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.