Shedding “Dark” on the Oxidation of Dissolved Organic Matter by Hydroxyl Radical in Arctic Soils and Surface Waters

Abstract:

Hydroxyl radical (•OH) is a very reactive and unselective oxidant produced ubiquitously by photochemical reactions of dissolved organic matter (DOM) or iron in sunlit surface waters. Once formed, •OH can oxidize DOM to produce CO₂ or low molecular weight compounds that bacteria convert to CO₂, thereby contributing to the mineralization of DOM in inland waters. This role of •OH in aquatic carbon cycling was thought to be limited to sunlit waters, but we recently demonstrated that •OH is also formed in the dark by a sunlight-independent pathway. We show that •OH is formed in soil and surface waters of the Alaskan Arctic during the oxidation of reduced DOM and ferrous iron. Across a gradient of soil waters from dry upland to wet lowland habitats, •OH formation rates increased with increasing concentrations of DOM and reduced iron, with highest •OH formation observed at oxic−anoxic boundaries in soil and surface waters. Comparison of measured vs. expected electron release from reduced moieties suggested that both DOM and iron contributed to •OH formation. At landscape scales, DOM oxidation to CO₂ by dark •OH pathway may be as important to carbon cycling as bacterial oxidation of DOM in arctic surface waters, and is likely much more important than photochemical oxidation of DOM by •OH in sunlit surface waters of the Arctic.