Temperate savannas contain herbaceous N2-fixing legumes presumed to replenish nitrogen (N) losses from fire. The ability of legumes to fill this role may depend on the factors that regulate symbiotic fixation, including soil nutrient constraints, fixation strategy and species identity. We tested whether these factors influence the fixation of seven legumes species native to longleaf pine savannas with two greenhouse experiments. In experiment 1, we applied phosphorus (P) and molybdenum (Mo) individually and in combination and found that P alone constrains legume growth and fixation in longleaf pine soils. In experiment 2, we added three levels of N fertilizer to plants and found that species differ in the fixation strategies (facultative vs obligate) they employ. Finally, we found that species vary in their capacity for fixation. Our results suggest that the ability of legume communities to replenish N losses depends on their species composition and the availability of soil P.