Soil Organic Matter Modulation during Redox Oscillations: Insights from Iron Speciation and Microbial Metabolism

Soil organic matter (SOM), the primary terrestrial carbon reservoir, critically regulates global carbon cycling. In reservoir riparian zones, hydrological-fluctuation-induced redox oscillations restructure the SOM transformation dynamics. Iron (Fe) minerals and microbial consortia are recognized as primary SOM regulators under redox conditions, while their specific functions remain elusive. Herein, we investigated the influences of Fe speciation and microbial metabolism on the SOM transformation through redox incubations on riparian soils. The results demonstrated that the redox oscillations induced cyclic Fe(II)/Fe(III) transformation that liberated Fe-bound organic carbon and generated hydroxyl radicals (•OH). Enhanced organic substrate bioavailability upregulated microbial glycolysis and the tricarboxylic acid cycle (TCA cycle) pathways, resulting in a 2.5- to 5.3-fold increase in CO2 flux rate during oxic phases. However, Fe mineral crystallization and SOM transformation (e.g., lignin accumulation and labile organic substrate depletion) downregulated microbial energy metabolism (glycolysis, TCA cycle, and oxidative phosphorylation) compared to the static anoxic condition. The weakening of electron-donating capability decreased Fe reduction efficiency from 91.7% to 28.5%, thereby suppressing organic carbon release and •OH yields. Collectively, the effects of redox oscillations on the SOM transformation gradually diminished with cycle numbers. This study elucidated Fe mineral-microbe-SOM mechanistic linkages during redox oscillations, advancing theoretical frameworks for riparian ecosystem management.