Alleviating heavy metal migration risks by adding biochar during sludge amelioration of forest soil: Evaluating...

The land application of municipal sludge risks heavy metals (HMs) migration via runoff, necessitating effective immobilization strategies. Although biochar can immobilize HMs and reduce erosion, its feedstock-dependent efficacy and role in multi-media transport (surface runoff, interflow, sediment) remain unclear. We evaluated sludge applied alone (SS) or combined with 5 % sludge biochar (SSC), eucalyptus leaf biochar (SSE), or pine sawdust biochar (SSP) in field-simulated rainfall experiments. Biochar properties critically influenced the "soil structure-hydrology-speciation" nexus, dictating HM fate. SSC and SSP reduced surface runoff (7.3-8.4 %) and sediment erosion (12.4-18.3 %), whereas SSE increased the losses. SSC and SSP significantly promoted particle aggregation, increased the > 1 mm particle size fraction by 29.5-50.4 % and reduced the loss of soluble Cd in both surface runoff and interflow by 20.5-44.7 %. In contrast, strongly hydrophobic SSE promoted the leaching of soluble Cd, where the Cd concentration in interflow was 22.58 times higher than that in surface runoff, identified as a critical hidden risk pathway. Sediments were the primary migration carriers, accounting for 44.2-79.2 % of the total loss, and enriched with fine particles (< 0.054 mm). Ecological risk assessment revealed a discrepancy between concentration-based (WQI) and flux-based (RI) indices: though interflow WQI values were all greater than 300 (Unfit for drinking), only the presence of SSE treatment posed a moderate potential ecological risk (RI = 152.4), with Cd being the primary risk factor (E_i > 80). This study underscores the importance of feedstock-specific biochar selection, recommends SSC for optimal risk mitigation, and advocates monitoring interflow and bioavailable HMs to ensure sludge utilization safety.