Multiple isotopes reveal the nitrate sources in Aojiang River Basin, Eastern China

Excessive levels of nitrate (NO 3 − ) in water pose a significant environmental challenge on a global scale. Hence, it is of utmost importance to determine the sources of NO 3 − and evaluate their corresponding contributions as a means to improve aquatic quality. To address this issue, a recent research undertaking was conducted in the Aojiang River basin located in Eastern China. The study involved the collection of surface water samples throughout both the dry and wet seasons. Various hydrochemical indicators, along with dual NO 3 − isotopes (δ 15 N–NO 3 − and δ 18 O–NO 3 − ), were utilized to determine the sources of NO 3 − contamination and assess the relative contributions made by various sources. The findings revealed that nitrification, a process involving the conversion of nitrogen, was the primary nitrogen transformation mechanism within the study area throughout the sampling period. According to the isotopic analysis of NO 3 − , it was revealed that the primary contributors to the elevated NO 3 − were manure and sewage (M&S), soil nitrogen (SN), and nitrogen fertilizer (NF). Furthermore, the results obtained from the MixSIAR model, a Bayesian model that employs stable isotope analysis within the R software, demonstrated that the proportional contributions of atmospheric deposition (AD), NF, M&S, and SN to NO 3 − were 2.84%, 19.63%, 44.67%, and 34.87%, respectively, during the dry season. In the wet season, the corresponding contributions were 3.61%, 22.32%, 32.37%, and 41.70%, respectively. This study serves as valuable evidence for advancing our understanding of the sources, transport, and transformation of nitrogen in the Aojiang River basin, thereby facilitating improved management strategies for nitrogen contamination.