<p>Excessive nitrate (NO₃⁻) in water poses a global environmental challenge. Identifying NO₃⁻ sources and their contributions is crucial for improving water quality. We collected surface water samples in the Aojiang River basin, Eastern China, during dry and wet seasons. Hydrochemical indicators, δ¹⁵N–NO₃⁻ and δ¹⁸O–NO₃⁻ were used to identify NO₃⁻ sources and assess their contributions. The findings revealed nitrification as the primary nitrogen transformation process. Isotopic analysis identified manure and sewage (M&amp;S), soil nitrogen (SN), and nitrogen fertilizer (NF) as major NO₃⁻ contributors. The MixSIAR model analysis showed proportional contributions of atmospheric deposition (AD), NF, M&amp;S, and SN to NO₃⁻ during the dry season as 2.84%, 19.63%, 44.67%, and 34.87%, respectively. In the wet season, the contributions were 3.61%, 22.32%, 32.37%, and 41.70%, respectively. This study enhances understanding of nitrogen sources and transformations in the Aojiang River basin, aiding better nitrogen contamination management.</p>