ABSTRACT
In the last six decades, nitrogen fertilization has been a powerful tool in increasing the grain yield of food crops, especially wheat, rice, and corn. However, in the current agricultural and economic environment farmers must optimize the application of nitrogen fertilizers to avoid pollution by nitrate and to preserve their economic margin (Hirel et al., 2001). In this context, nutrient use efciency, including nitrogen in crop plants, has special importance due to economic and environmental implications. The importance of improving nutrient use efciency in modern agriculture or crop production is higher because of the use of large amounts of chemical fertilizers. It has been estimated that almost 1011 kg of N per annum is applied as fertilizer worldwide, a 20-fold increase over the past 50 years, at a cost of 50 billion US dollars (Glass, 2003). The majority of crops except nitrogen-xing legumes receive an application of N; the major requirements are for the production of seeds (Mengel et al., 2006) and forage (Kingston-Smith et al., 2006). However, crop plants are only able to convert 30-40% of this applied N into useful food products such as grain (Raun and Johnson, 1999). There is therefore extensive concern in relation to the N that is not used by the plant, which is lost by leaching of nitrate, denitrication from the soil, and the loss of ammonia to the atmosphere, all of which can have deleterious environmental effect (Vitousek et al., 1997; Glass, 2003). The possibility of more precise use of N in crop production is now being taken seriously (Day, 2005).
