Quantification of the contribution of biological nitrogen fixation to Cratylia mollis using the 15N natural abundance technique in the semi-arid Caatinga region of Brazil

This study was performed to investigate the capacity of the woody perennial Cratylia mollis, a legume endemic to the semi-arid region of the North–East of Brazil, to nodulate, and obtain N from BNF using the 15N natural abundance technique. To estimate the 15N abundance of the N-derived from soil, the leaves of several (4–6) non-legume and non-nodulating legume species growing in close proximity to the Cratylia were analysed for δ15N. The δ15N values of these reference plants were high (from +9 to +16‰) and showed relatively small differences between species at each site/sampling time. At the irrigated site at both samplings, and at the non-irrigated site sampled in the rainy season, the 15N abundance of the Cratylia mollis leaves was far lower (+1 to +5‰) strongly suggesting that the legume obtained large proportional contributions from BNF. As was to be expected, no nodules were found on the Cratylia plants at the non-irrigated site in the dry season, and the 15N abundance of the Cratylia mollis plants were very similar to that of the reference plants, consistent and nodulation and BNF being limited by water deficit at this time.