Identifying nitrate sources and nitrogen transformation in a karst river–groundwater system using dual nitrate isotopes and a Bayesian (SIAR) mixing model

Karst basins have permeable surface-subsurface connections that can accelerate nitrogen transport. The Lipu River Basin in southern Guilin is affected by intensive agriculture and industrial activity, but its nitrate sources and transformations remain poorly resolved. We integrated dual nitrate isotopes (δ 15 N-NO 3 ⁻ and δ 18 O-NO 3 ⁻), hydrochemical data, and a Bayesian SIAR mixing model. River-water TN was significantly higher in winter (7.09 ± 6.59 mg/L) than in summer (2.95 ± 1.48 mg/L). River-water NO 3 ⁻-N was lower in summer than in winter and lower than groundwater only during summer; the winter river-groundwater difference was not significant. The highest individual groundwater NO 3 ⁻-N values occurred near the industrial park, but the land-use group difference was not significant (Kruskal-Wallis, p = 0.48). The SIAR model identified manure and sewage as the largest modeled summer nitrate source, contributing 49% at the basin scale. These results identify elevated winter river loads and groundwater nitrate hotspots near tributaries and industrial zones as management priorities.