Congratulations to Adrianna Trusiak who defended her dissertation on Friday, February 21, 2020.
Advisor: Rose Cory
Interactions between iron and organic carbon (OC) in soils influence the amount of soil OC that is oxidized to carbon dioxide (CO2), a greenhouse gas warming our planet. Although both microbial or abiotic iron redox reactions can oxidize soil OC to CO2, the role of abiotic iron redox reactions in the oxidation of soil OC to CO2 remains poorly understood. Oxidation of reduced ferrous iron (Fe(II)) by dissolved oxygen produces hydroxyl radical (•OH), a reactive oxidant that may oxidize dissolved OC (DOC) to CO2. Production of •OH from Fe(II) oxidation has been well-studied in controlled laboratory experiments, but it is unknown whether this process is an important pathway for the oxidation of DOC to CO2 in soils. To address this knowledge gap, the oxidation of Fe(II) and the subsequent •OH and CO2 production were measured in arctic soil waters. •OH was produced in all soil waters studied in the Arctic, and the oxidation of Fe(II) by dissolved oxygen was found to be the main source of •OH. The •OH produced from this reaction oxidized DOC to CO2 in controlled laboratory experiments and in soil waters. The production yield of CO2 from the oxidation of DOC by •OH varied by 2- to 50- fold possibly due to differences in DOC chemical composition. On a broader, landscape scale, Fe(II) production rates, and thus •OH and CO2 production rates, varied by landscape age and vegetation type. For example, Fe(II) production rates were higher in the upland, older mineral-rich soils with tussock vegetation than the lowland, younger organic-rich soils with wet sedge vegetation. In all soils, the magnitude of •OH and CO2 production depended on the balance of (i) the rates of Fe(II) oxidation by dissolved oxygen and (ii) the rates of Fe(II) production. Dissolved oxygen supplied to the soils with rainfall oxidized Fe(II), resulting in higher •OH and CO2 production than under static, waterlogged conditions. During rainfall events, Fe(II) was continuously detected despite oxidizing conditions, suggesting that Fe(II) production exceeded its oxidation. Under static, waterlogged conditions, Fe(II) oxidation, and thus •OH and CO2 production, was limited by the supply of dissolved oxygen to the soils. On a landscape scale in the Arctic, the rates of CO2 production from DOC oxidation by •OH in soils were comparable to the rates of CO2 production from microbial respiration of DOC in surface waters. Thus, this dissertation research demonstrated a novel pathway for soil OC oxidation where abiotic interactions between iron and OC can be an important source of CO2 to the atmosphere. As the Arctic warms, permafrost soils are thawing and releasing high concentrations of iron and OC that are susceptible to oxidation. The conversion of this permafrost OC to CO2 will result in positive and accelerating feedback to climate change. The results from this thesis improve our ability to predict this feedback by identifying the controls on the magnitude of the CO2 produced from iron-mediated OC oxidation in soils.