Quantification of the contribution of biological nitrogen fixation to Cratylia mollis using the N 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.