Abiotic mechanisms of reactive oxygen species regulate dissolved organic carbon turnover at the molecular level in wetlands

Reactive oxygen species (ROS) serve pivotal roles in elemental geochemical processes, particularly in fluctuating oxic–anoxic environments. Dissolved organic matter (DOM) is a critical component in mediating soil carbon turnover. However, the contribution and underlying mechanisms of ROS-driven DOM transformation in wetlands remain unclear. In this study, the effects of ROS (i.e. •OH, O 2 •− and H 2 O 2 ) on DOM transformations at the molecular levels and their corresponding contributions in CO 2 emissions were explored vis microcosm experiments using wetlands samples under perennial and seasonal waterlogged, and perennial dry conditions. We estimated ROS-driven CO 2 emissions contribute 23.2%-26.9% of total CO 2 emissions, demonstrated ROS implication can not be ignored in wetland carbon cycling·H 2 O 2 facilitated the transformation of DOM from tannins to lignins/cram-like compounds (from 21 to 34) under aerobic conditions, thereby promoting DOM mineralisation. Under anaerobic conditions, the reaction between H 2 O 2 and Fe(II) generated •OH through the Fenton-like reactions, and the increasing •OH levels favoured the production of tannins and aromatic fractions, resulting in DOM accumulation. Density functional theory (DFT) calculations identified that DOM served as an electron receptor from ROS (i.e., H 2 O 2 ) in aerobic environments through electrophilic reactions, and as an electron donor to ROS (i.e., •OH) that Fe and redox-metastable constituents through nucleophilic reactions under anoxic conditions. Overall, ROS played an important role in promoting the transformations of organic carbon at the DOM molecular level under shifting oxic–anoxic interfaces.