Soil δ¹⁵ N spatial distribution is primarily shaped by climatic patterns in the semiarid Caatinga, Northeast Brazil

Soil nitrogen isotopic composition (delta N-15(soil)) is an invaluable tool as it integrates nitrogen (N) transformations in soils. In addition to serving as a baseline to understand the N cycle, spatial representations of delta N-15(soil) across landscapes (or isoscapes) is a multi-purpose tool useful to investigate, for example, plant-microbe interactions, animal migration and forensics. We investigate the climatic and edaphic controls of delta N-15(soil) utilising data from 29 geographical locations sampled across the semiarid Brazilian Caatinga biome. The sampling covered a mean annual precipitation (P-A) gradient ranging from 0.51 to 1.36 m a(-1) and eight soil types originating from three different geological origins. Our data show that the combination of higher aridity and lower seasonality (psi) leads to higher values of delta N-15(soil). Moreover, soil total carbon had a positive relationship with delta N-15(soil), appearing within the best-supported models according to the information-theoretic approach undertaken here. The contribution to the plant communities by the Fabaceae trees expressed as their basal area was not related to delta N-15(soil) values, suggesting that the magnitude of biological N fixation in the Caatinga is not large enough to be reflected in the soil. In addition, considering P-A in a categorical fashion, i.e., 'high' (> 0.8 m a(-1)) and 'low' P-A (< 0.8 m a(-1)), we found that, within the wetter category, delta N-15(soil) was positively related to several soil properties (i.e., clay content, effective cation exchange capacity, exchangeable calcium, silt content, pH(H2O), total phosphorus and sum of bases) and negatively related to sand content. Our study provides new insights into the functioning of semiarid ecosystems from a pedo-isotopic perspective and contributes to the overall understanding of the N cycle in the Caatinga region, with the potential to support the development of new conceptualisation of biogeochemical process and testing of global models that simulate N and C cycles.