Scale-dependent dynamics of nutrient export under rainfall–runoff events in the central Three Gorges Reservoir Area, China

Understanding cross-scale nutrient transport dynamics is critical for managing non-point source pollution in large, heterogeneous watersheds. This study integrated multi-scale, event-based rainfall monitoring across five representative sub-watersheds and the outlet of a typical agricultural watershed in the central Three Gorges Reservoir Area (TGRA). A linear mixed-effects modeling framework was used to quantify the relative contributions of tributary gullies to nutrient export and to evaluate the effects of rainfall and sediment-related drivers. Results revealed pronounced spatial heterogeneity and scale-dependent patterns. Total nitrogen export was primarily controlled by short-duration, high-intensity rainfall at the sub-watershed scale, whereas total phosphorus export was strongly associated with sediment mobilization and transport. Sub-watersheds dominated by intensive agriculture exhibited the highest nutrient loads (total nitrogen: 5.63 kg/km² total phosphorus: 0.43 kg/km²), indicating high nutrient export. In contrast, forested and grassland sub-watersheds showed minimal nutrient exports, functioning as nutrient sinks. Importantly, nutrient concentrations at the watershed outlet did not linearly reflect upstream inputs, suggesting substantial attenuation and transformation along flow pathways. Agricultural areas showed a significantly positive correlation with nutrient concentrations (p < 0.001), while grassland and forest-dominated regions displayed weak correlations. These findings underscore the need for sub-watershed-scale, site-specific nutrient management strategies to effectively control diffuse pollution in the TGRA.