Unlocking the microplastics amplification effect: How biochar counters heavy metal risks by driving speciation shifts and reversing soil property degradation.

Microplastics and heavy metal co-contamination exacerbate risks to soil-plant ecosystem, yet remediation mechanisms remain unclear. This study demonstrated polystyrene microplastics reduce soil pH and soil organic (SOM), while increasing redox potential and zeta potential, suppressing urease, alkaline phosphatase, and sucrase activity, causing declines in soil total nitrogen, available phosphorus and potassium, inhibiting lettuce growth and increasing Pb and Cd significantly. Applying wood chip and straw biochar reversed soil pH to alkaline condition, enhanced reductivity, increased SOM by 47.68-50.46%, restored enzyme activity by 73.35-242.76%, improved nutrient availability, and significantly reduced Pb and Cd accumulation. Key mechanisms first revealed involve biochar synchronously correcting microplastic-induced soil acidification, organic matter loss, and dispersion through synergistic pH, SOM, and zeta potential adjustments. Biochar alleviated ecological hazards through dual pathways whereby increased SOM activated soil enzymes and phosphorus availability, promoted chlorophyll synthesis, and increased biomass by 23.46-29.23%; while it mitigated plant antioxidant enzyme increases, reduced malondialdehyde and free proline, alleviated membrane damage. Biochar facilitated Pb(II) conversion from exchangeable to organic-bound and residual fractions by enhancing two key factors: pH and SOM, achieving up to 89.04%, thereby reduced stem and leaf Pb accumulation by 25.38-29.45%, yet failed to fully offset Cd activation by microplastics. Wood chip biochar outperformed straw biochar in Pb immobilization and oxidative stress mitigation; both offered limited Cd remediation. This study elucidates the mechanisms by which organic matter hubs and pH govern the regulation of heavy metal speciation, providing a theoretical bases for the targeted remediation of co-contaminated soils.