ABSTRACT
Nitrogen (N) is one of the most yield-limiting nutrients for crop production in the world. It is also the nutrient element applied in the largest quantity for most annual crops (Huber and Thompson, 2007). Systems of agriculture that rely heavily on soil reserve to meet the N requirements of plants cannot long be effective in producing high yields of crops (Stevenson, 1982). Except for legumes, which have the ability to fix their own N, N must be supplied to plants for growth. It is usually added as a fertilizer and is required for all types of soils (Clark, 1982). To increase crop yields, growers worldwide apply over 80 million metric tons of nitrogen fertilizers per year (Epstein and Bloom, 2005). Use of inorganic N fertilizers has had its most substantial beneficial effect on human health by increasing the yield of field crops and nutritional quality of foods needed to meet dietary requirements and food preferences for growing world populations (Galloway and Cowling, 2002; Galloway et al., 2002). Ridley and Hedlin (1980) concluded that increased use of N fertilizer has had the most dramatic influence on increasing crop yields since the 1950s, in combination with disease-resistant cultivars to a lesser effect. Similarly, Camara et al. (2003) reported that historically, few if any technologies have increased winter wheat yield in the United States more than N fertilization. The main reasons for N deficiency are high-quantity uptake by crop plants compared to other macronutrients (except K in some crops such as rice), including in grains or seeds, and its loss by leaching, denitrification, volatilization, soil erosion, and surface runoff. In addition, N is immobilized by soil microbes and undecomposed plant residues, which may cause temporary deficiency. Nitrogen loss in the form of NH3 by plant canopy has been reported (Fageria et al., 2005). Furthermore, in intensive cropping systems, where a no-tillage system is adopted, depletion or loss of organic matter has been reported (Johnson et al., 2006), which may result in N deficiency in crop plants.
