Legacy nitrogen impeding the achievement of nitrogen management targets: Evidence from China's first cross-provincial ecological compensation watershed

The persistent impact of legacy nitrogen (N) remains a critical obstacle to improving water quality, even as anthropogenic N inputs have declined in many regions. This study quantified long-term (1988–2020) N inputs and riverine outputs (2000–2020) and developed a N dynamic fate model to analyze N variations in China’s first cross-provincial ecological compensation (EC) watershed. The model explained 99 % of the variability in riverine N flux between 2000 and 2020, identifying legacy N as the dominant contributor (53.8 %) and a major impediment to water quality improvement. Analysis of anthropogenic N fate revealed that 5.2 % and 70.7 % of non-point source inputs were lost via riverine export, and denitrification, biomass uptake, and wood removal, while 24.1 % accumulated in soil, groundwater, and sediment. In contrast, 70 % of point-source N was removed through in-stream denitrification. Legacy N storage increased steadily from 2000 to 2020 (from 81.3 to 187.6 kg N ha⁻¹ yr⁻¹), predominantly originating from historical sources (86.4 %). Notably, 90 % of initial legacy N was depleted within 7 years, suggesting a potential 8-year time lag in the watershed. Projections for 2021–2050 under four scenarios highlight persistent challenges in reducing N fluxes due to the combined effects of rainfall and legacy N, emphasizing the need for long-term management strategies. Overall, this study provides critical insights into N dynamics and underscores the importance of addressing legacy nutrient sources for effective watershed management and policymaking. • Quantified long-term N inputs and riverine outputs in China’s first cross-provincial ecological compensation watershed. • Developed a dynamic N fate model to quantify legacy N storage and trace anthropogenic N fate at the watershed scale. • Investigated the sources of long-term riverine N fluxesand identified key factors hindering water quality improvement. • Predicted riverine N fluxes over the next 30 years under various scenarios.