Reactive Oxygen Species Formation in Paddy Soils under Alternating Redox Conditions: An Iron-Microbe Synergy Driving Mechanism and Environmental Impacts

Abstract Alternating wetting and drying cycles in paddy soils drive active biogeochemical processes and profoundly influence pollutant migration and transformation. This study systematically investigates the formation and mechanisms of reactive oxygen species (ROS) under anaerobic–aerobic alternation in five typical paddy soils, as well as their implications for the attenuation of tetracycline (TC) and associated antibiotic resistance genes. The results demonstrate that ROS generation primarily arises from microbially mediated Fe(II) accumulation during flooding, followed by Fenton-like oxidation upon reoxygenation. The cumulative hydroxyl radical (•OH) yields during the 12 h aerobic phase following 30 days of flooding followed the order: MaAnShan (1175.7 μmol/kg) > QiongHai (794.8 μmol/kg) > SuiHua (711.8 μmol/kg) > ChenZhou (391.6 μmol/kg) > GuiLin (297.0 μmol/kg). Iron-reducing bacteria, including Anaeromyxobacter, Desulfosporosinus, and Geomonas, were identified as the primary drivers of Fe(III) reduction. The variation in ROS production potential among soils was primarily governed by poorly crystalline iron content and indigenous iron-reducing bacteria, with alkaline pH and high organic matter content serving as key inhibitory factors. The ROS generated under fluctuating redox conditions contributed substantially to TC degradation (24.8–66.9%) and tetA gene attenuation (60.4–79.2%). Collectively, this study elucidates the biogeochemical mechanisms underlying ROS formation in paddy soils during drying–wetting cycles and their role in pollutant attenuation, offering a theoretical framework for advancing our understanding of contaminant dynamics in agricultural wetlands and for guiding the development of targeted pollution control strategies.