Root-Bacteria Interactions: Symbiotic N2 Fixation

The importance of biological nitrogen fixation (BNF) to world food security is unquestionable. Since the dawn of farming, symbioses capable of BNF have been instrumental in both supplying food and improving soil health (Vance et al., 2000; Van Kessel and Hartley, 2000). Today earth’s 6 billion people consume an average of nearly 11 g N per day or ~24 million Mt annually (Frink et al., 1999). Worldwide, legumes provide at least 33% of humankind’s N needs. In the tropics and subtropics, plant sources provide up to 80% of the dietary N requirements. Legumes are grown on ~275 million hectares, or nearly 11 % of the arable land (Peoples et al., 1995; Smil, 1999; Socolow, 1999). They fix ~40-60 Tg N2 year−1 (Smil, 1999; Socolow, 1999), an amazing amount since the total amount of nitrogenase in the world is only a few kilograms (Delwiche, 1970).