Multiple Isotopes Reveal the Nitrate Sources in Aojiang River Basin, Eastern China

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&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&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.