N2 fixation by Sesbania rostrata and Sesbania sesban estimated using 15N and total N difference methods

N2 fixation by the stem-nodulated legume Sesbania rostrata was compared in waterlogged and non-waterlogged conditions with that of a non-stem-nodulated Sesbania species, Sesbania sesban. To estimate N2 fixation, two methods were used, the 15N direct isotope dilution method and the difference method. Due to stem nodulation S. rostrata fixed much more N2 in 60 days (0.78-0.68 g N2 per plant in waterlogged soils or 0.62-0.59 g N2 in drained soils) than S. sesban (0.06-0.05 g N; per plant or0.13-0.12 g N2). There was a satisfactory agreement between the isotopic and diflerence method estimates. From these values it was calculated that about 83–109 kg N2 and 7–18 kg N2 ha−1 would be fixed in 60 days by S. rostrata and S. sesban respectively. The results of this experiment confirm the high N2-fixing potential of the stem-nodulated legume S. rostrata as compared to that of a non-stem-nodulated species, S. sesban.