The Influence of Burning and Light Availability on N2-fixation of Native Legumes in Longleaf Pine Woodlands1

Abstract

Three dominant legume species (Tephrosia virginiana (L.) Pers., Rhynchosia reniformis DC., and Centrosema virginianum (L.) Benth.) found in frequently burned longleaf pine woodlands were grown in a garden plot and treated with a factorial arrangement of shade treatments (±) and burning (±) treatments. Nitrogen fixation was measured using 15N Isotope dilution method with whole plant recovery (including root systems). Burning treatments decreased total biomass (P < 0.001) and increased root to shoot ratios (P < 0.001). N2-fixation rates were generally reduced by burning, but there was a complex interaction among species, shade, and burn treatments. For Tephrosia, contrasts of shade (F1,42 4.48, P < 0.05) and burn (F1,42 4.42, P < 0.05) indicate that both main treatment effects significantly reduced N2-fixation rates. Relative to all other treatment combinations, Rhynchosia had elevated levels of N2-fixation under the no shade by burn treatment. Centrosema showed no effect of shade on N2-fixation (F1,42 1.65, P > 0.05), but burns significantly lowered N2-fixation rates for both shaded and unshaded plants (F1,42 19.31, P < 0.001). Despite the diverse response of species N2-fixation activity to treatments, a regression of all individuals regardless of species shows a curvilinear relationship of % N derived from the atmosphere to total plant biomass. The diverse response of these herbaceous legumes to light and fire challenges the conventional wisdom that legumes maintain functional redundancy as a guild across environmental gradients within longleaf pine woodlands.