Abstract Heavy metal contamination in global agricultural soils has posed severe ecological and health risks. However, little is known about the long-term effects of soil management on the bioavailable concentration and the speciation of heavy metals, especially via physicochemical and microbial processes. Utilizing a 14-year field trial, we showed that high-dosage biochar (HBC) effectively reduced heavy metal bioavailability by 2–91%, outperforming low-dosage biochar (LBC) and straw amendments. Both HBC and LBC drove residual Cd, Zn, and Pb toward reducible fractions, whereas straw exhibited no significant impact. Partial least squares-structural equation modeling and variance partitioning analysis indicated that the concentration and speciation of metals were co-regulated by physicochemical and microbial properties, with microbial attributes dominating bioavailability (30% variance) and physicochemical governing speciation (12%). Specifically, biochar reduced bioavailability by increasing the cation exchange capacity (CEC), soil organic carbon (SOC), and free iron oxides, coupled with enriching Entomophthoromycota and Nitrospirae while suppressing Bacteroidetes and Verrucomicrobia. Conversely, straw increased bioavailability by decreasing CEC but enhancing enzyme activity alongside Bacteroidetes or Verrucomicrobia. For metal speciation, biochar drove the transformation of speciation by enhancing SOC, aromatic compound levels, and Zoopagomycota, but suppressing Ascomycota and Latescibacteria. By evaluating the coupling index of heavy metal immobilization and carbon sequestration, we showed that HBC had a higher score (0.703) than LBC (0.361) and straw (0.396). This indicated that HBC can more effectively immobilize heavy metals than LBC and straw, and achieve extra benefits in promoting carbon sequestration. Our results provided insights into adjusting soil management practices to achieve soil multi ecosystem functions and improve agricultural sustainability. Highlights Coupling index shows HBC can synergistically immobilize heavy metal and sequester C. HBC reduced heavy metal bioavailability and drove residual toward reducible fractions. Heavy metal bioavailability was dominated by Entomophthoromycota and Bacteroidetes. Heavy metal speciation was governed by SOC content and aromaticity. Graphical Abstract
Ma et al. (2026) studied this question.