Climatic control effect on the soil nitrogen isotopic composition in Alisols across the physiographic regions of Pernambuco State, Northeast Brazil

Soil nitrogen dynamics in terrestrial ecosystems can be evaluated by analyzing the natural abundances of N isotope ratio mass spectrometry (δ¹⁵N) because δ¹⁵N integrates decades of organic matter transformations in the soil. On a large scale, the δ¹⁵N of soil organic matter across climate gradients may provide critical insights into understanding regional patterns of N cycling. However, the mechanisms that control N cycling in some tropical regions still present critical gaps, particularly in dry ecosystems. We evaluated the variation of soil δ¹⁵N in Alisols along the climatic gradient of three physiographic regions in Northeast Brazil: Zona da Mata (humid, rainfall well distributed throughout the year), Agreste (subhumid, rainfall concentrated in 5–7 months), and Sertão (semiarid, rainfall in highly intense events, concentrated over 3–5 months). Soil samples were collected at the 0–20 cm depth layer and analyzed for physical and chemical characteristics, including soil δ¹⁵N, by mass spectrometry. A linear model with physical biological, chemical, and climatic variables was generated to explain the spatial distribution of the soil δ¹⁵N. The map modeled values δ¹⁵N for Pernambuco State was produced using all observed values and the best-performing explicit spatial predictive variables in the model selection – pH, clay content, mean temperature, mean precipitation, and potential of evapotranspiration. Soil δ¹⁵N was positively related to clay content and negatively associated with the pH, temperature, rain, and evapotranspiration, expressing differences in nitrogen dynamics along the climatic gradient. In the Sertão, the low and erratic rainfall combined with a neutral pH and high temperature provide adequate conditions for nitrogen transformations and cause high soil δ¹⁵N.