Native legumes are >10% of the plant species in fire maintained longleaf pine- (Pinus palustris) wiregrass (Aristida stricta)ecosystems and are important in the nitrogen cycle. The environmental controls for productivity and N₂-fixation of legumes on a particular site-type are unknown. The objectives of this study were to observe the differences in biomass accumulation, density, and N₂-fixation (using δ¹⁵N) of legume species located on xeric and mesic sites where irrigation has been supplied for three years versus a reference. The experiment was conducted using a nested split-plot experimental design, using four, 50m irrigated plots in each of two site-types, xeric and wet-mesic. All legumes with any part that touched or crossed each transect were clipped at the soil surface. All legumes and a non-leguminous reference species were collected and analyzed for biomass, δ¹⁵N natural abundance and percent N. Legumes within belt transects were counted to give density and total biomass in both irrigated and reference areas. Although Rubus cuneifolius, the non N₂-fixing reference species, was present on all sites and treatments, only three legume species were present on both xeric and mesic sites; five legume species were found on the xeric and eight on the mesic site. Irrigation effects were significant overall on the xeric sites, but not on mesic sites, and in all cases were species dependent. Against comparisons with reference δ¹⁵N values of -2.8417 for R. cuneifolius, a ubiquitous non N fixing species, the species of legumes had a range of -0.4207 to -1.3008 on xeric sites versus -0.0719 to -1.3213 on mesic sites. Although comparisons of legumes with reference species indicate high levels of N₂-fixation in several species, several species also exhibit lower levels of N₂-fixation in these field conditions. Two legume species and the reference species responded to irrigation on the xeric site.