ABSTRACT
Nitrogen fixation is the reduction of atmospheric nitrogen to ammonia. It is the basic phenomenon that makes nitrogen available for the biosphere. Biological N2 fixation (BNF) is the most important way for N to enter soil. It is an efficient source of fixed N2, which plays an important role in land remediation. Deficiency in mineral N often limits plant growth, therefore symbiotic relationships have evolved between plants and a variety of microorganisms (Freiberg et al., 1997). Nitrogen is a key input to most crops and pastures that do not belong to the leguminous family. Because nitrogen in its various forms is usually costly, it is an important decision for farmers to provide it to their crops in the most economic way and to choose between chemical fertilizers and legumes to provide the nitrogen (Schilizzi and Pannell, 2001). Similarly, because there are various types of fertilizers and different species of legume crops, the primary question farmers may ask is, Which fertilizers and legumes provide nitrogen in the most economic way? The N2-fixing Rhizobium-legume symbioses are important sources of BNF, and they are more effective as a biofertilizer than other N2-fixing systems(e.g., heterotrophs), associative bacteria, and actinorhizal plants (e.g., FrankiaCasuarina and Frankia-Atriplex symbioses) (Abdel-Ghaffar, 1989; Wullstein, 1989; Caucas and Abril, 1996; Sayed et al., 1997; Zahran, 1999). Values estimated for various legume crops and pasture species commonly fall in the range of 200 to 300 kg N·ha-1 per year (Peoples et al., 1995). The nodulated wild legumes have potential for N2 fixation, reforestration, and control soil erosion, and are also useful in providing a vegetational cover (biofertilizer) for degraded lands and arid ecosystems (Ahmad et al., 1984; Jha et al., 1995; Requena et al., 2001).
