Riverine nutrient non-stationarity in China: Dissolved oxygen links external loading to internal nitrogen and phosphorus feedbacks.

Riverine nutrient pollution is intensifying under climate warming and anthropogenic loading, yet the mechanisms that generate spatial decoupling between nitrogen (N) and phosphorus (P) across large river networks remain insufficiently resolved. Here, we analyzed 101,576 monthly hydrochemical observations from 3149 monitoring sites across 319 Chinese cities to evaluate whether dissolved oxygen (DO) is associated with spatially divergent responses of riverine N and P to climatic and anthropogenic forcing. N and P showed pronounced spatial decoupling: N accumulated mainly in northern and central water-limited rivers, whereas P hotspots clustered in eastern coastal urban agglomerations. Geographically weighted regression explained 61% and 65% of the spatial variability in N and P, respectively, indicating spatially non-stationary relationships between nutrient concentrations and their climatic, anthropogenic, and oxygen-related predictors. Warming-associated deoxygenation was linked to constrained aerobic N transformation and removal, whereas hypoxia was associated with enhanced redox-sensitive release of legacy P from sediments. These oxygen-associated patterns were spatially concentrated in northern high-N systems and eastern high-P clusters, but were less pronounced in southern high-flow rivers with greater metabolic buffering capacity. Our findings suggest that DO is closely associated with spatially non-stationary nutrient-driver relationships, and that climate-resilient river management should prioritize nutrient-load reduction while using oxygen-state management as a targeted, system-specific complement where technically and ecologically feasible.