Oxygen Vacancies on Ferrihydrite Promote Surface Polymerization of Decomposing Straw-Derived Dissolved Organic Matter: Implications for Soil Carbon Sequestration

Soil organic matter (SOM), a key redox-active component, interacts with natural minerals at interfaces, critically influencing the transformation and sequestration of organic carbon. However, the key effects of oxygen species at iron (oxyhydr)oxide defect sites on SOM molecular activation and transformation under an anoxic environment (e.g., paddy overlying water) are poorly understood. This study investigates molecular-level interfacial interactions between dissolved organic matter (DOM) extracted from rice straw at different stages of decomposition and ferrihydrite (Fh) under anoxic conditions. More decomposed DOM exhibited a 12.75% higher SUVA 254 removal rate under anoxic conditions than under oxic conditions, a process involving both adsorption and oxidation. This enhanced removal was attributed to vacancy defects on Fh, which activated persistent free radicals in the highly decomposed DOM, increasing electron transfer and triggering chain reactions involving carbon-centered radicals and hydroxyl radicals. Fourier transform ion cyclotron resonance mass spectrometry analysis of the molecular transformation pathways mediated by free radicals revealed that macromolecular polymer formation on the Fh surface was induced by the transformation between phenols and quinones including aryloxyl coupling and Schiff base formation. These findings highlight the importance of mineral defects in modulating redox-active organic matter evolved during decomposition under anoxic and oxic conditions, providing molecular-level evidence for Fh-induced free-radical reactions that aggregate DOM. This research deepens our understanding of how redox-active components formed during the decomposition of exogenous organic matter stabilize the soil organic carbon pool.