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Increased use of nitrogenous (N) fertilizer has significantly altered the global N-cycle and produced nitrogenous gases of environmental consequence. While nitrous oxide (N2O) emissions contribute to global greenhouse gas accumulation and the stratospheric ozone depletion, degradation of ground water quality by N use in agriculture is fundamentally a nitrate leaching problem. Despite these evident negative environmental impacts, consumption of N fertilizer cannot be reduced in view of the food security for teeming population in the developing countries. Various strategies, from agronomic to genetic engineering, have been tried to tackle this problem. Split application of N, use of slow-release fertilizers, nitrification inhibitors and the use of organic manures are some agronomic techniques adopted. One of the important goals to reduce N-fertilizer application can be effectively achieved by choosing N-efficient (i.e., Which can grow under low N conditions), ensuring their optimum uptake of applied N by application of adequate amounts of fertilizer nutrients in a balanced manner and knowing the molecular mechanisms for their uptake as well as assimilatory pathways. Newer approaches like QTL and proteomics could also help us in understanding these processes fully, hence could contribute greatly in enhancing NUE and reduction of N pollution in the environment.
Key words: Reactive Nitrogen, NUE, QTL, Proteomics, N-pollution.
Nitrogen (N) represents one of the most important nutrients found in terrestrial ecosystems. It is an important constituent of a number of complex organic molecules viz., proteins, nucleic acids etc. Atmosphere is the main reservoir of nitrogen (N2), which stores around one million times more N than contained in all the organisms. Oceans and organic matter in soil are the other major store houses of nitrogen. N is often considered as an important limiting nutrient for plant growth and development, despite its remarkable abundance in the atmosphere. This is the reason for the past half a century, supply of nitrogen through fertilizers has been an influential application for increasing the growth and yield of cultivated plants such as cereals. To meet the increasing demand for food, farmers apply more fertilizers in their bid to increase the agricultural productivity. Fertilizer nitrogen has provided food security particularly to developing nations including India, as the cereal production has kept pace with its ever-increasing population. Today, India occupies the third rank in the world in fertilizer N consumption and second in fertilizer N production (FAI, 2008). The consumption of fertilizer nitrogen in India increased from a mere 55 thousands metric tons in 1950-51 to over 14.2 million tonnes in 2007-08 and is still increasing (FAI, 2008). With the current rate of N fertilization, the requirement of nitrogen will be 22-25 million tonnes per year in 2020 (FAI, 2008). However, it is remarkable that utilization of applied fertilizer nitrogen in field by most cereal crops does not exceed 50% and around 70% of the total nitrogenous fertilizer is applied for rice and wheat cultivation (Abrol et al. 1999). Therefore with the increase in agricultural food production worldwide in last 50 years the N fertilization of crop plants has increased more than 20-fold (Shrawat and Good 2008). However, the use of this fertilizer is generally inefficient, as lesser amount of applied N (around 30-40%) is actually utilized by cereal crops, and the major part (60 - 70%) is lost from the plant-soil system which has caused severe impacts on the ecosystems of the non-agricultural neighbouring bacteria, animals and plants. As a result of leaching, the unused N fertilizer causes impacts like eutrophication of freshwater (London,2005) and marine ecosystems (Beman et al. 2005). In addition, gaseous augmentation of N oxides reacting and affecting with stratospheric ozone and the volatilization of toxic ammonia into the atmosphere (Stulen et al. 1998) and has also been linked to unused N fertilizers. The toxic effects of nitrate are due to its endogenous conversion to nitrite and this ion has been implicated in the occurrence of methaemoglobinemia, gastric cancer and many other diseases (Anjana et al. 2007).
Presently the human population is more than 6.5 million, which is expected to increase around 10 billion by 2025 (Hirel et al. 2007), therefore, the major challenge will be to reach a highly productive agriculture without degrading the quality of our environment. Efficient farming techniques and choosing plant varieties/genotypes that have better nitrogen use efficiency (NUE) could be the tools to tackle this problem. The development of such varieties /genotypes, through conventional plant breeding techniques or by using recombinant DNA technology, will be more proficient with a better understanding the physiological, genetic and molecular bases of NUE among cereal crops. Therefore, there is an urgent need of a 'second green revolution' that does not rely on exhaustive use of inorganic fertilizers rather would aim at improving crop yields in soils by developing varieties with better adaptation to low-fertility soils. (Yan et al. 2006). In the present chapter, we have discussed the inflow and effects of reactive N in the environment and then summarized the strategies adopted to develop the crop varieties/genotypes with high NUE.
Reactive Nitrogen Inflows:
Reactive nitrogen (Nr) is usually referred to all the nitrogen species that are biologically active, photo-chemically reactive and radiatively important N compounds in the atmosphere and biosphere of the earth (Galloway et al. 1995). Thus, Nr includes reduced inorganic forms of N (NH3, NH4+), oxidized inorganic forms (NOx, 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 favourable 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 neighbouring 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 has 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, 1999). Processes such as adsorption, fixation, immobilization and microbial assimilation of added NH4-N in soils are of great importance as they affect nitrogen use efficiency and have the corresponding environmental repercussions (Kissel et al. 2004).
In 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 et al. 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 30-40% 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% to 41%, the soil at wheat harvest retained 19% to 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 soi1-plant system) were about 42 % in rice and 33 % in wheat grown on a typical sandy loam soil in northwest 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.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 and Singh 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 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, 2004b) have shown that NH3 volatailization 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 NH3 concentration in flood water control the extent of NH3 loss from flooded soils (Singh and Singh 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 one 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 non-ideal 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 appled N was lost by leaching and these losses were reduced to 8% when urea was blended with neem cake (Singh et al. 1995b).