Electron Transfer Reversibility of Mineral-Associated Organic Matter Dominated by Redox-Active Metastable Substances in a Wetland–Grassland Transition Zone

Abstract

Soil organic matter is redox-active, capable of responding to redox fluctuations, and acts as a biogeobattery to facilitate biogeochemical cycling. However, whether the electron-transfer capacities of mineral-associated organic matter (MAOM) are reversible under controlled chemical redox-switching conditions remains unclear. Mediated electrochemical reduction/oxidation (MER/MEO) analysis showed that MAOM from the transition zones (e.g., between wetland and grassland ecosystems) exhibited the highest electron-accepting and -donating capacities (EAC and EDC) and reversible electron transfer capacity under varying redox states. Specifically, when conditions shift from oxidizing to reducing, the EAC of MAOM decreased in tandem with an equivalent increase in EDC. Conversely, when conditions revert to oxidizing, EAC increased while EDC decreased, demonstrating a fully reversible electron transfer behavior. Multiple complementary analyses reveal that MAOM in the transition zone was enriched in redox-active metastable phases (RAMPs), including redox-active carboxyl functional groups and mineral phases. Mineral phases were the major source of total electron exchange capacity (EEC), whereas organic matter functioned as an important modulator of coupled redox dynamics. MAOM features co-occurrences of iron–carbon (Fe–C) covalent bonding and abundant carboxyl functional groups, as well as mixed-valence iron phases (e.g., green rust and magnetite) and poorly crystalline iron (oxyhydr)oxide minerals (e.g., ferrihydrite and goethite), all of which contribute to electron transfer reversibility. These results suggest that MAOM contains RAMPs that support electron transfer reversibility under varied redox conditions, thereby influencing the overall biogeochemistry and persistence of the soil ecosystem.