Abstract: As the anthropogenic nitrogen input to the global ecosystem is experiencing an unprecedented rise, quantifying the relationships between biological nitrogen fixation (BNF) and available nitrogen is not only essential to estimating ecosystem nitrogen budgets, but also crucial to optimizing fertilization strategies. In this paper, research results on the effects of available nitrogen on BNF at scales from molecular, individual, to community level, were synthesized and compared. We found that the studies at molecular and individual scales are more systematic than communityscale studies, but are severely limited by the cultivability of diazotrophs; while the studies at the community scale employed the cultureindependent methods, and thus are more efficient. However, at the community scale, very few studies focused on the expression of nitrogenase genes, and the study system is still far from perfect. Therefore, future studies should pay more attention to the regulation of available nitrogen on the nitrogenase gene expression at community scale, with special emphasis on improving the communityscale research system of BNF.