Risk Assessment of Soil Nitrogen and Phosphorus Loss in Danjiangkou Reservoir Area Based on Geochemical Survey

This study aims to reveal the spatial distribution characteristics and loss patterns of soil nitrogen (N) and phosphorus (P) in the Danjiangkou Reservoir area, identify high-risk zones of N and P loss, and evaluate the mitigation effect of the Grain for Green Program on N and P loss.

Based on the 1:250000-scale soil geochemical survey comprising 5251 topsoil sampling points in the reservoir area, Kriging interpolation and ArcGIS spatial mapping were employed to characterize the spatial distribution patterns of soil N and P. A comprehensive risk assessment method for N and P loss was developed using the Soil and Water Assessment Tool (SWAT) model by integrating source intensity, transport risk, and receptor sensitivity, and the spatiotemporal characteristics of N and P loss loads were simulated. Three Grain for Green scenarios, with afforestation proportions of 10%, 30%, and 50%, were established to evaluate their mitigation effects on N and P loss.

The distribution of soil N and P showed a “high in the south and low in the north” pattern, with high-value areas located in the southern mountainous forest regions and low-value areas concentrated in the northern and central agricultural hilly and valley areas. High N/P loss-load zones were mainly located in the northern and central agricultural areas, whereas high-sensitivity zones showed the opposite spatial pattern. The loss loads exhibited significant seasonal variation, with losses during the flood season (May–October) accounting for 80.4% and 82.8% of the annual N and P losses, respectively. The Grain for Green Program significantly reduced N and P loss; when the afforestation proportion reached 50%, the loss loads of total nitrogen (TN) and total phosphorus (TP) decreased by 70.32% and 71.87%, respectively.

Rainfall is the primary driving force of soil N and P loss, whereas land use and agricultural fertilization are the key anthropogenic factors. The Grain for Green Program can reduce the risk of N and P loss by decreasing surface runoff and soil erosion, and these findings provide a scientific basis for nonpoint-source pollution control and ecological management in the reservoir area.