Source trancing analysis of nitrogen in the upper reach of Xin’an River Basin based on SWAT model

Non-point source nitrogen (N) pollution has presented significant challenges to the management of water resources and threatened environmental sustainability. The Xin’an River Basin is a crucial strategic water source in the Yangtze River Delta, and total nitrogen (TN) is the main pollution factor threatening the water quality in this region. This paper simulated the migration and transformation process of non-point source N pollution in the upper reaches of Xin’an River Basin based on SWAT model, and identified the main sources of TN through scenario analysis. The results indicated that the SWAT model accurately captured the rainfall-runoff processes and TN transport and transformation dynamics in the study area, with Ens and R² values exceeding 0.7 during both the calibration and validation periods. The annual TN load from hillslope into the rivers ranged from 7427.68 t to 16854.39 t during 2003–2018. The temporal variation of TN load was consistent with precipitation, while the spatial distribution was mainly affected by land use. Scenario simulation results revealed that atmospheric N deposition, fertilizer N application, and livestock N input represented the primary N sources in the area. It is noteworthy that, although atmospheric N deposition contributed a smaller input quantity in the model, its conversion rate (10.57 %) surpassed that of N fertilizer application (4.11 %) and livestock N input (9.41 %). Meanwhile, the reduction of atmospheric N deposition has a more significant effect on TN load mitigation and TN concentration at the outlet section than the reduction of fertilizer N application, especially during normal water years. The analysis highlighted that the trend of TN concentration variation at the outlet under different scenarios and hydrological conditions further underscored the importance of optimizing N management strategies based on hydrological years. These findings offer critical insights and a robust scientific basis for the effective management of non-point source N pollution in the upper reaches of the Xin’an River.