Influence of pO2 on Iron Redox Cycling and Anaerobic Organic Carbon Mineralization in a Humid Tropical Forest Soil

Ferrous iron (Fe-II) oxidation is an important pathway for generating reactive Fe-III phases in soils, which can affect organic carbon (OC) persistence/decomposition. We explored how pO(2) concentration influences Fe-II oxidation rates and Fe-III mineral composition, and how this impacts the subsequent Fe-III reduction and anaerobic OC mineralization following a transition from oxic to anoxic conditions. We conducted batch soil slurry experiments within a humid tropical forest soil amended with isotopically labeled Fe-57(II). The slurries were oxidized with either 21% or 1% PO2 for 9 days and then incubated for 20 days under anoxic conditions. Exposure to 21%pO(2) led to faster Fe-II oxidation rates and greater partitioning of the amended Fe-57 into low-crystallinity Fe-III-(oxyhydr)oxides (based on Mossbauer analysis) than exposure to 1% pO(2). During the subsequent anoxic period, low-crystallinity Fe-III-(oxyhydr)oxides were preferentially reduced relative to more crystalline forms with higher net rates of anoxic Fe-II and CO2 production-which were well correlated-following exposure to 21% pO(2) than to 1% pO(2) This study illustrates that in redox-dynamic systems, the magnitude of O-2 fluctuations can influence the coupled iron and organic carbon cycling in soils and more broadly, that reaction rates during periods of anoxia depend on the characteristics of prior oxidation events.