Coupling Hydrological Connectivity with Environmental Thresholds for Targeted Legacy Phosphorus Management in Subtropical Watersheds.

Legacy phosphorus (P) stored in soils from historical human activities can be remobilized under changing hydrological conditions, undermining water quality management and causing delayed pollution impacts. However, current management strategies lack the evaluation of legacy dynamics and the consideration of spatiotemporal lags, which limits the effectiveness of pollution control. In this study, we developed a novel management framework by integrating hydrological connectivity with interpretable machine learning. Using the aggregated index of connectivity, we elucidate how hydrological connectivity governs legacy P transport and identify environmental thresholds that modulate this relationship. Results revealed a phosphorus transport lag of more than 10 years in the Xiangjiang River Basin, with legacy P contributing more than 80% of the total phosphorus flux. Agricultural areas with high connectivity values were identified as primary sources of legacy P, whereas forested areas with low connectivity values temporarily retained phosphorus but posed sudden-release risks. We further found that precipitation and ground surface temperature were key environmental factors with critical thresholds influencing the relative importance of hydrological connectivity in driving legacy P transport. Accordingly, we propose a dynamic, threshold-based management framework incorporating hydrological connectivity and environmental drivers to guide the targeted mitigation of long-term legacy P impacts across subtropical watersheds.