Drought induces changes to redox chemistry and C exports from boreal riparian soils

Extreme summer droughts can drastically lower water tables and lead to oxygenation of normally anoxic soils in boreal ecosystems. In organic rich riparian soils, this creates a dynamic redox environment, driving changes in soil organic matter stability and the export of redox sensitive elements (e.g., C, N, S, Fe, etc.) to surface waters.  We hypothesized that the destabilization of redox cycles and the activation of oxidative soil enzymes during drought periods can lead to prolonged periods of altered soil biogeochemical processes that drive element export from terrestrial to surface water systems upon rewetting. Here we simulated a soil core drying-rewetting event, to ask how riparian soil solution biogeochemistry changes during two months post drought. To three drought treatments (dry, semi-dry, wet), we also added a root exudate treatment (exudates or no exudates) to simulate the effects of riparian vegetation on microbial organic matter decomposition. We found that following drought, dissolved organic carbon (DOC) concentrations initially decreased, due to the increased acidity caused by the oxidation of reduced S to SO₄^2-. As other preferred electron acceptors (O₂, NO₃^2-, Fe³⁺) were gradually reduced, reduction of SO₄^2- lead to increases in DOC concentrations, which after 2 weeks surpassed concentrations in the control (wet) treatment, and continued increasing until the end of the experiment. Once SO₄^2- was depleted and CO₂ became the preferred electron acceptor, methane (CH₄) in solution also increased to concentrations higher than those in control treatments. Peroxidase activity was increased post drought and remained elevated throughout the experiment, suggesting that microbial organic matter breakdown was enhanced, and could explain why DOC concentrations in drying treatments eventually surpassed those in wet controls. While the root exudate treatments produced mixed results, an increase of labile C supply appeared to increase extracellular enzymatic activity and serve as an alternative electron acceptor, thereby suppressing methanogenesis. Our results show that drought drastically changes the biogeochemistry of boreal riparian soils and that upon rewetting this can eventually lead to increased lateral exports of both organic and inorganic C. Changes in C biogeochemistry are seemingly caused by shifts in redox chemistry and by changes in microbial decomposition of soil organic matter induced by the oxygenation of riparian soils. Since this has implications for surface water chemistry, further study is needed on the length of drought effects to establish the duration of this influence of stream and riparian biogeochemistry.