NATURAL-ABUNDANCE OF N-15 IN TROPICAL PLANTS WITH EMPHASIS ON TREE LEGUMES

Natural abundance of N-15 (delta N-15) of leaves harvested from tropical plants in Brazil and Thailand was analyzed. The delta N-15 values of non-N 2-fixing trees in Brazil were +4.5 +/- 1.9 parts per thousand, which is lower than those of soil nitrogen (+8.0 +/- 2.2 parts per thousand). In contrast, mimosa and kudzu had very low delta N-15 values (-1.4 approximately +0.5 parts per thousand). The delta N-15 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 delta N-15 values of non-N2-fixing trees in Thailand were +4.9 +/- 2.0 parts per thousand. Leucaena leucocephala, Sesbania grandiflora, Casuarina spp. and Cycas spp. had low delta N-15 values, close to the value of atmospheric N2 (0 parts per thousand), pointing to a major contribution of N2 fixation in these plants. Cassia spp. and Tamarindus indica had high delta N-15 values, which confirms that these species are non-nodulating legumes. The delta N-15 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.