Increased nitrogen (N) deposition, resulting from the combustion of fossil fuels, production of synthetic fertilizers, growth of N(2)-fixing crops and high-intensity agriculture, is one of the anthropogenic factors most likely to cause global biodiversity changes over the next century. This influence may be especially large in temperate zone forests, which are highly N limited and occur in regions with the highest levels of N deposition. Within these ecosystems, N(2)-fixing plants, including legumes, may be more sensitive to N deposition than other plant species. Though it has long been recognized that the competitive edge conferred by N(2)-fixation diminishes with increasing soil N availability, the conservation implications of increased N deposition on native N(2)-fixers have received less attention. We focus on Desmodium cuspidatum, which has experienced dramatic population losses in the last 30-40 years in the northeastern United States. We explore competition between this regionally threatened legume and a common non-N(2)-fixing neighbor, Solidago canadensis, across a gradient of N deposition. Our data show that increased N deposition may be detrimental to N(2)-fixers such as D. cuspidatum in two ways: (1) biomass accumulation in the non-N(2)-fixer, S. canadensis, responds more strongly to increasing N deposition, and (2) S. canadensis competes strongly for available mineral nitrogen and can assimilate N previously fixed by D. cuspidatum, resulting in D. cuspidatum relying more heavily on energetically expensive N(2)-fixation when grown with S. canadensis. N deposition may thus reduce or eliminate the competitive advantage of N(2)-fixing species growing in N-limited ecosystems.