrate (KNO3)

It contains 13% N, quick-acting and increases the soil pH.

Calcium ammonium nitrate

It is a combination of NH4NO3 and calcium carbonate and contains 25-26%

(CAN)

N.

reactions that various inorganic N compounds undergo in soil-crop-atmosphere system of crop production. The N cycle, as typically described, begins with N in its simplest stable form, dinitrogen (N 2), and follows it through the processes of fixation, mineralization, immobilization, nitrification, ammonia volatilization, leaching, runoff, and plant assimilation and completes with denitrification.

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Figure 18.1. The Nitrogen Cycle—The dynamic interchange among various N forms in the soil-cropatmosphere system

In the soil, N becomes available to plants in several ways. 1. 2. 3. 4. 5. 6.

Soil reservoirs: parent material, soil organic matter. Biological N fixation: by symbiotic and nonsymbiotic N fixing organisms. Industrial fixation: commercial fertilizers. Crop residues, green manures, farmyard manure, compost Proteins, amino acids, amino sugars. Non-biological N fixation (precipitation/deposition). Lightning fixes N into various oxides through rain and snow deposit. Typically, this is less than 10 kg of total N ha -1 yr-1 Microorganisms have a central role in almost all aspects of N availability to crop plants. Some bacteria convert N2 into ammonium by the process termed N2 fixation; these bacteria are either free-living or form symbiotic associations with plants or other organisms such as protozoa. Other bacteria bring about transformations of ammonium to nitrate, and of nitrate to N2 or N2O gases. Many bacteria and fungi degrade organic matter, releasing fixed N for reuse by other organisms. All these processes contribute to the N cycle. At any one time a large proportion of the total fixed N will be locked up in the biomass or in the dead remains of organisms (shown collectively as organic matter). But the only N available to support new growth will be that which is supplied by N fixation from the atmosphere or by the release of ammonium or simple organic N compounds through the decomposition of organic matter (mineralization). The reactive N in these systems is in a constant dynamic exchange among the various forms.

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18.4.1. Nitrogen Transformations in Soil Nitrogen undergoes many transformations in soil and these are mediated by different soil microbes. Nitrogen added through organic sources (green manure, animal manures and crop residues) undergoes mineralization so that it can be utilized by plants. Three types of processes affect excess N not utilized by the crop plants: Microbial – e.g. nitrification, denitrification, immobilization; biological N fixation, Chemical – e.g. non-biological N fixation (precipitation, clay fixation), hydrolysis; volatilization  Physical – e.g. leaching and run-off. Their relative impact on the supply of N for future crops depends upon weather, soil conditions, and other factors.  

18.4.1.1. Nitrogen Mineralization The process of breakdown of organic N compounds by the microorganisms to the mineral N forms is termed as mineralization. The process takes place essentially in three steps. The first step of mineralization is called aminization, in which microorganisms (primarily heterotrophic bacteria and fungi) break down complex proteins to simpler amino acids, amides, and amines. Proteins  R+-NH2 + R-OH

…(18.1)

Heterotrophic microorganisms require organic compounds as sources of carbon and energy. Autotrophic microorganisms can derive energy from the oxidation of inorganic elements or compounds such as iron (Fe), sulfur (S), NH 4+, NO3-, or from radiant energy. They derive carbon from carbon dioxide (CO2). Ammonification is the second step of mineralization in which amino (-NH 2) groups are converted to ammonium. R-NH2 + H2O  NH3 + R-OH

…(18.2)

Again, microorganisms (primarily autotrophic) accomplish this action. Nitrification is the third step of mineralization. The ammonium released through mineralization is subject to several fates in the soil. It may be converted to nitrites and nitrates by the process of nitrification. It may be absorbed directly by higher plants. It may be utilized by heterotrophic organisms in further decomposing organic carbon residues.  It may be fixed in a biologically unavailable form in the lattice of certain expanding type clay minerals. In general, conditions that are good for microbial activity, such as warm temperature (25-35 oC) and moist soil, will promote more rapid breakdown of organic N. The rate at which N becomes available is determined by the complexity and stability of the organic matter and by microbial activity. It may occur in days, or it may take years if the N is in a very stable form. However, some organic N is in forms that are difficult for microbes to digest and release under any circumstances. The NH 4+-N has a positive charge which is important in explaining its behaviour in the soil. Because soil particles have a negative charge, there is an attraction between NH 4+ and soil particles which prevents NH 4+ from moving with water flowing through the root zone. For this reason, there is little concern about NH4+-N moving past the root zone and ending up in ground water. However, NH 4+ -N does not remain in the soil for long. It gets converted to NO 3-- N.   

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18.4.1.2. Nitrogen Immobilization Immobilization is process that converts inorganic N to organic N and is a reverse reaction of mineralization. Immobilization, or the temporary tying up of inorganic N by soil microorganisms, decomposing plant residues or organic manures, is not strictly a loss process. Immobilized N will be unavailable to plants for some time, but will eventually become available as residue decomposition proceeds and populations of microorganisms decline. Immobilization occurs when organic materials added to soil contain low amounts of N and/or have a high C:N ratio and NO3--N and /or NH4+-N present in the soil is used by the growing microbes to build proteins. There is often a net gain of N during the growing season because the additional N in the residue will be the net gain from immobilization-mineralization processes. The duration of the nitrate disappearance period during immobilization depends on environmental factors such as moisture and temperature and the quality of substrate (C:N ratio, lignin, polyphenol content, etc.). As a general rule the balance between N mineralization and immobilization is a function of the relative availability of C and N of substrate and metabolic needs of microbial biomass. The C:N ratio of organic residues ranges from 10:1 for young leguminous plant tissues to as high as 200:1 for straw of some cereal grain crops. Those with C:N ratio greater than 25:1 are associated with net immobilization of N. Plant tissues that are low in N content generally are more resistant to decomposition and require a longer time before the N is available to plants. When plant residue with a wide C:N ratio is incorporated into the soil, microbial decomposition starts. Microorganism populations increase greatly as evident by increased release of CO2 leaving the soil through respiration (Figure 18.2). The microorganisms take N from the soil for proteins. Consequently, for a time the concentration of inorganic N in the soil declines, and may be deficient for plant growth. As residue decomposes, the C:N ratio narrows. At a ratio of approximately 20:1, N becomes available for plant use. Fertilizer N immobilization can be reduced by deep placement or placing fertilizers below crop residues instead of incorporating fertilizer into the soil with residue.

Figure 18.2. Nitrogen immobilization-mineralization during decomposition of organic materials in soil

18.4.2. Assimilation of N by plants Crop removal accounts for more than 50% of the N that leaves the soil system. Total uptake of N (soil + fertilizer) by cereals ranges from 100 to 180 kg N ha -1 depending on the grain yield potential. Plants use N in NO3- and NH4+ forms. If any preference exists, it is usually for NH4+ early and NO3- late in the growing season. Growth is optimized with a mixture of both NH4+- and NO3--N, with NH4+ used preferentially for synthesis of amino

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acids and proteins. Some plants can also directly use urea, although in most cases urea-N will hydrolyze to NH4+-N prior to uptake. For plants to take up N, it must move with water toward the root - a process called mass flow. Consequently, nitrate-N that has moved below the root zone has potential to move up into the root zone as surface horizons of soil dry out and crops use water deeper in the profile. Conversely, plants may exhibit symptoms of N deficiency even though the soil contains adequate amounts of N, if moisture and consequently mass flow of N is limited. A small fraction of total N uptake by plants may be diffusion and root interception when high levels of NH 4+-N are present in the root zone.

18.4.3. Non-biological Fixation of N Non-biological N fixation is also known as electro-chemical or photochemical N fixation. In this process atmospheric N is converted into NO2, NO3, and NH3 with help of lightning and radiation. The lightning and radiations split the molecular N 2 into N atoms. It then combines with hydrogen or oxygen of atmospheric water forming the NH 3 or nitric oxide or nitrous oxide. These oxides then get hydrated and form nitrous and nitric acids. These acids and NH3 are washed off along with rain into the soil. There these acids combine with metallic ions to form metallic nitrites or nitrates.

18.4.3.1. Fixation of Ammonium- N by Clay Minerals One of the possible fates of NH4+-N in soils is its fixation by clays with an expanding lattice. The fixation of NH4+ is defined as the adsorption or absorption of ammonium ions by the mineral or organic fraction of the soil in a manner that these are relatively not exchangeable by the usual methods of cation exchange. Ammonium fixation is greatest in 2:1 type clay minerals such as illite, vermiculite, and montmorillonite. Clay minerals possess negative charges balanced by cations, for example, NH 4+ or K+ (Figure 18.3). The physics of NH4+ is closely related to that of K+ because both ions have similar ionic radii and low hydration energy. It comes about by a replacement of NH 4+ for interlayer cations (Ca2+, Mg2, Na+, H+) in the expanded lattice or clay minerals. The electrostatic energy between NH 4+ (or K+) and the negative charges in the crystal sheets is greater than the hydration energy of NH 4+. The NH4+ ion readily sheds its hydration water shell and enters the lattice void, where fixation occurs. Vermiculite, with its high surface charge density, has isomorphous substitution mostly in the tetrahedral sheets of the clays, whereas montmorillonite, with its lower surface charge, has substitution mostly in the octahedral sheet. Inorganic cations, NH 4+

Figure 18.3. Schematic diagram depicting the different forms of NH4+on illite clay mineral

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(radius of 0.143 nm) or K+ (radius of 0.133 nm), can be fixed in the hexagonal cavity (radius of 0.140 nm) of silicate tetrahedral sheets of the tetrahedral-substituted or octahedral-substituted layer charges. As a rule, fixation occurs to a much greater extent in subsoils than in topsoils. Ammonium fixation and release can play a crucial role for the efficiency of fertilizer N as it affects the indigenous soil N supply towards crop N uptake. Some soils are able to bind NH4+ in such a manner that these cannot be easily replaced by other cations. In soils with high NH4+ fixation capacity a part of the NH 4+ supplied through NH4+-forming or NH4+-containing fertilizers may be bound in clay mineral interlayers. Increasing NH 4+ fixation can be a way of building up an available N pool in soils to optimize crop recovery and minimize N losses into the environment as the NH 4+ ions after penetration into the clay mineral interlayers are not available for nitrification and are thus protected against leaching. Fixation and release of NH 4+-N is dependent upon chemical equilibria between the amounts of NH4 + in soil solution, exchangeable NH4 + and fixed NH4 + (Eq. 18.4) Fast

Slow

Very slow

NH4+ (soil solution)  NH4+(exchangeable)  NH4+(intermediate)  NH4+(fixed) …(18.4) Both soluble and exchangeable NH 4+ are readily available for plant uptake. The intermediate NH4+ is considered to occupy interlayer sites on the clay lattice and is exchangeable with K+ and H+ when release of NH4+-N is increased with the expansion of clay lattice. Fixed NH4+ can be grouped into recently fixed and native fixed NH 4+(resulting from geological process). The recently fixed NH 4+ is mainly derived from mineral N fertilizers, but it may also originate from mineralization of SOM. Generally, the content of fixed NH4+ amounts to 10–90 mg kg-1 in coarse-textured soils, 60–270 mg kg-1 in mediumtextured soils (loess, marsh, alluvial sediment, basalt), and 90–460 mg kg -1 in fine textured soils (limestone, clay stone). Fixation is usually fast and occurs within the first few hours after fertilizer application. The fixation rate is controlled mainly by ion diffusion and declines with time until the equilibrium point is approached. About 50-60% of added NH4+ can be fixed within a short period of time. The NH 4+ fixation appears to be less significant in the waterlogged than in aerobic soils. The release of fixed NH4+-N ranges from 4% to 25% in different soils. A substantial portion of fixed NH4+ in soils is potentially available to crops. The magnitude of NH 4+-N release strongly depends on the length of crop growth period and plant density. Most of the differences in NH4+-N release can probably be attributed to both the variable pool sizes of ‘native fixed NH4+’ and ‘recently fixed NH4+’. Apparently, recently fixed NH4+ resulting from fertilizer application is more available to plants than native fixed NH 4+, which is more tightly held. Differences in clay mineralogy and K saturation of the minerals also influence the release of fixed NH 4+-N. Soils containing vermiculite and low contents of exchangeable K+ release significantly higher amounts of fixed NH 4+ as compared to soils with no vermiculite and high percentage of K + saturation of the clay minerals. 18.4.3.1.1. Factors influencing NH4+ fixation: The amount and the capacity of soils to fix NH4+ is related to parent material, texture, clay content, clay mineral composition, the K concentration in soil solution, the degree of K saturation of the exchange complex of the soil colloids, K saturation of the interlayers of 2:1 clay minerals, pH, temperature, and soil moisture conditions. Generally, vermiculite and illite have the greatest capacity to fix NH4+, while montmorillonite fixes less NH 4+and holds it less tenaciously. As compared to montmorillonite, beidelite is a high fixing smectite, due to the isomorphic substitution in

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the tetrahedral layer. Kaolinites, belonging to the 1:1 type of clay minerals, are not able to bind NH4+ ions in their interlayers because hydrogen bonds which join the interlayers allow only very little dilatation of the narrow interlayer space. Fixation increases with increasing temperature and NH4+ concentration. The amount and method of fertilizer N application that affect concentration of NH 4+ in soil solution may promote either release or fixation of NH4 +. Freezing and thawing and wetting and drying cycles of soils increase NH4+ fixation. Other factors influencing the capacity of soils to fix NH 4+ include: presence of various cations, presence of associated anions, base saturation and soil organic matter. It increases with increase in pH and base saturation. Anions affect NH 4+ fixation by affecting soil pH. A combination of various factors present will determine the rate and magnitude of NH4+ fixation. When the K content of a soil is high, fixation of NH 4+ will be low due to competition for fixation sites. Addition of K + prior to NH4+ can depress NH4+ fixation, and addition of NH4+ prior to or at the same time as K+ can reduce the K+ fixation. Soil moisture can reduce NH4+ fixation as clay minerals are expanded under wet conditions. In dry soils, the interlayer space is reduced and NH 4+ fixation also increases. The redox potential (Eh) may have an important effect on the fate of NH 4+ in flooded paddy soils. At a low Eh, octahedral Fe 3+ in the clay minerals is assumed to be reduced, resulting in a higher negative charge of the unit cell and therefore a higher Coulombic attraction between the interlayer cations and the silicate layers. A further prerequisite for the marked NH4+ fixation in flooded soils is the microbial reduction of Fe 3+, followed by the dissolution of Fe-oxides coated on the surface of clay minerals at a low Eh, promoting the diffusion of Fe3+ ions into the interlayers of the clay minerals. Because of the reversible oxidation and reduction of Fe oxides in paddy soils, this mechanism may be of special importance for fixation of NH4+.

18.4.4. Urea Hydrolysis Urea applied to soil, is converted to ammonium carbonate by an enzyme called urease. When urea is applied to the soil, it is converted relatively rapidly (within a few days) to ammonia and CO2 by bacterial enzyme urease found in most of the soils. The carbonate ions result in a relatively short-lived increase in soil pH around the fertilizer granule. Moisture is necessary to start the hydrolysis reaction. In the first step, urea is transformed by the enzyme urease into the unstable ammonium carbamate (NH 2COONH4), and then to ammonia (Eq. 18.5). NH2-CO-NH2 + H2O  NH2COONH4  2NH3+ CO2

…(18.5)

NH3 +H2O  NH4OHNH4+ + OH-

…(18.6)

The rate of urea hydrolysis increases as temperature increases. Thus hydrolysis is normally completed within two days at a temperature of 30 °C and it may take about 10 days at a temperature of 5 °C. This transformation has a major drawback in that it leads to very high volatilization losses of ammonia if urea is applied at the soil surface. Such ammonia losses will occur particularly on soils poor in sorption capacity, without plant cover and with a high pH. Urea can cause severe germination and seedling damage due to release of NH3 and NO2– toxicity when the amount placed near the seed is too large. The protonization of NH3 to NH4+ leads to a slight rise in pH value (Equation 18.6). A pH rise of >8.0 in the vicinity of NH 3 or urea fertilizer granules will inhibit Nitrobacter organisms. Nitrosomonas are not so sensitive and function up to pH 9 so that NO 2accumulates in the soil. The oxidation of NH 4+ to NO3- will cause the pH to fall below 8; one mole of NH4+ on nitrification releases two moles of H +.

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18.4.5. Nitrification The term nitrification refers to the conversion of NH 4+ to NO3-. This is brought about by the nitrifying bacteria, which are a specialized group of soil microorganisms gaining their energy by oxidizing NH4+, while using CO2 as their source of carbon to synthesize organic compounds. Organisms of this sort are termed chemoautotrophs - they gain their energy by chemical oxidations (chemo-) and they are autotrophs (self-feeders) because they do not depend on organic matter. Nitrification is a two-step process; Nitrosomonas (obligate autotrophic bacteria) convert NH 4+ to NO2- (Eq. 18.7). The second step of nitrification occurs through Nitrobacter and Nitrosolobus species, which convert NO2- to NO3- (Eq. 18.8). This step rapidly follows NH4+ conversion to NO2-, and consequently nitrite concentrations are normally low in soils. 2NH4+ + 3O2 2NO2- + O2

Nitrosomonas

 Nitrobacter/Nitrosolobus

 

2NO2– + 2H2O

…(18.7)

2NO3-

…(18.8)

The nitrifying bacteria are found in most soils of moderate pH, but are not active in highly acidic soils. Soil temperature is the main factor influencing the rate of nitrification in agricultural soils. The warmer the soil, the faster ammonium-N will be converted to nitrate-N. Winters in most part of India generally do not get cold enough to stop the nitrification process entirely. However, the process is slowed down during cooler months and typically occurs in the range of several weeks to months. Under summer conditions, the transformation can occur within a few days to weeks. Because NO 3- is present in the soil solution, it is easily taken up by plant roots. When more NH 4+-N is applied than the crop will use before the next irrigation, the excess NH 4+ will be converted to NO3-. When the next irrigation is applied, any excess water that moves below the root zone will carry NO3- -N with it. Some heterotrophs, mostly fungi, can also produce NO 3-.

18.4.6. Biological Fixation of N Biological N fixation is the conversion of unreactive dinitrogen (N 2) gas to NH3 by microorganisms, which can readily undergo chemical reactions. Although unavailable to most plants, large amounts of N2 can be used by leguminous plants via biological N fixation. Biological N fixation occurs symbiotically (dinitrogen-fixing bacteria, such as nodule-forming Rhizobium bacteria in conjunction with legumes) and non-symbiotically (free living organisms such as photosynthetic bacteria, blue-green algae, and free-living Azotobacter species). The most important N2-fixing agents in agriculture are the symbiotic associations between legumes and rhizobia. Nitrogen-fixing legumes can be utilized as a seed crop, a green manure or as the main N input into a pasture by growing it in association with grass. The group of microorganisms that fixes N 2 is also called biofertilizers. 18.4.6.1. Mechanism of biological nitrogen fixation: Biological N fixation is an important source of N in agriculture and it has been estimated that more than 140 Tg of N is fixed per year by the N-fixing microorganisms in natural terrestrial ecosystems (90 Tg in agricultural lands and 50 Tg in forest and non-agricultural lands). Interestingly, these total values are similar to fertilizer N produced by the Haber–Bosch process. Biological N fixation can be represented by Eq. 18.9, in which two moles of NH 3 are produced from one mole of N2 gas, at the expense of 16 moles of ATP and a supply of electrons (e -) and protons (H+): N2 + 8H+ + 8e- + 16ATP  2NH3 + H2 + 16ADP + 16 Pi

…(18.9)

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This reaction is performed exclusively by prokaryotes (the bacteria and related organisms), using an enzyme termed nitrogenase. At least two protons are reduced to hydrogen gas for each molecule of N2 fixed. This enzyme consists of two proteins - an iron protein and a molybdenum-iron protein. The reactions occur while N 2 is bound to the nitrogenase enzyme complex. The Fe protein is first reduced by electrons donated by ferredoxin. Then the reduced Fe protein binds ATP and reduces the molybdenum-iron protein, which donates electrons to N2, producing HN-NH. In two further cycles of this process (each requiring electrons donated by ferredoxin) HN-NH is reduced to H 2N-NH2, and this in turn is reduced to 2NH3. Depending on the type of microorganism, the reduced ferredoxin which supplies electrons for this process is generated by photosynthesis, respiration or fermentation. There is a remarkable degree of functional conservation between the nitrogenase proteins of all N-fixing bacteria. The nitrogenase enzyme complex is highly sensitive to O2. It is inactivated when exposed to oxygen, because this reacts with the iron component of the proteins. Although this is not a problem for anaerobic bacteria, it could be a major problem for the aerobic species such as cyanobacteria (which generate O 2 during photosynthesis) and the free-living aerobic bacteria of soils, such as Azotobacter and Beijerinckia. These organisms have various methods to overcome the problem. For example, Azotobacter species have the highest known rate of respiratory metabolism of any organism, so they might protect the enzyme by maintaining a very low level of O 2 in their cells. Azotobacter species also produce large amounts of extracellular polysaccharide (as do Rhizobium species in culture). By maintaining water within the polysaccharide slime layer, these bacteria can limit the diffusion rate of O 2 to the cells. In the symbiotic N-fixing organisms such as Rhizobium, the root nodules can contain O 2-scavenging molecules such as leghaemoglobin, which shows as a pink colour when the active Nfixing nodules of legume roots are cut open. Leghaemoglobin may regulate the supply of O2 to the nodule tissues. Some of the cyanobacteria have yet another mechanism for protecting nitrogenase. Nitrogen fixation occurs in special cells (heterocysts), which possess only photosystem I (used to generate ATP by light-mediated reactions) whereas the other cells have both photosystem I and photosystem II (which generates O 2when light energy is used to split water to supply H 2 for synthesis of organic compounds). 18.4.6.2. The nitrogen-fixing organisms: All the N-fixing organisms are prokaryotes (bacteria). Some of them live independently of other organisms - the so-called free-living N-fixing bacteria. Others live in intimate symbiotic associations with plants or with other organisms (e.g. protozoa). Examples are shown in Table 18.2. A range of the estimates of N2 fixation in various ecosystems is given in Table 18.3. 18.4.6.3. Symbiotic nitrogen fixation: The most familiar examples of N-fixing symbioses are the root nodules of legumes (peas, beans, clover, etc.). In these leguminous associations Table 18.2. Examples of N-fixing bacteria (*denotes a photosynthetic bacterium) Free living Aerobic Azotobacter Beijerinckia Klebsiella (some) Cyanobacteria (Blue green algae)*

Symbiotic with plants Anaerobic Clostridium Desulfovibrio Purple sulphur bacteria*, Purple non-sulphur bacteria*, Green sulphur bacteria, *Cyanobacterial association, *(Azolla)

Legumes Rhizobium, Bradyrhizobium, Azorhizobium (root and stem nodulating in Sesbania rostrata)

Other plants Frankia Azospirillum

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Table 18.3. Estimates of N2 fixation (kg N ha-1 yr-1 or crop-1) (adapted from Bacon 1995) Ecosystem

Range measured

Common range

0-80 0-240

0-15 0-50

10-150 20-460 10-450 15-680

10-50 50-150 30-150 50-250

i. Free living microbes in rice ii. Associative microbes in arable crops Symbiotic system i. Azolla in rice (per crop of azolla) ii. Green manure legumes iii. Grain legumes iv. Pasture legumes

the bacteria usually are Rhizobium species, but the root nodules of soybeans, chickpea and some other legumes are formed by small-celled rhizobia termed Bradyrhizobium (Figure 18.4). Nodules on some tropical leguminous plants are formed by yet other genera (Frankia). In all cases the bacteria ‘invade’ the plant and cause the formation of a nodule by inducing localized proliferation of the plant host cells. Yet the bacteria always remain separated from the host cytoplasm by being enclosed in a membrane - a necessary feature in symbioses. Among legumes, alfalfa and clovers have potential to fix more than 200 kg N and 120-135 kg N ha -1 yr -1 , respectively. Cowpeas and soybean have potential to fix 100-115 kg N ha-1, Figure 18.4. Root nodulation in soybean roots and peas fix 60-80 kg N ha-1 and peanuts fix 4550 kg N ha-1. Rhizobium accounts for the largest proportion (40%) of the total production in India. This is followed by Azotobacter. Because the Rhizobium bacteria that infect legume roots normally supply adequate N to the host plant, well-nodulated legumes rarely respond to additions of N fertilizer. Occasionally, however, a small dose of N fertilizer (starter dose) may be needed early in the season, presumably because there is lag in N fixation by the nodules. Use of biofertilizers can have a significant effect on the yield of most crops. 18.4.6.4. Free living microorganisms: In addition to the intimate and specialized symbiotic associations, there are several free-living N-fixing bacteria that grow in close association with plants. For example, Azospirillum species have been shown to fix N when growing in the root zone (rhizosphere) or tropical grasses, and even of maize plants in field conditions. Similarly, Azotobacter species can fix N in the rhizosphere of several plants. In both cases the bacteria grow at the expense of sugars and other nutrients that leak from the roots. Among anaerobic bacteria Clostridium can make only a small contribution to the N nutrition of the plant, because N-fixation is an energy-expensive process, and large amounts of organic nutrients are not continuously available to microbes in the rhizosphere. Table 18.4 shows the effect of Azotobacter on grain yields of different crops in India. However, their effectiveness is found to vary greatly, depending largely on soil condition, temperature and farming practices. 18.4.6.5. Cyanobacterial associations: The photosynthetic cyanobacteria form symbiotic associations with other organisms such as the water fern Azolla. The association with

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Table 18.4. Effect of Azotobacter on crop yield in India Crop

Food grains Wheat Rice Maize Sorghum Cotton Sugarcane

Increase in yield over yields obtained with chemical fertilizers (%) 8-10 5 15-20 15-20 7-27 9-24

Crop

Other crops Potato Carrot Cauliflower Tomato

Increase in yield over yields obtained with chemical fertilizers (%) 13 16 40 2-24

Azolla, where cyanobacteria (Anabaena azollae) are harboured in the leaves, has been shown to be important for N inputs in rice paddies, especially if the fern is allowed to grow and then ploughed into the soil to release N before the rice crop is sown.

18.4.7. Losses of N from Soils The N may be absorbed by crops, immobilized by the soil or lost from the soil system. The amount of N lost to the aqueous and atmospheric environments can become a serious pollutant and a conservation concern. The accumulation of excessive amounts of reactive N (NH3, NO2- and NO3-) in terrestrial and aquatic ecosystems as well as in the troposphere leads to significant costs to society that occur through direct and indirect negative effects on environmental quality, ecosystem services, biodiversity, and human health. Extents of N loss depend on several of soil, plant and climatic factors. Most of the problems are caused by the incorrect use of nutrients and their poor integration with other production inputs. The various means of N losses include: leaching of nitrate to ground water; volatilization of ammonia into the atmosphere; and as nitrous oxide (N 2O) to the atmosphere resulting from denitrification by soil organisms. In addition to these, soil and applied N can also be lost through soil erosion and surface runoff. 18.4.7.1. Ammonia Volatilization: Ammonia (NH 3) loss to the atmosphere is called ammonia volatilization. Significant losses from some surface-applied N sources can occur through the process of volatilization. Ammonia volatilization occurs when NH 4+ in the soil is converted to NH3 at pH above 7.0, which is then lost as a gas. Ammonia is an intermediate form of N during the process in which urea is transformed to NH 4+-N. Ammonia loss can be significant from the surface-applied fertilizers and organic manures containing NH4+-N or urea. The amount of total N loss from fertilizers containing urea due to NH3 volatilization can vary considerably, from 0 to 50% or more of the applied N. Typical losses from urea broadcast, without rain/irrigation for at least a week after application, may range from 10 to 20% of the applied N. Ammonia volatilization loss potential is the greatest with urea and anhydrous ammonia, intermediate with ureaammonium nitrate solution, and the least with ammonium nitrate, ammonium chloride and ammonium sulphate, particularly on non-calcareous soils. List of factors favouring NH3 volatilization from soils includes high urease activity, high pH (>7.0), high calcium carbonate (lime) content, low clay content, low organic matter, low CEC and buffering capacity, high soil or atmospheric temperature, high wind velocity, application of urea to initially moist soil followed by drying conditions, crop residue on the soil surface, no rainfall or irrigation after application, urea not incorporated into the soil (especially with no-till, high residue cover), and high rate of fertilizer

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application. Ammonia volatilization is directly proportional to NH 3/NH4+ concentrations in soil solutions, which is governed by pH of soil solution (pKa 9.25). Generally, NH 3 losses increase with increasing N rate. This may be linear or exponential such that the relative loss (as percent of the applied N) may decrease, remain constant, or increase with increasing application rates. Water management in rice and wheat fields influences the extent of N losses due to nitrification-denitrification and ammonia volatilization. Ammonia losses from urea application are not only a substantial economic loss for farmers, but ammonia lost to the atmosphere will be deposited by rain to land and water, causing acidification and eutrophication. Volatilization can be minimized by activities that move N into the soil, so that ammonia formed from urea hydrolysis can be attached to soil particles. These activities include: incorporation of N fertilizers into soil, irrigation and rainfall soon after application, deep placement (band placement) of the urea into the soil, and use of urease inhibitors. The depth to which incorporation is necessary depends on the soil characteristics, such as texture, density, CEC and SOM content, but for most soils it is 5 to 10 cm. The loss of N from rice soils will be determined by the concentration of NH 4 in the floodwater. However, in coarse-textured soils, NH 4-N in floodwater is readily transported to subsurface soil layers along with percolating water and thus less prone to ammonia volatilization. Ammonium-N placed at depth cannot move back to soil surface because there is always downward flux of percolating water under repeatedly irrigated rice culture. 18.4.7.2. Denitrification: Denitrification refers to the process in which NO3- and NO2- are converted to gaseous compounds (NO, N2O and N2) by soil microbes under conditions of low oxygen supply. The sequence usually involves the production of nitrite (NO 2-) as an intermediate step as shown in Equation (18.10). NO3–  NO2- NO  N2O  N2

(18.10)

Several types of bacteria perform this conversion when growing on organic matter in anaerobic conditions or waterlogged soils. Because of the lack of O 2 for normal aerobic respiration, they use NO3- in place of O2 as the terminal electron acceptor. The gases (NO, N2O and N2) released are defined as by-products or intermediate products of microbial nitrification and denitrification processes. The N2 gas can then move up through soil and into the atmosphere, which is composed mostly of N2. Using 15N labeled N fertilizers gaseous N losses from wetland paddy fields via nitrification-denitrification have been recorded in the range from 42-51% of applied N. However, the losses under upland conditions are quiet low and range from 10-20% of the applied fertilizer N. The magnitude of these losses varies greatly between systems and environments. The common denitrifying bacteria include several species of Pseudomonas, Alkaligenes and Bacillus. Their activities result in substantial losses of N into the atmosphere. The conditions favouring denitrification include (1) presence of bacteria possessing the metabolic capability for denitrification, (2) availability of suitable electron donors such as organic C compounds (in the form of oxidizable organic matter), amount of reduced S compounds or molecular hydrogen available, (3) absence or restricted availability of O 2, (4) ready supply of reducible N sources (NO3- and NO2-) to serve as terminal electron acceptor, (5) neutral or alkaline pH (7.0 or greater), and (6) temperatures above 20 °C. The occurrence of denitrification in various environments depends on the interaction of many limiting factors and is largely governed by the soil-crop management. Denitrification cannot take place without NO3-. High rates of denitrification from soil are frequently observed as ‘short pulses’ which occur when soil conditions become congenial. The effect of moisture on denitrification is largely due to its effect on aeration but as temperature decreases the

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minimum soil water content for denitrification to occur increases. Tropical climate provides congenial conditions (high rainfall and temperatures) for high N 2O and NO emission losses. Low field areas which are subject to ponding of water for sustained periods during the irrigation season often exhibit N deficiencies related to denitrification losses. Other pathways of N2O production from NH4+-based fertilizers applied to soils are known as nitrifier-denitrification and chemodenitrification of NO 2--N in addition to classical denitrification. Nitrifier-denitrification the production of N 2O by autotrophic ammonia-oxidizing bacteria, could be an important contributor to soil N 2O emissions (Figure 18.5). Chemodenitrification (denitrification without microbial activity) requires low pH, but may be significant in freezing soils with high salt concentrations and high NO2- content.

Figure 18.5. Emission of nitrous oxide during nitrification/denitrification in soils

In traditional paddy soils, nitrification in oxidized soil zones and floodwater converts the ammoniacal N formed by ammonification and hydrolysis of urea into NO 3-. The NO3can thereafter move into reduced soiled zones where it is readily denitrified to N 2 and N2O. The rate of nitrification is a primary determinant of denitrification losses in flooded soils. In coarse-textured soils, due to occurrence of frequent alternate aerobic-anaerobic cycles NO3- are rapidly produced under aerobic conditions and subsequently NO 3- gets denitrified under anaerobic conditions which develop due to application of irrigation. When rice is grown in these soils, applied urea-N may be preferentially lost via denitrification rather than ammonia volatilization. 18.4.7.3. Leaching and runoff losses of N: Nitrates having a negative charge are not held strongly by the soil and can leach down with percolating waters. The rate of NO 3movement downward depends on a variety of factors, including soil texture, concentration of NO3- in the soil profile, rainfall, root zone depth, irrigation amounts, and crop uptake of NO3-. Not all of the above conditions have to be met for NO 3- leaching to occur. However, NO3- leaching is at its maximum where all these factors are favourable and minimum where the reverse is the case. Coarse-textured soils have a lower water holding capacity and, therefore, a greater potential to lose NO 3- from leaching when compared with fine-textured soils. Leaching losses of N can be very high where N is applied to crops that have a shallow rooting system. A deep and extensive rooting system enables crops to utilize N more efficiently, thus minimizing the risk of leaching. Large quantities of NO3- accumulating in ecosystems are a cause for great concern, mainly from health and environmental viewpoints. The NO 3- lost by leaching or transported in surface runoff can result in reduced soil fertility, causes eutrophication of surface waters and increases concentrations in drinking-water. High content of N in lake water stimulates the growth of algae in lakes. The algal growth then depletes the O 2 in the water which is essential for fish. High NO3- levels in drinking water are associated with methemoglobinemia in infants or the blue baby syndrome. When leached, all anions (NO3-, SO42+ and Cl-) take along with them equivalent amounts of cations. Therefore, nitrate leaching can deplete the soil of exchangeable cations such as Ca 2+, Mg2+ and K+, which can increase soil acidity.

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Nitrogen can also be lost from agricultural lands through soil erosion and runoff. Losses through these processes do not normally account for a large portion of the soil N budget, but should be considered for surface water quality issues. Incorporation or injection of manure and fertilizer can help to protect against N low through erosion or runoff. Where soils are highly erodible, conservation tillage can reduce soil erosion and runoff, resulting in less surface low in N.

18.4.8. Nitrogen Transformations in Paddy Fields In rice soils, a thin oxidized layer is differentiated from the reduced plough layer at the soil surface 3-4 weeks after flooding (Figure 18.6). The thickness of the aerobic zone varies considerably depending on the soil organic matter content, photosynthetic activity by algae, etc. When NH4+-N comes to this oxidized layer, it is readily transformed into NO3--N, by nitrifying bacteria. As an anion, NO3--N is not retained by soil particles, and is readily washed with percolating water into the underlying reduced plough layer. Here, it undergoes denitrification and N is lost to the atmosphere. Fertilizer, mineralization of organic N in aerobic zone and diffusion of NH 4+-N from the underlying anaerobic zone are the major sources of NH4+-N in flooded soils. The rate of nitrification is a primary determinant of denitrification losses in flooded soils. As a way of minimizing this loss of N, deep placement of ammonium fertilizers is recommended. Estimates of the amount of biologically fixed N per crop of rice vary quite widely, but 30 to 40 kg ha -1 would be a reasonable figure. This amount of N is two or three times higher than the amount of N fixed in ordinary upland soils planted in non-leguminous crops. Interestingly, this amount of fixed N can explain the average yields of paddy obtained in unfertilized fields in India (2-3 t ha-1). Majority of N loss occurs within 7-10 days of fertilizer application. Studies on ideal rice (fine-textured with low percolation rate) soils suggest that NH 3 volatilization rather than denitrification is more important gaseous loss mechanism for fertilizer N, but the picture is quite opposite in highly permeable porous soils under rice. There exist two mechanisms in such soils due to which losses due to denitrification assume more importance than ammonia volatilization losses. Firstly, in porous soils under rice it is not possible to maintain continuous flooding. Rather there occur very frequent alternate aerobic-anaerobic cycles, which lead to very fast formation of nitrate under aerobic

Figure 18.6. Nitrogen transformations in a rice soil

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conditions and their subsequent denitrification under anaerobic conditions that, develop due to application of irrigation. Secondly, due to high permeability of coarse textured porous soils, urea as such is rapidly transported to subsoil where even after it is hydrolyzed to NH4+, it is not prone to losses via ammonia volatilization. Thus in nonideal porous soils under rice, there exists every possibility that applied urea-N is preferentially lost via denitrification rather than ammonia volatilization.

18.5. Optimizing N Management for Increasing N Use Efficiency (NUE) in Field Crops 18.5.1. Defining NUE terms and calculations The term N use efficiency (NUE) relates only to applied fertilizer N, although crops absorb N from different sources. Definitions of NUE in crops and cropping systems include: (a) partial factor productivity (PFPN), expressed as kg grain yield (or harvested product) per kg of N applied; (b) agronomic efficiency (AEN), expressed as the increase in grain yield over that of the zero-N control per unit of N applied; (c) internal or physiological efficiency (PEN), defined as the increase in grain yield over that of the zeroN control per unit of N uptake by the crop; and (d) apparent recovery efficiency (REN), defined as the percentage increase in N uptake by the crop in aboveground biomass per kg N applied. The first three indices relate to production efficiency where the output is harvested crop product and the input is N. However, the fourth index (REN) more accurately reflects the NUE as it gives the ratio of output N to input N indicating how well the given N-management strategy performs in recovering the applied N. The REN is commonly measured by establishing plots with and without application of nitrogen and is calculated using Eq. 18.11. REN = (NUN–NUC)× 100/ FN

…(18.11)

where NUN = total plant N uptake in aboveground biomass at maturity (kg ha -1) in a plot that received fertilizer N NUC = total plant N uptake in aboveground biomass at maturity (kg ha -1) in a plot that received no N FN = amount of fertilizer N applied (kg ha-1) The objective of nutrient management is to increase the overall performance of cropping systems by providing economically optimum nourishment to the crop while minimizing nutrient losses from the field. The NUE addresses some but not all aspects of that performance. Therefore, system optimization goals necessarily include overall productivity as well as NUE. The REN may be apparent recovery efficiency (RENa) or true recovery efficiency (RENt). RENt is determined with the help of labeled fertilizer, e.g. 15N to differentiate fertilizer N from indigenous soil N. The RENa is less accurate but more easily measured. It is the total N uptake (in aboveground parts of the crop at maturity) at a given fertilizer rate minus the uptake at zero fertilizer rate, divided by the amount of the nutrient applied. It is called 'apparent' because part of the total uptake will be from mineralized soil organic nutrient and the amount that was mineralized varies with the amount of fertilizer that has been applied. RENa is used by soil and environment scientists in finding out the part of nutrient taken up by crop and the part causing environmental pollution. PEN is used by plant physiologists and plant breeders in studying the efficiency of different crops or cultivars of a crop in utilizing the absorbed nutrients. AEN and REN require a record of N input and output, and data on plots

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without N input. REN is the most logical measurement to calculate NUE for environmental aspects, because it looks at the N uptake by the crop. The difference method for RENa is, however, only appropriate for long-term trials, because the indigenous fertility of soils (zero nutrient plot) can only be estimated over long periods of time. If it is used for annual trials, NUE will be underestimated because the crop yield in the unfertilized treatment is supported by nutrient applications from previous years. For long-term trials (of at least 10 years duration), the difference method gives an accurate estimate of the long-term contribution of fertilizer to crop yield. PFPN permits comparison of NUE in different countries or in different regions of a country. The term is useful in comparing the advantages of fertilizer use in experiments on tillage, irrigation, weed control etc., where a ‘no fertilizer control’ is typically not provided. Another terms used for NUE are N utilization efficiency, the ability of the plant to transfer N to the grain, predominantly present as protein and N harvest index (NHI), a ratio between N accumulated in grain to N accumulated in grain plus straw. The NHI is an important index in determining crop yields because it is positively associated with grain yield.

18.5.2. Soil N and Fertilizer N Use Efficiency Agricultural soils contain a large pool of N that exists in organic combinations and it is distinguished from applied N as soil N. Fertilizer N applications in agro-ecosystems rely on the premise that fertilization, rather than soil N supply is the major source for crop uptake. But in managed agro-ecosystems soil N plays a vital role in supplying N to crop plants and thus dictates the efficiency of applied fertilizer N. For example, if by applying 120 kg N ha-1 as fertilizers to rice or wheat total N uptake of 107 kg N ha -1 is recorded with RENa value of 43%, only 52 kg N ha-1 came from the fertilizer and 55 kg N ha-1was the contribution from soil N. Also, RENt values obtained with 15N-labelled fertilizers are often somewhat lower than RENa values estimated with the difference method because of confounding effects caused by pool substitution, i.e., immobilization of 15N fertilizer in microbial biomass and initial release of microbial-derived 14N. Thus, N contributions from soil in a given year/season and on a long-term basis can greatly alter REN because there is a large fertilizer N substitution of soil N. Although both soil N and applied fertilizer N contribute to plant available N pool consisting of NO3 and NH4 ions, this pool represents a very small fraction of total soil-N (Figure 18.7). For example, a typical irrigated soil under rice-wheat cropping system in

Figure 18.7. Relative contribution of soil and fertilizer N in uptake of N by crop plants

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the Indo-Gangetic plain in north-western India contains more than 2000 kg N ha -1 in the top 30 cm of soil where roots derive majority of N supply. The amount of N derived from indigenous resources during a single cropping cycle typically ranges from 30-100 kg N ha-1 that represents only 1.5 to 5% of total soil N. Although small in size, the indigenous N supply has a very high fertilizer N substitution value because of the relatively low REN of fertilizer N (Figure 18.7). When soil N content increases, it increases N uptake by the crop and the amount of sequestered N derived from applied N leads to a higher REN. Conversely, any decrease in soil N stocks will reduce overall N use efficiency and REN. A decrease in soil N supply is inherently detrimental to productivity. Crop yields may be sustained or even increased by using improved varieties or due to higher fertilizer application rates despite the lower incremental return per unit of N applied, but eventually, soil degradation is likely to lead to a decline or stagnation in yield.

18.5.3. Increasing Fertilizer N Use Efficiency in Field Crops There is a need to reduce the use of N fertilizer in agricultural systems by increasing NUE. The use efficiency of fertilizer N that constitutes more than 60% of total plant nutrients consumed in India, is very low: 30-40% in rice and 40-60% in other crops. Ladha et al. (2005) reported that average recovery efficiency of N (REN) in aboveground biomass (grain+ straw) in research plots was 44% in rice, 54% in wheat and 63% in maize. Not included in these figures is the fertilizer-N recovered in roots, N recovered in subsequently grown crops, and N that remains in the soil N. When residual N recovery in the subsequent crops was considered in the calculations, total REN from application of N averaged about 50 to 57% in research trials with cereals. The remainder is either stored in soil organic matter pools or lost from the cropping system. Average REN and AEN in irrigated rice on farmers’ fields in Asia are around 31% and 11.5 kg kg -1 as compared to 44% and 21.6 kg kg-1 in research trials, respectively. The lower NUE in farmers’ fields is usually explained by a lower level of management under practical farming conditions and greater spatial variability of factors controlling REN and other indices of NUE. It is reasonable to assume that, on a global scale, at least 50% of the fertilizer-N applied is lost from agricultural systems and most of these losses occur during the year of fertilizer application. Average PFPN for cereals has been estimated at 44 kg kg -1. Low NUE not only leads to financial loss to the farmers and government, it also creates environmental problems. Since most of the fertilizer-N is lost during the year of application, N and crop management must be fine-tuned in the cropping season in which N is applied in order to maximize system-level NUE. Numerous concepts and tools needed to increase NUE have been developed. These strategies can be: (1) those that enhance crop N demand and uptake (genetic improvements, management factors that remove restrictions on crop growth and N demand) and (2) management options that influence the availability of soil and fertilizerN for plant uptake. High NUE can also be achieved through balanced fertilization combined with other practices (e.g. improved varieties, water management, and plant protection) that stimulate maximum uptake of plant nutrients by the crop. Other methods include using soil and plant analyses for nutrients and constantly monitoring crop growth and development and applying amounts of nutrients that correspond as precisely as possible to crop needs and growing conditions. This can be achieved by choosing the most suitable type and rate of N fertilizer and the most appropriate application technique (for example, fertilizer placement or band application into the root zone, split application, foliar application, fertigation, so-called ‘spoon-feeding’), developing special types of fertilizers (new N forms, slow- and controlled-release fertilizers that avoid, or at least reduce N losses), and various forms of site-specific N management.

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Improved synchrony can be achieved by more accurate N prescriptions based on the projected crop N demand and the levels of mineral and organic soil N, by using decision aids to diagnose soil and plant N status during the growing season, or by using controlledrelease fertilizers or inhibitors. Improved fertilizer products can play an important role in the quest for increasing NUE, but their relative importance varies by regions and cropping systems. Prerequisites for the adoption of improved N management technologies are that these must be simple, provide consistent and large enough gains in NUE, involve little extra time and be cost-effective. If upcoming strategies leads to at least a small and consistent increase in crop yield with the same amount or less N applied, the resulting increase in profit is usually attractive enough for a farmer. The N supply for a crop comes not only from fertilizer, but also from manure and crop residues, and mineralization from soil. The optimal fertilizer N rate varies from field-to-field and from year-to-year due to variation in both crop N demand and soil N supply. The amount of N required by the crop is determined by the level of crop growth – the greater the growth, the higher the crop demand for N. Crop growth is influenced by management practices such as crop grown, variety and planting date, and also by soil and climatic conditions. The fine-tuning of the fertilizer doses for all crops based on precise soil-crop requirement estimations, as facilitated through soil analysis and crop diagnostic measures, would be a key element in precision N management.

18.5.3.1. Best N Management Practices The best management practices (BMPs) provide a framework to achieve cropping system goals of increased production, increased farmer profitability, enhanced environmental protection, and improved sustainability. The goal of fertilizer BMPs is to match nutrient supply with crop requirements and to minimize nutrient losses from fields. Selection of fertilizer BMPs varies by location, and those chosen for a given farm are dependent on local soil and climatic conditions, crop management conditions and other site-specific factors. Different N fertilizers are valued according to their total N-content and the different N-forms. The right N source must consider any nutrient interactions or compatibility issues, potential sensitivity of crops to the source, and any non-nutrient elements included with the source material. Efficiency of some products may be reduced due to leaching losses of nitrates or volatilization of ammonia under certain temperature and soil moisture situations. The right source may vary with the crop, the soil properties of the field, and options for method of application. The right N rate considers the supplying power of the soil in relation to the N requirement of the crop. Soil testing and plant analysis are important tools to help in such decisions. Plants require different rates of N at different stages of the growing season. Rate should be adjusted to help balance nutrient supply with crop removal at all times to avoid deficiency stress and economic loss. Excessive rates may lead to inefficiency in nutrient use and economic losses and environmental problems. The right rate should take into account all sources of nutrients, including soil supply (soil test), manure and other organic sources, crop residue, irrigation water and rainfall, etc. Rate comparison studies are an important part of determining the right rate. Rate studies are best done under the conditions for which the rate decision is being made, preferably on-farm rate studies. Timing has a major effect on the efficiency of N management systems. Nitrogen should be applied to avoid periods of significant loss and to provide adequate N when

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the crop needs it most. If significant losses, particularly those due to denitrification or leaching, are anticipated, split applications - the application of N fertilizers at multiple times during the growing season can help improve NUE and reduce losses, in which much of the N is applied after crop emergence, can be effective in reducing losses. Applying N fertilizer as close as possible to the time of uptake requirement by the crop is a good management strategy to maximize efficiency. Crop nutrient requirements change throughout the growing season as the crop grows from vegetative stages, through reproductive stages, and on to maturity. One of many examples of timing fertilizer applications based on stage of crop growth and nutrient needs is split application of N. An increasingly popular system for applying N to rice, wheat and maize is to divide the application into 2 or 4 different times, and often different application methods and fertilizer sources. After pollination, the effectiveness of the roots to take up N begins to decline, so it is important to have most of the total N requirement met and taken up by the plant at that time. Having the nutrients in the right place helps to ensures that plant roots can absorb enough of each nutrient at all the times during the growing season. Options for fertilizer application include: surface broadcast, band application or deep banding (usually 10 to 15 cm below the surface in the root zone) or foliar application. The right place of application depends upon the characteristics of the fertilizer material being applied. Anhydrous ammonia, for example must be injected into the soil deep enough to seal the gas from being lost to the atmosphere. Surface applied urea is especially susceptible to such losses. Urea may be surface applied, but volatilization losses can be substantial without sufficient rainfall within a few days to move the fertilizer into the soil.

18.5.3.2. Real time site-specific N management Substantial portions of applied N can be lost due to the lack of synchrony of plant-N demand with N supply. The timing of fertilizer N application is used to best match the demand of N by crop plants with supply. An important element of site-specific N management revolves around the development and use of diagnostic tools and models that can assess real N need of crop plants. More recently, some non-invasive, optical methods based on leaf greenness, absorbance and/or reflectance of light by the intact leaf, have been developed. These include chlorophyll meters, leaf colour charts (LCC), groundbased remote sensors and digital, aerial, and satellite imageries. Over the last decade, chlorophyll meter, LCC and hand held Green Seeker have been extensively tried to improve N use efficiency in cereals grown in different agro-ecosystems and regions. 18.5.3.2.1. Leaf colour chart: The International Rice Research Institute has developed a leaf colour chart (LCC) for N management in rice (Figure 18.8). The LCC is an inexpensive

Figure 18.8. The six panel leaf colour chart (left) and SPAD chlorophyll meter (right) for estimating greenness of leaves as an indicator of relative N content

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and simple tool to monitor leaf greenness and guide the application of fertilizer N to maintain optimal leaf N content. A standardized plastic LCC with six panels, now modified to have four panels, ranging in color from yellowish green to dark green, has been developed and promoted across Asia. The LCC can be used to determine the amount and timing of N fertilizer application in rice, wheat, maize and other crops. It is cheap, easily understood and implemented by farmers and advisers with a wide range of educational backgrounds and skills, and can thus be easily adapted to mass audiences. 18.5.3.2.2. Chlorophyll meter: The SPAD (Subsystem Positioning Aid Device) meter or chlorophyll meter has been used since the 1990s by researchers to help estimate the N status of plants. The SPAD meter instantly measures chlorophyll content or 'greenness' of plants to reduce the risk of yield-limiting deficiencies or costly over-fertilizing. The SPAD quantifies subtle changes or trends in plant health long before they’re visible to the human eye. Non--invasive, non-destructive measurement is made on green plants, by simply clamping the meter over leafy tissue, and receiving an indexed chlorophyll content reading in less than 2 seconds (Figure 18.8). A strong relationship between SPAD measurements and leaf N content suggests that N status of plants then be used to formulate supplemental N recommendations if needed. 18.5.3.2.3. Optical sensors: Other electronic tools that have become important for N management guidance are optical sensors and include GreenSeeker, the Crop Circle, and the Rapid Scan CS-45 sensors. They can be used as a single hand-held unit, or mounted on a tool bar as a gang of multi-row sensors. These tools emit standard wavelength light beams and measure the reflected light coming back to the unit from the leaves. The GreenSeeker sensor (Figure 18.9), emits light in two wavelengths, and then measures the reflectance from the crop canopy, and computes the NDVI (Normalized Differential Vegetation Index) value that relates to the amount of plant material in the field of view and its general vigour (greenness). The device uses a technique to measure the fraction of the emitted light in the sensed area that is returned to the sensor (crop reflectance) and calculates NDVI as: NDVI = (FNIR - FRED)/(FNIR +FRED), where FNIR and FRED are, respectively, the fractions of emitted NIR and red radiation reflected back from the sensed area. The NDVI value is then compared to a calibration dataset, such as an N rate comparison strip, to provide a relative indication of plant condition that can be used to predict response to additional N fertilizer. By calibrating with standard colour references and 'non-limited' N reference plots, an estimate of the N status of plants can be made and used to predict potential response to added N fertilizer. As cellular phone networks spread across rural areas throughout the world, tools such as this can potentially be used to improve N management wherever crops are grown.

Figure 18.9. GreenSeeker in use in the field (left) and the operation of the GreenSeeker (right)

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18.5.3.3. Controlled and slow release fertilizers Another possible way of improving NUE is the use of slow- (SRFs) and controlled-release (CRFs), or ‘stabilized’ N fertilizers, which hold the N until plants actually require it. Thus, these can reduce N losses and enhance NUE, decrease the labour costs and protect the environment. The uptake of N by cereals is generally ‘sigmoidal’ (S-shaped) and matching N demand with availability from fertilizers is shown diagrammatically in Figure 18.10. The ideal fertilizer should release nutrients in a sigmoidal pattern for optimal plant nutrition and reduce the N losses by processes that compete with the plant’s nutrient requirements. Several options are available to help make fertilizers more efficient by slowing their release, inhibiting conversion to forms that are less stable in the soil, or enhancing availability to plants. These include chemical additives, biological inhibitors, and coatings that physically constrain N activity in the soil. The use of these additives depends upon potential benefits in relation to the cost, considering appropriate agronomic, economic, and environmental factors. The application of CRFs and SRFs can potentially decrease fertilizer use by 20 to 30% of the recommended rate of a conventional fertilizer while obtaining the same yield. Compared to the large amount of fertilizers used throughout the world, the total use of SRFs and CRFs is still small and comprises only about 0.15% of the total use of nutrients. These are being expensive than conventional N fertilizers due to the additional labour and equipment required to produce coated fertilizers and the cost of the coating materials.

Figure 18.10. The ‘ideal fertilizer’: the nutrient release is synchronized with the crop’s nutrientrequirements (Adapted from Lammel 2005)

18.5.3.3.1. Controlled release fertilizers 18.5.3.3.1.1. Stabilized N fertilizers: A fertilizer to which a N stabilizer has been added is called stabilized N fertilizer. Strictly speaking stabilized fertilizers are only those to which an N stabilizer has been added during production. The N stabilizers are substances (such as nitrification and/or urease inhibitors), which when added to the fertilizer extend the time that the N component of the fertilizer remains in the soil in the urea-N or NH 4+-N forms). Depending upon the thickness of the coating and its components, the protection can be for a few days to a few months. Though nitrification and urease inhibitors are recognized as N stabilizers; nitrification inhibitors in some publications are designated as slow- or controlled-release fertilizers. Nitrification inhibitors slow the bacterial conversion of ammonium to nitrate, and urease inhibitors reduce the enzymatic breakdown of urea

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into ammonium and nitrate. Both processes help reduce potential losses of N to the atmosphere, allowing more time for the crop plants to utilize it. 18.5.3.3.1.2. Nitrification inhibitors: Nitrification inhibitors delay/inhibit the bacterial oxidation of the ammonium ion (NH 4+) by depressing over a certain period of time (4 to 10 weeks) the activity of Nitrosomonas bacteria in the soil (Figure 18.11). The objective of using nitrification inhibitors is to control the loss of nitrate by leaching or the production of nitrous oxide (N2O) by denitrification from the topsoil by keeping N in the ammonium form longer and thus increasing the NUE. Furthermore, nitrification inhibitors – by delaying the conversion of ammonium to nitrate – avoid undesirable high nitrate levels in plants used for human and animal nutrition. Nitrification inhibitors favour the partial ammonium nutrition of plants because plants need less energy to incorporate ammonium into amino acids; nitrate has to be reduced first to ammonium and this requires energy. Nitrification inhibition is based on product’s ability to tie-up copper - a critical metal used by nitrification bacteria. The important nitrification inhibitors include N-Serve® nitrapyrin (2-chloro-6-[trichloromethyl] pyridine), DCD (dicyandiamide), 4-amino-1,2,46-triazole-HCl (ATC), 3,4-dimethylpyrazole phosphate (DMPP), carbon disulphide (CS 2) and ammonium thiosulphate (NH4)2S2O3).

Figure 18.11. Nitrfication inhibitors inhibit the activity of Nitrosomonas responsible for the conversion of NH4+ to NO2-

In India, encapsulated calcium carbide (CaC2) pellets are mixed with fertilizer to produce a controlled release fertilizer. CaC2 particles are successfully coated with waxes (1:1). On contact with water these pellets slowly disintegrate and acetylene (C 2H) is released, which has proved effective in inhibiting the nitrification of NH 4+-based fertilizers in rice and wheat. Nitrapyrin and DCD are the proven nitrification inhibitors. Nitrapyrin can delay nitrification by 82%, and DCD by 53% in 14 days. Nitrapyrin can be added to any ammonium fertilizer such as ammonium sulphate, ammonium nitrate, urea, and anhydrous ammonia.However, incorporation of nitrapyrin into conventional N fertilizers during their production process is difficult due to its high vapour pressure. 18.5.3.3.1.3. Neem cake coated urea: Neem (Azadirachta indica) cake coated urea (NCCU), which was shown to have nitrification inhibiting properties has been developed in India. It increased yields by 6-11% in rice and rice-wheat cropping systems. The major factor responsible for N regulation is nitrification inhibition by the triterpenes in neem. Since coating of urea with neem cake was industrially not feasible due to the large volumes involved, a neem oil microemulsion (neem seed extract) technique was developed in India. This technique or its modification is currently being used by different fertilizer companies in India.

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18.5.3.3.1.4. Urease inhibitors: A chemical that inhibits hydrolytic action on urea by the enzyme urease is called as urease inhibitor. They are used to slow down the conversion of urea to ammonium and nitrate. They can inhibit urea hydrolysis for 2 weeks or more depending on soil and climatic conditions. Warm temperatures and wetter conditions cause urease to repopulate faster. The principal advantages of urease inhibitors are: a significant reduction in volatilization losses of ammonia resulting in an increase in plantavailable soil N; an improvement of N-use efficiency from urea and urea-containing fertilizers; a reduction of seedling damage; and a decrease in the emission of N oxides and nitrous oxide. Inhibition of urease activity is due theoretically to a tie-up of soil nickel - a critical metal constituent of urease enzyme. Most commonly used urease inhibitor is NBPT (N-(n-butyl) thiophosphoric triamide) (commercial name Agrotain®). It competes for active sites on the urease enzyme and ties up activity for about 10 days, depending on weather conditions. Other potential urease inhibitors are phenyl phosphorodiamidate (PPD), hydrquinone (HQ) and cyclo hexyl phosphoric triamide (CHPT). NBPT is at present the only urease inhibitor of commercial and practical importance in agriculture. It can be injected into molten urea before granulation, applied to the surface of granules or prills in batch or continuous-processes. Agrotain Plus is a dry concentrate containing the urease inhibitor NBPT plus the nitrification inhibitor DCD. It reduces volatilization losses of ammonia as well as N losses from denitrification and leaching. Super Urea is a granulated urea containing the urease inhibitor NBPT plus the nitrification inhibitor DCD.

18.5.3.3.2. Slow release fertilizers (SRFs) The condensation of urea with aldehydes (and particularly with formaldehyde) is one of the most common methods for preparing SRFs. Urea formaldehyde (UF) is the most popular organic-N compound used for the slow release of N, and the most widely used of all SRF/CRFs. Four types of SRFs according to the mode of release control: (i) diffusion, (ii) chemical reaction (decomposition), (iii) swelling, and (iv) osmosis. Microbially decomposable N products such as urea-formaldehyde (ureaform), isobutylidenediurea (IBDU), and crotonaldehydediurea (CDU) are the most common SRFs. IBDU is in second place after UF in terms of worldwide production. IBDU contains about 31% N, most of which (about 90%) is water-insoluble. The SRFs are characterized by their activity index (AI). The AI of CRFs is defined as: AI = (CWIN – HWIN) / CWIN x100, where CWIN is cold water insoluble N and HWIN is hot water insoluble N. The AI provides an estimate of the fraction of relatively long-lasting release (about six months). As a result of technological developments, a shift has occurred over the past decade in AI values from 40% to 55 - 60%. The SRFs are mainly used on professional turf, in nurseries, greenhouses, on lawns and for gardens and landscaping, and in high-value crops. 18.5.3.3.2.1. Sulphur coated urea (SCU): One of the more common coatings is sulphur, which is used to protect urea granules from dissolving. The sulphur coating is an impermeable layer that slowly degrades through microbial, chemical and physical processes. Due to the simplicity and relatively low cost of using sulphur as a coating material, SCU has become the most commonly used coated-urea product in many parts of the world. Although, SCU has proved highly efficient source of N in rice, it is not popular due to high cost of sulphur in India. 18.5.3.3.2.2. Polymer coated urea (PCU): In PCU, urea is coated with special polymer coating – special to each manufacturer (e.g. resins or mineral-based products that act as semipermeable membranes or impermeable membranes with tiny pores), which is a hydrophobic (water insoluble). Water moves in through coating to dissolve urea and N

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diffuses out through porous polymer membrane. These coatings can protect the fertilizer particles from a few weeks to several months depending on the coating and the conditions in the soil. These products release the nutrients at a time as close as possible to the time of uptake required by the plant thereby increasing NUE. By keeping more of the nutrient in the crop, losses to the environment are also reduced. Though polyolefin-coated urea (POCU or Meister®) is more expensive than conventional N fertilizers, it can contribute to innovative fertilizer applications, whereby total production costs can be reduced by 30 to 50%. The N release is not significantly affected by soil properties such as pH, salinity, soil texture, microbial activity, redox potential or CEC. Another coated SRF is sulphur + polymer-coated urea. 18.5.3.3.2.3. Urea supergrnules (USG): These are ~1 cm diameter granules/pellets weighing ~1 g. USG outperforms commercial urea prills in rice on most soils except the highly porous coarse textured soils. In addition to the advantage of N placement, which reduces volatilization losses, placement of such a high amount of urea at a micro-locus produces very high concentrations of NH 3, inhibiting nitrification. Availability of appropriate machine to place USG in paddy fields is the major limitation for their use in Indian agriculture. Use of USG by rice farmers is becoming popular in Bangladesh. 18.5.3.3.2.4. Difference between slow- and controlled-release fertilizers: The term CRF is applied to fertilizers in which the factors controlling the rate, pattern and duration of release are well known and controllable during their preparation. CRFs are typically coated or encapsulated with inorganic or organic materials that control the rate, pattern, and duration of plant nutrient release. Most importantly, the release rate of a CRF fertilizer is designed in a pattern synchronized to meet the changing crop nutrient requirements. The CRFs provide the plant with available nutrients for a longer time compared to normal fertilizers, such as urea. The SRFs involve a slower release rate of nutrients than conventional water-soluble fertilizers and CRFs, but the rate, pattern, and duration of release are not controlled because they depend on microbial organisms whose effectiveness is dependent on temperature and moisture conditions. Nitrogen products decomposed by microbes are commonly referred as SRFs. These extend their bioavailability significantly longer than normal N fertilizers such as ammonium nitrate and urea. The delay of initial availability or extended time of continued availability may occur by a variety of mechanisms. These include controlled water solubility of the material by semi--permeable coatings, occlusion, chemical forms or by slow hydrolysis of water-soluble low molecular weight compounds.

18.6. Integrated Nutrient Management Integrated plant nutrient management (IPNM) or integrated plant nutrient supply (IPNS) is yet another approach to enhance NUE. The use of organic manures as source of nutrients and their general benefit to the soil dates back to the beginning of settled agriculture. With escalating prices of fertilizers and increasing awareness about soil health and sustainability in agriculture, organic manures and many diverse organic materials, have gained importance as components of IPNM strategies. The basic concept underlying IPNM is the maintenance and possible improvement of fertility and health of the soil for sustained crop productivity on long-term basis and use fertilizer nutrients as supplement to nutrients supplied by different organic sources available at the farm to meet the nutrient requirement of the crops. Consequently, major focus in sustainable agricultural systems is on the management of SOM and plant nutrients through integrated use of mineral

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fertilizers with organic inputs such as animal manures, biological N fixation, crop residues, green manures, sewage sludge, and food industry wastes. Legumes as a green manure or a dual purpose crop (grain+ green manure) such as mungbean (Vigna radiata) and cowpea residue left after one picking of pods applied to rice can contribute up to 40-120 kg N ha-1 in rice-wheat system and thus reduce fertilizer N application to crops. Legume residues decompose fairly fast under subtropical and tropical conditions especially under submerged conditions and make the N in them readily available to the growing rice crop. Similarly, application of different organic manures can supply 40 to 120 kg N ha-1 in different cereal-based cropping systems. Apart from soil and climatic conditions, and cropping system, the N supply from organic sources depends on their C:N ratio and rate of application.

18.7. Conclusions Nitrogen exists in the nature primarily as N2 gas, which makes up about 78% of the Earth’s atmosphere. So as to make N usable by plants, it must be converted to ammonium or nitrate forms. Organic forms of N make up a very high percentage (>90%) of the total N found in the soil; only a small part is in inorganic forms also called as mineral N (NH 4+ and NO3-). Manufacturing of fertilizer N begins with anhydrous ammonia which is produced from air and natural gas by the Haber-Bosch process. The major (>80%) source of N fertilizers available in India is urea. Hydrogen ions released during nitrification of the NH4+-based fertilizers cause acidity in soils. The N cycle includes the processes of fixation, mineralization, immobilization, nitrification, ammonia volatilization, denitrification, leaching, runoff, and plant assimilation. Microorganisms convert organic N present in soil organic matter, crop residues, and manures to inorganic forms of NH 4+N and NO3--N and the process is termed as mineralization. Immobilization occurs when organic materials added to soil contain low amounts of N and/or have a high C:N ratio (> 25:1). The fixation of NH4+occurs through the process adsorption by the mineral or organic fractions of the soil. About 50-60% of added NH 4+ can be fixed within a short period of time. The release of fixed NH4+ -N ranges from 4% to 25% in different soils. Urea is converted relatively rapidly to ammonia and CO 2 by bacterial enzyme urease found in most of the soils. This transformation increases soil pH and leads to very high volatilization losses of ammonia, particularly when urea is surface-applied. Biological N fixation is an important source of N in agriculture and it has been estimated that more than 90 Tg of N is fixed per year by the N fixing microorganisms in agricultural lands. Rhizobium accounts for the largest proportion (40%) of the biologically fixed in India. Use efficiency of fertilizer N is very low in India; 30-40% in rice and 40-60% in other crops. It is reasonable to assume that at least 50% of the fertilizer-N applied is lost from agricultural systems and can become a serious pollutant. High NUE can be achieved by choosing the most suitable type and rate of N fertilizer and the most appropriate application technique (for example, fertilizer placement or band application into the root zone, split application, fertigation), and developing special types of fertilizers (new N forms, slow- and controlled-release fertilizers that avoid, or at least reduce N losses). Improved fertilizer products can play an important role in the global quest for increasing NUE, but their relative importance varies by regions and cropping systems. These developments can help in improving the NUE of three major crops of rice, wheat and maize. Compared to the large amount of fertilizers used in India, the total use of slow and controlled release fertilizers is still small being expensive than conventional N fertilizers.

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Study Questions Q. 1.

Discuss native fixed and recently fixed NH 4-N in soils.

Q. 2.

Calculate the total amount on N (kg ha -1)present in 20 cm layer of soil (0.08% total N and bulk density 1.5 Mg m-3).

Answer: Total weight of soil in 20 cm depth = 102x 102 x 1.5 x 0.20 m= 0.3 x 104 Mg or 3 x 106 kg Total N in 20 cm layer= 0.08/100 x 3x 106 = 2400 kg ha-1 Q. 3.

List sources of direct N2O emissions from agricultural soils

Q. 4.

Discuss that nitrification is a necessary evil.

Q. 5.

Explain how nitrification can be regulated in soils.

Q.6.

Describe the ammonia volatilization and methods to reduce its losses from the soil.

Q. 7.

Identify environmental conditions that determine the rate of ammonia volatilization from soils.

Q 8.

Discuss the differences in N transformations in ideal (fine-textured) and non-ideal (coarse-textured) soils under rice cultivation.

Q. 9.

Explain how the N fertilizers cause soil acidification?

Q. 10. Explain how the temperature, pH and redox potential affect the relative concentration of NH4+ and NO3-- N in soils? Q. 11. Discuss the effect of straw management (surface placement versus incorporation ) on N immobilization in soils. Q. 12. Identify crop and soil conditions under which conventional and slow-release fertilizers are recommended. Q. 13. Compare and contrast nutrient availability from the fertilizers, organic manures and plant residues. Q. 14. Describe how timing of application affects the effectiveness of fertilizer N. Q. 15. Describe relative efficiency of applying N fertilizers to soils via broadcast with and without incorporation. Q. 16. Describe the water quality implications of improper application of nitrogen to soil. Q. 17. Grain yield of rice crop for no nitrogen and 150 kg N/ha treatments was 3.5 t/ha and 9.0 t/ha and total N uptake (grain + straw) was 45 kg N/ha and 135 kg N/ha, respectively. Calculate the partial factor productivity, agronomic efficiency and recovery efficiency of N. Answer: (i) Partial factor productivity of N= 9000/150= 60 kg/kg N (ii) Agronomic efficiency of N= 9000-3500/150= 36.7 kg grain/kg N (iii) Recovery efficiency of N= 135-45/150 x 100= 60% Q. 18. Define real time or need based nitrogen management for increasing N use efficiency.

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References Freney, J.R. and Simpson, J.R. (1983) Gaseous Loss of Nitrogen from Plant-Soil System. MartinusNijhoff/ Dr W. Junk. The Hague, Netherlands. Ladha, J.K., Pathak, H., Krupnik, T.J., Six, J. and van Kessel, C. (2005) Efficiency of fertilizer nitrogen in cereal production: retrospects and prospects. Advances in Agronomy 57, 85-156. Lammel, J. (2005) Cost of the different options available to the farmers. Current situation and prospects. IFA International workshop on Enhanced- Efficiency Fertilizers, Frankfurt. International Fertilizer Association, Paris, France.

Further Suggested Readings Bacon, F.E. (1995) Nitrogen Fertilizers in the Environment. Marcel Dekker Inc. New York. 608 pp. Bagyaraj, D.J. and Rangaswami, G. (2007) Agricultural Microbiology. Prentice Hall of India Pvt. Ltd.440 pp. Brady, N.C. and Weil, R.R. (2007) The Nature and Properties of Soils, 14th Edition. Pearson Education Inc., Upper Saddle River, NJ, USA. Havlin, J. L, Tisdale, S.L., Nelson, W.L. and Beaton, J.D. (2013) Soil Fertility and Fertilizers: An Introduction to Nutrient Management, Eighth Edition. Prentice Hall; 528 pp. Schepers, J.S. and Raun, W. R. (eds) (2008) Nitrogen in Agricultural systems. Agronomy Monographs 49. American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America, Madison, Wisconsin.