Spatiotemporal variations in groundwater nitrate and its sources regulated by external inputs and soil profile...

Nitrate (NO₃^-) in groundwater poses a risk to human health, and its dynamics are influenced by land use/land cover (LULC) and hydrological periods. Thus, understanding these variations is essential for designing effective management strategies. This study presents a new soil perspective on groundwater NO₃^- dynamics in agricultural basins by combining a Bayesian isotope mixing model (MixSIAR) with soil profile analysis to quantify NO₃^- sources and identify key soil drivers. Results showed that groundwater NO₃^- concentrations differed among LULC types, consistent with soil NO₃^--N accumulation in the 0-100 cm soil profile, mainly influenced by fertilizer and manure inputs. Soil organic nitrogen (SON) was the primary contributor during the dry period, likely due to the priming effect induced by fertilization, whereas manure/sewage (M&S) dominated during the normal and wet periods. NO₃^- leaching was mainly regulated by SON and silt content in the subsurface soil. During the wet period, intense rainfall accelerated NO₃^- transport, diminishing the retention capacity of fine-textured soils. These findings advanced the understanding of NO₃^- fate in agricultural basins and established a foundation for developing targeted strategies.