Impact of intensive human activities on nitrate pollution in river basins: A spatiotemporal isotopic analysis

Human activities are key drivers of nitrogen pollution in river basins. However, a comprehensive understanding of the dynamic spatiotemporal variations in nitrogen pollution sources and their proportions at a watershed scale under intensive human activities is still limited. This study provides new insights by investigating nitrogen pollution in the Yiluo River Basin, a major agricultural tributary of the Yellow River. We used dual stable isotopes of nitrate (δ¹⁵N-NO₃⁻ and δ¹⁸O-NO₃⁻) and a MixSIAR model to analyze water samples collected across three periods from November 2022 to August 2023. The results revealed contrasting temporal trends: nitrate concentrations in surface water generally decreased over the study period, whereas groundwater nitrate levels peaked in May 2023, coinciding with the peak agricultural season. Spatially, nitrogen levels were consistently highest in the Yi River, lower in the Luo River, and showed accumulation in the downstream Yiluo River. The MixSIAR model identified soil nitrogen (SN), manure and sewage (MS), and chemical fertilizers (CF) as the primary nitrogen pollution sources, with atmospheric deposition (AP) playing a minor role. Critically, the proportional contributions of these sources varied significantly in both space and time. Upstream regions were predominantly influenced by soil nitrogen (up to 43.2 %), the midstream by chemical fertilizers (up to 30.4 %) especially during the growing season, and the downstream by manure and sewage (up to 54.2 %), reflecting increased urbanization. These findings underscore the dynamic nature of nitrogen pollution and demonstrate the necessity of developing targeted, region-specific strategies to achieve effective nitrogen management in intensively managed river basins.