Free-Radical Cascade Mediates Humus Formation in Mineral-Rich Organic Matrices

Abstract Humus formation in nature is traditionally viewed as a lengthy microbial enzymatic process, yet the abiotic pathways remain unclear. Here, by employing iron-mineral-rich composting as a model system that amplifies and accelerates natural humification, we reveal a free-radical cascade that drives abiotic humification. Magnetite addition amplified hydroxyl radical (•OH) production 1.5-fold (1289.4 μmol·kg–1) and enriched humus precursors (amino acids, polyphenols, reducing sugars), ultimately yielding 1.5 times more humic acid (135.6 mg/g) with higher molecular weight and stronger aromatic condensation. Mass spectrometry combined with density functional theory calculations identified multiple environmentally persistent free radicals (EPFRs), including semiquinone and phenoxy radicals as key intermediates that couple precursors into macromolecular humus. Simulated transformation experiments with selective microbial inactivation and •OH scavenging demonstrated that the abiotic radical pathway contributed approximately 33.4% to humus formation. We propose a free-radical cascade: •OH-driven oxidative fragmentation of organic matter followed by EPFR-mediated humus precursors coupling. These findings establish an unrecognized abiotic dimension of humification, reshaping our mechanistic understanding of organic carbon stabilization and offering nature-based strategies for soil carbon sequestration and remediation in organic-mineral interaction environments.