Evolution Characteristics and Driving Factors of Net Anthropogenic Nitrogen and Phosphorus Inputs in the Typical Plateau Basins in the Upper Yangtze River Basin

Most existing studies on anthropogenic N and P inputs in the Yangtze River Basin focus on the middle-lower main stem and large lake regions, while long-term fine-scale assessments of small and medium plateau tributaries in the upper reaches remain limited. This study was based on the Net Anthropogenic Nitrogen and Phosphorus Input (NANI) & (NAPI) models to systematically quantify total anthropogenic N/P inputs and their sources, and to characterize spatiotemporal dynamics. The random forest model was employed to disentangle and quantify key driving factors; additionally, the multiple linear regression (MLR) model was employed to construct prediction equations, and key-factor scenarios were set to predict N and P input evolution trends under reduced fertilizer application and population change. Results indicate that: (1) from 2009 to 2023, the mean annual NANI and NAPI in the study area were (8741.97 ± 1715.37) kg·km−2·yr−1 and (2016.01 ± 610.68) kg·km−2·yr−1, respectively, exhibiting an overall “first increase, then decrease” trend. Their spatial distribution patterns were highly coupled. The spatial distribution was highly heterogeneous, with high values concentrated in the central dam regions and low values in mountainous areas. (2) Across all sub-basins, food and feed N input and fertilizer application were the primary sources of NANI, accounting for 50.87% and 43.94% of the total on average, respectively, while food and feed P input and fertilizer application dominated NAPI with average shares of 31.31% and 68.69%. (3) Population size, fertilizer application intensity, and livestock/poultry breeding volume were the primary drivers of regional NANI and NAPI. (4) Scenario prediction results show that fertilizer reduction significantly reduces N and P inputs, whereas population growth does not directly elevate N and P loads. Overall, N and P inputs in the Longchuan River Basin are dominated by human activities, with spatiotemporal variations closely linked to agricultural activity intensity and population distribution. These findings provide scientific support for targeted precise zonal N/P control in the Longchuan River Basin, and also offer a reference for non-point source pollution control research and management policy-making in similar plateau basins of the upper Yangtze River.