Past research focused on the effects of soil fertility on legume nitrogen fixation. We utilized a management-induced fertility gradient across grain farm fields in the Finger Lakes region of New York to understand the relative importance of soil nitrogen availability and plant species interactions in regulating nitrogen fixation in annual and perennial legumes grown in mixtures and monocultures. This research was funded by USDA NRI, Land Institute Natural Systems Agriculture Graduate Fellowship, and small grants from the Cornell NSF IGERT in Biogeochemistry and Environmental Biocomplexity.