Abstract

Ferrous iron (Fe^II) oxidation is an important pathway for generating reactive FeIII 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 FeII 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.

Citation

Chen, Chunmei, Christof Meile, Jared Wilmoth, Diego Barcellos, and Aaron Thompson (2018): Influence of pO2 on iron redox cycling and anaerobic organic carbon mineralization in a humid tropical forest soil. Environmental Science & Technology 52 (14): 7709-7719. DOI: 10.1021/acs.est.8b01368

This Paper/Book acknowledges NSF CZO grant support.