ABSTRACT
Low nitrogen (N) availability is usually the main soil nutrient factor limiting growth and yield of many crop plants. Application of chemical N fertilizer is frequently an important management tool used to increase crop yield (Andrews et al., 2003). However, overuse of fertilizer N has contributed to soil and water pollution and a range of environmental problems, including rapid eutrophication of fresh waters and increased atmospheric ammonia and nitrogen oxide concentrations. During the 20th century, especially after 1950, chemical N fertilizers were used abundantly with the objective of enhancing yields of most of the major crops. Nevertheless, fertilizers sometimes cause over-luxuriant growth and little increase or even decrease in seed yield, despite the additional cost and labor (Tewari et al., 2004). Further, application of chemical fertilizers may inhibit the symbiotic N2 fixation process of leguminous plants and the root-nodule bacteria (rhizobia). High doses of chemical N fertilizer severely depressed nodule formation (nodule number and size) and N2 fixation activity in some legumes (Abdel-Wahab et al., 1996; Zahran, 1999). Therefore, the adoption of agronomic and biological technology
to ensure optimum N2 fixation, through new N fertilization techniques or inoculation methods of rhizobia, is essentially important for increasing yield of legume crops (Graham and Vance, 2000). Global terrestrial biological N2 fixation (BNF) has been reported to be between 100 and 200 million tons of N per year. Agricultural crops in fields fix and contribute about 4 0 -4 8 million tons N year1 (Jensen and Hauggaard, 2003). In comparison, 83 million tons N year1 is fixed industrially in fertilizer production. Atmospheric N2 is a renewable resource and, therefore, BNF in agricultural systems is a free and sustainable source of N in cropping systems (Jensen and Hauggaard, 2003). Further, BNF either alone or in combination with fertilizers and animal manure, could be a better solution to supply N to the cropping systems in the future.
