Reactive nitrogen (Nr) is usually referred to all the nitrogen species that are biologically active, photochemically reactive, and radiatively important N compounds in the atmosphere and biosphere of the earth (Galloway 1995). Thus, Nr includes reduced inorganic forms of N (NH3, NH4+), oxidized inorganic forms (NO^, HNO2, N2O, NO3-), and organic compounds (urea, amines, proteins, nucleic acids). There are numerous sources in environment that contribute to Nr and total nitrate content of natural waters, e.g., atmosphere, geological features, anthropogenic sources, atmospheric nitrogen fixation, and soil nitrogen. However, detailed hydro geological investigations conducted have indicated a heterogeneous pattern of nitrate distribution. Soils with low water-holding capacity (sandy soil) and high permeability, movement of pollutants like chloride and nitrate is much quicker than in clayey soil. This is probably the main cause for high nitrates in areas with sandy soil. Vegetables account for more than 70% of the nitrates ingested in the human diet. The remainder of nitrate in a typical diet comes from drinking water (21%), meat and meat products (6%) (Prasad 1999).
The form of added N plays a role in regulating N losses and influencing NUE. Among these forms, NO3 is the most susceptible to leaching, NH4 the least, and urea moderately susceptible. Ammonia and urea are more susceptible to volatilization loss of N than fertilizers containing NO3. Urea is the most widely used N fertilizer in India. The studies showed the importance of selecting ammonium-based N fertilizer early in the season to reduce N leaching due the mobility of urea and nitrate source in irrigated rice and wheat systems (Prasad and Prasad 1996). Nitrate containing fertilizers when applied to rice proved less efficient because nitrate is prone to be lost via denitrification and leaching under submerged soil conditions in normal and alkali soils (Prasad 1998). In saline soils, however, it is beneficial to use NO3 containing N fertilizers as it compensates the adverse effects of Cl- and SO42- on absorption of NO3 by plants (Choudhary et al. 2003).
Nitrogen losses from soil-plant system. Once inorganic N has appeared in the soil, it can be absorbed by the roots of higher plants or still metabolized by other microorganism during nitrification. This process is carried out by a specialized series of actions in which a few species of microorganisms oxidize NH4+ to NO2 or NO2- to NO3-. Ammonium ion reacts with excess hydroxyls in soil solutions, which leads to N losses to the atmosphere by NH3 volatilization (Wood et al. 2000) . This represents an important source of N loss in agricultural soils under favorable conditions. Due to extensive use of N fertilizers and nitrogenous wastes, the amount of N available to plants significantly exceeds the N returned to the atmosphere by gaseous losses of N through volatilization and denitrification (Martre et al. 2003). Minimizing drying of surface soil and providing additional source of urease enzyme can minimize NH3 volatilization. A portion of this excess N is leached out in the soil profile as NO3- or carried in runoff waters. These are conductive conditions for N losses in agricultural soils, thus reducing the NUE (Delgado et al. 2001). With transport of N in water ways and neighboring ground-water systems, the N concentration could exceed the levels acceptable for human consumption. Nitrate in soil profile may be leached into groundwater when percolating water moves below the rooting depths of crop and provides leaching potential. Paramasivam et al. (2002) have reported a potential leaching of NO3- in arid regions and sandy soils. Losses of N by leaching are affected by local differences in rainfall, water-holding capacity of soil, soil-drainage properties, and rates of mineralization of soil organic N (Delgado et al. 1999). Processes such as adsorption, fixation, immobilization, and microbial assimilation of added NH4-N in soils are of great importance as they affect NUE and have the corresponding environmental repercussions (Kissel et al. 2004).
3n many field situations, more than 60% of applied N is lost due in part to the lack of synchrony of plant N demand with N supply. The remainder of the N is left in the soil, or is lost to other parts of the environment through leaching, runoff, erosion, NH3 volatilization, and denitrification. The cereal NUEs are 42% in developed and 29% in developing countries (Raun and Johnson 1999). Many 15N studies have reported N fertilizer losses in cereal production from 20 to 50% with higher values in rice than in wheat (Ladha 2005) . Prasad (1998) reported that apparent recovery of N applied to wheat varies from 40 to 91%. It has been estimated that rice and wheat N recovery efficiency ranging from 30 to 40% are occurring
in irrigated conditions. An N recovery efficiency exceeding 40% is expected to occur in response to improved N management practices. In a rice-wheat cropping system of Punjab, recovery of 15N by the first wheat crop was 30-41%, the soil at wheat harvest retained 19-26%, and the succeeding rice recovered 5.2% of the 120 kg N ha-1 applied (Singh and Singh 2001). Total losses of applied N (not recovered from soil-plant system) were about 42% in rice and 33% in wheat grown on a typical sandy loam soil in north-west India.
The main causes of for low N recovery are usually attributed to (1) ammonia volatilization, (2) denitrification, (3) leaching, and (4) runoff and erosion (Fig.10.1). Loss of N via NH3 volatilization can be substantial from surface-applied urea in both rice and wheat, which can exceed 40%, and generally greater with increasing soil pH, temperature, electrical conductivity, and surface residue (Singh et al. 2003; Choudhary et al. 2003). Water management in rice and wheat fields influences the extent of N losses due to nitrification-denitrification and NH3 volatilization. Available research results from ideal rice soils suggest that NH3 volatilization rather than denitrification is an more important gaseous loss mechanism for fertilizer N applied to continuously flooded, puddled rice soils of the tropics. 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 NH3
volatilization losses. Firstly, in porous soils under rice it is difficult to maintain continuous flooding. Rather there occur very frequent alternate aerobic-anaerobic cycles, which lead to very fast formation of nitrate under aerobic conditions and their subsequent denitrification under anaerobic conditions that develop due to application of irrigation (Singh and Singh 2001). 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 NH3 volatilization (Sangwan et al. 2004a). Sangwan et al. (2004a, b) have shown that NH3 volatilization losses from urea increases with the increase in soil salinity, sodicity, and the rate of N applied. The losses of fertilizer N as NH3 in rice decreased with increasing floodwater depth and depth of placement (Singh et al. 1995a), and with the application of organic manures (Sihag and Singh 1997). Alkalinity, pH, and NHi concentration in flood water control the extent of NH3 loss from flooded soils (Singh et al. 2003). Sarkar et al. (1991) reported a loss of 15-20% of applied N when urea was broadcast in a wheat field. Prasad (1999) reported a marked reduction in the loss of applied N when the urea was deep placed as compared with surface broadcast on a moist soil. They have reported 13.5% N losses as ammonia after 1 week of urea application under submerged conditions. The high pH or alkalinity resulted in high losses of ammonia by volatilization, which can be nearly 60% of applied N at field capacity. Submergence decreases pH as well as losses to 35% of applied N. The reclamation of sodic soils using gypsum has been found to decrease N losses through ammonia volatilization (Choudhary et al. 2003). The timing of fertilization and irrigation could further influence the losses of urea applied to porous soils. If applied on the wet soil surface following irrigation, as much as 42% of the applied 15N was lost, most likely due to volatilization (Sangwan et al. 2004a). Singh et al. (1995b) showed that application of urea before irrigation increased the NUE by 20% as compared to its surface application after irrigation or broadcast application and surface mixing of urea at field capacity in a clay loam soil.
In nonideal porous soils under rice, there exist every possibility that applied urea N is preferentially lost via denitrification rather than NH3 volatilization. Direct measurement of denitrification losses made by Aulakh et al. (2001) showed that denitrification is a significant N loss process under wetland rice amounting to 33% of the applied N. In excessive N fertilizer application (i.e., at rates in excess of that needed for maximum yield in cereal crops), NO3 leaching can be significant, particularly from the coarse-textured soils. Residual N is then available in soil profile for potential leaching. High levels of NO3-N in the region's groundwater have been reported by Singh et al. (1995b). There is not much information available on leaching losses of N. In a pot culture study, the leaching loss was 11.5% of the applied urea N and was reduced to 8.7% when urea was coated with neem cake (Prasad and Prasad 1996). In a field study at Pantnagar on a silty clay loam soil, 12% of the applied N was lost by leaching and these losses were reduced to 8% when urea was blended with neem cake (Singh et al. 1995b).
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