Natural abundance of 15N in tropical plants with emphasis on tree legumes

Abstract

Natural abundance of 15N (δ 15N) of leaves harvested from tropical plants in Brazil and Thailand was analyzed. The δ 15N values of non-N2-fixing trees in Brazil were +4.5±1.9‰, which is lower than those of soil nitrogen (+8.0±2.2‰). In contrast, mimosa and kudzu had very low δ 15N values (−1.4≈+0.5‰). The δ 15N values of Panicum maximum and leguminous trees, except Leucaena leucocephala, were similar to those of non-N2-fixing trees, suggesting that the contribution of fixed N in these plants is negligible. The δ 15N values of non-N2-fixing trees in Thailand were +4.9±2.0‰. Leucaena leucocephala, Sesbania grandiflora, Casuarina spp. and Cycas spp. had low δ 15N values, close to the value of atmospheric N2 (0‰), pointing to a major contribution of N2 fixation in these plants. Cassia spp. and Tamarindus indica had high δ 15N values, which confirms that these species are non-nodulating legumes. The δ 15N values of Acacia spp. and Gliricidia sepium and other potentially nodulating tree legumes were, on average, slightly lower than those of non-N2-fixing trees, indicating a small contribution of N2 fixation in these legumes.