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The supply of nitrogen often limits the growth of plants, influencing the composition of plant communities, and ecosystem productivity (Boring et al. 1988, Vitousek and Howarth 1991). A number of processes tend to reduce the biological availability of N in ecosystems, notably the strong link between organic N and recalcitrant C com-pounds in soils, and the mobility of N out of ecosystems by hydrologic and atmospheric pathways (especially leach-ing and denitrification). However, the capacity of biologi-cal nitrogen fixers to convert N2 to organic N is substantial where symbiotic N fixers are abundant, and enough to maintain N pools in ecosystems and to replenish N losses (Vitousek et al. 2002). Consequently, nitrogen fixation by autotrophs can be an important mechanism of nitrogen accumulation in ecosystems (Sprent and Sprent 1990). Plant species with symbiotic N2-fixing bacteria can fix 50 to 200 kg N ha–1 y–1 in pure stands (Boring et al. 1988