Soil δ15N spatial distribution is primarily shaped by climatic patterns in the semiarid Caatinga, Northeast Brazil

Abstract

Soil nitrogen isotopic composition (δ¹⁵N_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 δ¹⁵N_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 δ¹⁵N_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 (ψ) leads to higher values of δ¹⁵N_soil. Moreover, soil total carbon had a positive relationship with δ¹⁵N_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 δ¹⁵N_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, δ¹⁵N_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.

Keywords: Dry ecosystems; Nitrogen cycle; Soil nitrogen; Soil nitrogen modelling; Stable nitrogen isotopes.