Heavy rainfall shunts heavy metal transport from runoff to dominant sediment pathways in sludge-amended forest soils

The application of sewage sludge (SS) to forest soils offers a valuable waste management solution, but heavy rainfall can significantly alter heavy metal (HMs) transport pathways, destabilize soil aggregates, and accelerate erosion, thereby enhancing pollution risks to water bodies. This study examines how different rainfall intensities (60, 90, 120, and 150 mm·h⁻¹) influence the migration of six HMs (Ni, Cd, Pb, Cr, Cu, Zn) in SS-amended subtropical forest soils, focusing on their distribution in surface runoff, interflow, and sediment. Through nine simulated rainfall experiments, we found that SS amendment elevated HMs concentrations across three migration pathways, with sediment accounting for over 91.0 % of cumulative HMs transport. As rainfall intensity rose from 60 to 150 mm·h⁻¹ , the mass proportion of < 0.05 mm aggregates decreased by 48.1-58.6 %, while that of ≥ 0.25 mm aggregates increased by 12.1-23.7 %. Under extreme rainfall (≥120 mm·h⁻¹), the comprehensive ecological risk from the three HMs transport pathways was "moderate", primarily attributable to sediment and Cd. However, when rainfall intensity was below 150 mm·h⁻¹ , the risks associated with runoff and interflow were negligible. Although SS application reduced interflow and sediment loss, it heightened HMs pollution, driven largely by Cd and fine aggregates. These findings underscore the importance of sediment-bound HMs dynamics, particularly Cd, in guiding the safe use of SS in subtropical forests under increasing rainfall intensity.