Multi-scale quantification of non-point source nitrogen in a coastal river using an integrated multi-method...

Excessive nitrogen inputs from human activities enter coastal waters primarily via riverine transport and have become a major stressor on coastal ecosystems; meanwhile, quantitatively distinguishing non-point source (NPS) contributions remains technically challenging. This study develops an integrated multi-method source-tracing framework that combines the pollutant discharge coefficient method (PDCM), isotopic analysis, and hydrological modeling to downscale nitrogen contribution to administrative management units in the Xiaoqing River (XQR) Watershed, China. The framework systematically analyzes NPS contributions and their spatiotemporal dynamics under varying hydrological conditions. Nitrogen loads and source contributions at the pollution control unit derived from the PDCM and the Soil and Water Assessment Tool (SWAT) exhibited moderate positive correlations (p < 0.01), improving pollution source targeting. Isotopic analysis and SWAT simulations jointly indicated a distinct seasonal shift in dominant nitrogen sources: urban domestic sewage dominated point sources (PSs) during the dry season (∼ 45 %), whereas agricultural NPS prevailed in the wet season (> 65 %). Additionally, the SWAT model quantified the spatial distribution of NPS across the district and county and revealed seasonal differences in nitrogen transport pathways to the river. Overall, the integrated framework substantially improves the accuracy and comprehensiveness of NPS source tracing, providing a robust, administrable scientific basis for refined nitrogen management in complex coastal watersheds.