Ferrous iron (Fe) oxidation is an important pathway for generating reactive Fe phases in soils, which can affect organic carbon (OC) persistence/decomposition. We explored how pO concentration influences Fe oxidation rates and Fe mineral composition, and how this impacts the subsequent Fe 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. The slurries were oxidized with either 21% or 1% pO for 9 days and then incubated for 20 days under anoxic conditions. Exposure to 21% pO led to faster Fe oxidation rates and greater partitioning of the amended Fe into low-crystallinity Fe-(oxyhydr)oxides (based on Mössbauer analysis) than exposure to 1% pO. During the subsequent anoxic period, low-crystallinity Fe-(oxyhydr)oxides were preferentially reduced relative to more crystalline forms with higher net rates of anoxic Fe and CO production-which were well correlated-following exposure to 21% pO than to 1% pO. This study illustrates that in redox-dynamic systems, the magnitude of O 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.

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Soils from humid forests undergo spatial and temporal variations in moisture and oxygen (O2) in response to rainfall, and induce changes in iron (Fe) and carbon (C) biogeochemistry. We hypothesized that high rainfall periods stimulate Fe and C cycling, with the greatest effects in areas of high soil moisture. To test this, we measured Fe and C cycling across three catenas at valley, slope, and ridge positions every two days for a two-month period in a rainforest in Puerto Rico. Over 12 days without rain, soil moisture, FeII, rapidly reducible Fe oxides (FeIIIRR), and dissolved organic C (DOC) declined, but Eh and O2 increased; conversely, during a 10-day period of intense rain (290 mm), we observed the opposite trends. Mixed-effects models suggest precipitation predicted soil moisture, soil redox potential (Eh), and O2, which in turn influenced Fe reduction/oxidation, C dissolution, and mineralization processes. The approximate turnover time for HCl-extractable FeII was four days for both production and consumption, and may be driven by fluctuations in FeIIIRR, which ranged from 42% to 100% of citrate–ascorbate-extractable FeIII (short-range order (SRO)-FeIII) at a given site. Our results demonstrated that periods of high precipitation (hot moments) influenced Fe and C-cycling within day-to-week timescales, and were more pronounced in humid valleys (hot spots).

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