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

Abstract

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 pO₂ 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 ⁵⁷Fe^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% pO₂ led to faster Fe^II oxidation rates and greater partitioning of the amended ⁵⁷Fe into low-crystallinity Fe^III-(oxyhydr)oxides (based on Mössbauer analysis) than exposure to 1% pO₂. 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% pO₂ than to 1% pO₂. This study illustrates that in redox-dynamic systems, the magnitude of O₂ 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.