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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Manganese (Mn) is an abundant element in terrestrial and coastal ecosystems and an essential micronutrient in the metabolic processes of plants and animals. Mn is generally not considered a potentially toxic element due to its low content in both soil and water. However, in coastal ecosystems, the Mn dynamic (commonly associated with the Fe cycle) is mostly controlled by redox processes. Here, we assessed the potential contamination of the Rio Doce estuary (SE Brazil) by Mn after the world’s largest mine tailings dam collapse, potentially resulting in chronic exposure to local wildlife and humans. Estuarine soils, water, and fish were collected and analyzed seven days after the arrival of the tailings in 2015 and again two years after the dam collapse in 2017. Using a suite of solid-phase analyses including X-ray absorption spectroscopy and sequential extractions, our results indicated that a large quantity of Mn II arrived in the estuary in 2015 bound to Fe oxyhydroxides. Over time, dissolved Mn and Fe were released from soils when Fe III oxyhydroxides underwent reductive dissolution. Due to seasonal redox oscillations, both Fe and Mn were then re-oxidized to Fe III , Mn III , and Mn IV and re-precipitated as poorly crystalline Fe oxyhydroxides and poorly crystalline Mn oxides. In 2017, redox conditions (Eh: −47 ± 83 mV; pH: 6.7 ± 0.5) favorable to both Fe and Mn reduction led to an increase (~880%) of dissolved Mn (average for 2015: 66 ± 130 μg L −1 ; 2017: 582 ± 626 μg L −1 ) in water and a decrease (~75%, 2015: 547 ± 498 mg kg −1 ; 2017: 135 ± 80 mg kg −1 ) in the total Mn content in soils. The crystalline Fe oxyhydroxides content significantly decreased while the fraction of poorly ordered Fe oxides increased in the soils limiting the role of Fe in Mn retention. The high concentration of dissolved Mn found within the estuary two years after the arrival of mine tailings indicates a possible chronic contamination scenario, which is supported by the high levels of Mn in two species of fish living in the estuary. Our work suggests a high risk to estuarine biota and human health due to the rapid Fe and Mn biogeochemical dynamic within the impacted estuary.

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