Biochar-mediated divergence in arsenic and lead fate within a soil-plant-aphid continuum: The immobilization-mobilization trade-off and disrupted trophic transfer

Soil metal(loid) contamination threatens ecosystem health through arthropod-mediated trophic transfer, yet remediation strategies rarely consider these ecotoxicological cascades. This study examines the mechanistic pathways by which pure and co-pyrolyzed biochars-Japanese knotweed (JK), pig manure (PM), and pig manure-Japanese knotweed hybrid (P1J1)-alter As and Pb partitioning across the soil-faba bean-aphid continuum. Results revealed that JK biochar significantly (p < 0.05) enhanced faba bean growth (biomass and height). In contrast, PM and P1J1 suppressed plant growth due to elevated electrical conductivity (6-7 times higher than the control) with increased salinity stress, despite drastically reducing soil soluble Pb by 93-94 %. However, all biochar treatments increased As solubility (by 1.6- to 13.8-fold), attributed to phosphate competition and pH-driven desorption from soil oxides. Pb was predominantly retained in roots (especially with PM and P1J1), minimizing transfer to aphids, while As remained root-localized without aerial translocation. Aphid biomass and fecundity were highest in unamended soil, which also showed the greatest Pb biomagnification (transfer coefficient: 2.39), posing risks to higher trophic levels. Biochar amendments reduced aphid Pb uptake but did not influence the declining efficacy of plant-based repellents (mugwort/knotweed extracts). These repellents demonstrated an initial repellency of > 80 % that decreased to approximately 55 % over time, indicating limited efficacy as pest management tools. The hybrid P1J1 exhibited PM-like effects (salinity increase, As mobilization) rather than synergistic benefits. These results quantify trade-offs: biochars disrupt Pb trophic transfer but may enhance As mobility and salinity stress, requiring careful biochar selection for contaminated agroecosystems.