Molecular‐weight dependent redox cycling of humic substances of paddy soils over successive anoxic and oxic alternations

Humic substances (HSs) are critical to regulating methane cycling in temporary anoxic systems because their redox‐active functional groups can sustainably shuttle electrons from microorganisms to oxygen (O₂) over successive anoxic and oxic alternations. Whether the relative amount of these redox‐active functional groups involved in redox cycles is associated with the overall chemical structures of HS still remains poorly understood. In this study, variations in the reducing capacities (RCs) of HSs extracted from paddy soils were monitored over consecutive cycles of reduction by iron‐reducing microorganisms and oxidation by O₂. Our results show that about 10–40% of the microbially reduced redox‐active moieties within HS, which were reoxidised by O₂, cannot restore the same redox state as those before microbial reduction. The restoration extents of RCs of HSs after microbial reduction and O₂ reoxidation were greatly negatively correlated with average molecular weight (AMW). The microbially reduced HSs having large AMWs (>1,000 Da) were more kinetically difficult to restore by O₂ from a reduced state to an oxidised state than were those having small AMWs (<1,000 Da) possibly because the former, which underwent process of microbial reduction and O₂ reoxidation, were more easily sheltered by other parts of HS structures than were the latter. Nevertheless, the unsheltered redox‐active groups that accounted for about 50–85% in HSs can be fully reversible and sustainable to switch between a reduced state and an oxidised state over successive redox cycles. Our results can help to understand the redox dynamics of HSs in biogeochemical cycles in environments.