In the fire-maintained longleaf pine (Pinus palustris) -wiregrass (Aristida stricta) ecosystem, native legumes comprise greater than 10 percent of the understory flora.  These legumes may contribute substantial amounts of fixed atmospheric N² to the nutrient cycling of this ecosystem.  The objectives of this study were to  compare the species-specific and legume population responses to irrigation by biomass and N²-fixed in paired irrigated and adjacent control woodland areas in wet-mesic and xeric site types.  N²-fixation was measured using the δ¹⁵N natural abundance method. Irrigation did not significantly affect number of legumes per hectare or mean biomass per species, nor did irrigation significantly impact legume N²-fixation (δ¹⁵N) for either of the site-types.  However, the non-leguminous (reference) plants did show a significant shift in δ¹⁵N at wet-mesic and xeric sites, which may reflect a change in soil N-mineralization rate.  For both site types and in both treatments, δ¹⁵N was significantly different between legumes and reference species, which indicates that all of the legumes are fixing-N².  Aboveground tissue N-content ranged from 1.9-2.9 percent N for legumes and 1.2-1.4 for the non-legumes at the xeric site, and at the wet-mesic sites, percent N for legumes ranged from 1.9-3.0, and non-legumes averaged 1.2 percent N. Growth and N²-fixation were not limited by soil moisture during the year that this study was conducted.  Evidence from this study indicates that fixed N² from native legumes are in important source of N for this longleaf pine-wiregrass ecosystem.