Natural 15N abundance in shrub and tree legumes,Casuarina, and non N2 fixing plants in Thailand

The leaves and nodules from the shrub and tree legumes, particularly, Aeschynomene spp., Sesbania spp., Mimosa spp. and Leucaena spp., and Casuarina spp. and the leaves from neighbouring non-fixing plants were analyzed for their natural abundances of ¹⁵N ( ¹⁵N).The ¹⁵N in the leaves of non-fixing plants was +5.9% on average, whereas those from shrub legumes and Casuarina spp. were lower and close to the values of atmospheric N₂, suggesting the large contribution of N₂ fixation as the N source in these plants. The ¹⁵N values of the leaves from tree legumes except for Leucaena spp. were between the shrub legumes and non-fixing plants, which suggests that the fractional contribution of fixed N₂ in tree legumes may be smaller than that in the shrub legumes. Casuarina spp. was highly dependent on N₂ fixation. The ¹⁵N values of the nodules from most of the shrub legumes investigated were higher than those of the leaves.