<p>Nitrate pollution in the Ling River Basin has emerged as a pressing environmental issue requiring immediate attention. To effectively prevent and control nitrogen pollution in the watershed, this study systematically investigated the sources, migration and transformation mechanisms of nitrate in surface water through an integrated approach combining hydrochemical analysis, nitrogen and oxygen isotope techniques, and a Bayesian mixing model (SIAR). The results indicate that the dominant anion in the water is HCO₂⁻, while Ca²⁺ is the predominant cation, classifying the hydrochemical type as HCO₃–Ca.Mg, with the water exhibiting a slightly alkaline nature. The distribution patterns of hydrogen and oxygen isotopes (δD-H₂O and δ¹⁸O–H₂O) suggest that atmospheric precipitation is the primary source of surface water recharge. Nitrate concentrations remained relatively stable across the wet, normal, and dry seasons. Isotopic analysis of nitrogen and oxygen further revealed mixed sources of nitrate, including soil nitrogen, chemical fertilizers, domestic sewage, and atmospheric deposition. Among these, nitrification is the dominant process in the nitrogen cycle of the basin, whereas denitrification plays a minor role. Quantitative analysis based on the SIAR model showed slight seasonal variations in the contribution rates of nitrate sources: in the wet season, soil nitrogen, chemical fertilizers, domestic sewage, and atmospheric deposition accounted for 50.0%, 23.0%, 17.3%, and 9.7%, respectively; in the normal season, the contributions were 50.0%, 25.9%, 15.0%, and 9.1%, respectively; and in the dry season, 54.6%, 18.8%, 17.8%, and 8.8%, respectively. Therefore, soil nitrogen and chemical fertilizers are identified as the primary sources of nitrate pollution in the Ling River Basin. This study provides critical theoretical support for the control of agricultural non-point source pollution and the protection of aquatic environment.</p>