Data 2

The application of sewage sludge as a soil conditioner may lead to the migration of heavy metals (HMs) and pollution via runoff, necessitating effective fixation strategies. Biochar can immobilize HMs and reduce erosion, but its efficacy depends on feedstock, and the mechanisms governing multi-media HM transport (surface runoff, interflow, sediment) remain unclear. Through field-simulated rainfall experiments, we compared sludge alone (SS) with sludge combined with 5% of sludge biochar (SSC), eucalyptus leaf biochar (SSE), or pine sawdust biochar (SSP). Results showed that biochar properties critically influenced the "soil structure-hydrology-speciation" nexus, dictating HM fate. SSC and SSP significantly reduced surface runoff (7.3-8.4%) and sediment erosion (12.4-18.3%), while SSE increased losses. SSC promoted aggregation (>1 mm fraction increased by 29.5%) and Cd precipitation (33.6% reduction). SSP relied on high specific surface area (461.6 m2·g-1) for adsorption. In contrast, SSE's hydrophobicity enhanced DOM-Cd complex leaching, identifying interflow as a critical and previously underestimated risk pathway (interflow Cd concentration was 2,258.2% higher than in surface runoff). Sediment was the main migration carrier (44.2-79.2% of total loss), driven by fine particulate enrichment (<0.054 mm). Ecological risk assessment showed a discrepancy between concentration-based (WQI) and flux-based (RI) indices: though interflow WQI > 300 (unfit for drinking), only SSE interflow posed moderate potential ecological risk (RI=152.4). Cd was the primary risk factor (Ei>80). This study underscores the importance of feedstock selection, recommending SSC for optimal risk mitigation, and advocates enhanced monitoring of interflow and bioavailable HMs to ensure sludge application safety.