Assessment of N2 fixation capability of native legumes from the longleaf pine–wiregrass ecosystem

Abstract Longleaf pine ( Pinus palustris ) is a major ecosystem in the southeastern United States. Native legumes species are viewed as an important component of successful restoration of the longleaf pine–wiregrass ( Aristida stricta ) ecosystem as a result of their ability to fix atmospheric nitrogen (N 2 ). However, virtually no measurements exist on the N 2 fixation potential of the various native legume species in this ecosystem. The objective of this study was to compare the N 2 fixation capability under ambient and half ambient light conditions of nine native legume species: Centrosema virginianum , Clitoria mariana , Crotalaria rotundifolia , Lespedeza hirta , Mimosa quadrivalvis , Orbexillum lupinellus , Rhyncosia reniformis , Tephrosia virginiana , and Chamaecrista nictitans . Their N 2 fixation activity was indexed using five approaches (nodule biomass, total plant N content, acetylene reduction assay, N transport and storage product analysis, δ 15 N natural abundance) since no single measure by itself has proven to be a completely reliable result. Based on the results of these five approaches, a decrease in light levels by half had no impact on the N 2 fixation potential of any of the species. At least under a relatively open longleaf pine canopy, native legumes could be important contributors to the N balance. Considering all measures, two legumes proved consistently to be superior in N 2 fixation capability: Mimosa quadrivalvis and Centrosema virginianum .