Divergent controls on nitrogen and phosphorus export regimes across Chinese catchments

Effective mitigation of watershed non-point source pollution is hindered by the spatially divergent export of nitrogen (N) and phosphorus (P). While concentration-discharge (C-Q) relationships can mathematically separate these complex export regimes into baseline accumulation and hydrological mobilization, it remains unclear to what extent the controls on N and P diverge. We evaluated C-Q dynamics across 136 catchments in four major Chinese basins (2021-2024) using Random Forest models and SHAP analysis to identify the environmental factors associated with these export regimes. TN and TP were associated with distinct controls. TN mobilization behaved as a source-controlled process associated with soil organic carbon (SOC, mean |SHAP| = 0.049), consistent with biogeochemical processing in the shallow subsurface rather than fixed vertical nutrient contrasts. In contrast, TP mobilization was associated with hydroclimatic conditions (e.g., evapotranspiration) and anthropogenic alteration of hydrological pathways, consistent with transport-limited behavior. Baseline accumulations showed the same divergence. The TN baseline was strongly associated with the spatial fragmentation of historical legacy sources (patch density, mean |SHAP| = 0.181), whereas the TP baseline was most associated with population pressure, though only weakly (mean |SHAP| = 0.009). By emphasizing dynamic biogeochemical processes, these findings refine the static shallow-deep hypothesis and point to nutrient-specific mitigation, targeting biogeochemical source factors for N and physical transport pathways for P.