Persistent free radicals on biochar surfaces redirect straw-derived carbon toward mineral-associated organic matter

Returning crop residues to soil is a central strategy for increasing soil organic carbon (SOC), yet whether biochar-associated persistent free radicals (PFRs) actively regulate the stabilization of residue-derived carbon remains unresolved. Here, graded PFR activities were generated by selective radical quenching while maintaining similar biochar physicochemical properties, and 13C-labeled straw was used to trace residue-derived carbon dynamics in two contrasting agricultural soils. Biochar PFRs enhanced hydroxyl radical (·OH) production in a dose-dependent manner, with the highest-PFR treatment generating 6.1- and 18.1-fold higher ·OH levels than the straw-only control in black soil and fluvo-aquic soil, respectively. This abiotic oxidation accelerated early-stage (0–14 d) straw carbon mineralization without increasing net carbon loss. Instead, it redirected straw-derived carbon toward aromatic intermediates that preferentially accumulated as mineral-associated organic matter (MAOM), increasing the proportion of MAOM carbon derived from straw from 59.4% to 64.2% in black soil and from 53.8% to 60.0% in fluvo-aquic soil. Structural equation modeling further demonstrated that PFRs promoted MAOM formation primarily through ·OH-driven chemical transformation, whereas microbial communities acted mainly as secondary responders. These findings demonstrate that biochar-derived PFRs redirect, rather than simply accelerate, straw-derived carbon turnover by coupling abiotic oxidation with microbial processing. This study establishes a mechanistic link between biochar surface redox chemistry and soil carbon persistence, providing a mechanistic basis for designing biochars with enhanced carbon sequestration potential.