Impacts of Climate–Land Use Interactions and Topographic Thresholds on Nitrogen and Phosphorus Export Risks in Mountainous Watersheds: A Case Study in Yunnan, China.

Elucidating the drivers and risk evolution of nitrogen (N) and phosphorus (P) export is fundamental to non-point source (NPS) pollution management. However, existing research focuses primarily on factor contributions or statistical thresholds, often failing to translate attribution results and thresholds into spatial risk diagnostics. This disconnect between mechanistic understanding and spatially targeted management constitutes a critical knowledge gap. This study addresses this knowledge gap by integrating InVEST-NDR modeling, counterfactual factor decomposition, XGBoost-SHAP interpretation, and segmented regression to quantify the relative contributions, nonlinear responses, and threshold relationships of N and P export drivers in Yunnan Province. An ESI-based spatial risk classification framework—integrating environmental sensitivity, source pressure, and climate–land use interaction increments—was developed to delineate priority management zones. The results indicate that: (1) Land use and land cover (LULC) change contributed most (57.2%) with strong temporal persistence; climate effect (33.5%) showed pronounced interannual variability. (2) Cropland area and precipitation exhibited continuously enhancing responses; distance from rivers (DFR) and elevation (DEM) displayed L-shaped threshold responses, with export sensitivity significantly increasing when DFR < 2.3 km and DEM < 1.9 km. (3) Significant synergistic associations exist between cropland change and environmental conditions; agricultural source pressure overlapping with near-river, low-elevation, and high-precipitation zones amplified N and P export risk. (4) Compound high-risk zones accounted for 6.4% in 2020, concentrated in river corridors and agricultural basins, representing priority areas for NPS control. Projected risk patterns diverge substantially across Shared Socioeconomic Pathways (SSPs), with high-emission scenarios posing considerably greater adaptive management challenges. This study quantifies the relative contributions of climate and LULC change to N and P export dynamics and identifies spatial risk patterns, providing evidence-based guidance for targeted NPS pollution management in mountainous watersheds under climate change.