From source emission quantification to riverine load attribution: Systematic tracing of nitrogen flow in the complex river catchment to inform management strategy

• Develop a load attribution framework integrating emission inventory, process model, and scenario analysis. • Construct a high-resolution N emission inventory for point and non-point sources in WRB (2015–2020). • Emission inventory and riverine loads yield different N management recommendations. • Provide targeted, time- and space-specific control measures based on TN load attribution. Understanding the complete nitrogen (N) transport processes, from land-based emissions to riverine loads at specific site, is essential for effective watershed N management. However, attributing total nitrogen (TN) loads remains a challenge due to the interplay of multiple pollution sources and complex hydrological transport mechanisms. This study presents an integrated framework that combines an emission inventory, a watershed process model, and scenario analysis to trace N fluxes and quantify TN load contributions in the Wei River Basin (WRB), China. A high-resolution emission inventory was constructed at county, grid, and sub-basin levels, covering seven key pollution sources from 2015 to 2020. A watershed N fate model was developed and validated using multiple datasets to establish the relationships between emissions from both point and non-point sources and the resultant N loads. Scenario analysis was then employed to apportion TN loads to their respective sources. Comparisons between two source attribution approaches—one relying solely on inventory-based emissions and the other incorporating source-load response dynamics—revealed distinct implications for watershed management. The emission inventory identified livestock and poultry breeding (LPB) and chemical fertilizer (FER) as the dominant N sources, concentrated primarily in the upper and middle reaches of the WRB. However, source-load response analysis indicated that point-source discharges from LPB, along with urban and rural domestic wastewater, were the primary contributors to riverine TN loads. The overall TN contribution hierarchy was determined as follows: rural domestic sewage (29.4 %) > urban domestic sewage (26.4 %) ＞ soil N (18.2 %) > LPB point-source discharge (17.6 %) ＞ LPB manure fertilizer (6.04 %) > nitrogen fertilizer (1.61 %) ＞ atmospheric deposition (0.691 %) > industrial wastewater treatment (0.006 %). Seasonal variations in TN loads were significant, with a sharp increase during the summer monsoon due to the enhanced non-point source runoff and the mobilization of soil N pools. Conversely, during winter and spring, steady point-source discharges from domestic wastewater dominated due to intermittent tributary flow. Furthermore, our findings suggest that reducing soil N inputs may have limited immediate impact on mitigating riverine TN pollution, as large soil N reservoirs and delayed non-point source responses hinder short-term effectiveness. These insights underscore the need for long-term, integrated watershed management strategies to effectively control TN pollution and improve water quality in the WRB and comparable river systems.