Linking water and oxygen availability to biogeochemical redox cycling and carbon release in Arctic soils

Across the Arctic, the seasonally thawed soil active layer is deepening as permafrost continues to degrade. Formation of thermokarst features collapsed soil resulting from permafrost thaw can form conduits for water and dissolved organic carbon (DOC), leading to biogeochemical environments that contrast adjacent upland soils that are still underlain by permafrost. Hydrological transitions due to thermokarst formation can promote changes in oxygen availability, microbial soil organic carbon (SOC) decomposition, and biogeochemical redox cycling, ultimately impacting the magnitude and proportion of carbon dioxide (CO2) and methane (CH4) released from Arctic soils. As these changes become more frequent and widespread, it is increasingly important to understand the controls on SOC decomposition in these dynamic environments and effectively model their interconnected hydrobiogeochemial processes. This study focuses on how soil water saturation and drainage impact oxygen availability, microbial utilization of alternative terminal electron acceptors (e.g., Fe(III)), biogeochemical cycling, and CO2 and CH4 fluxes from upland and thermokarst soils. These processes were investigated in two column experiments with soils obtained from a thermokarst site near Council, Alaska. Soils were packed into an insulated column with a controlled thermal gradient and instrumented with optical oxygen sensors, volumetric water content sensors, and MicroRhizon samplers. Water was drained from the soil columns and deionized water was reintroduced at the top of the column as precipitation in a series of saturation and drainage cycles. Throughout the experiments, CO2 and CH4 were measured in the column headspace. Porewater samples were used to measure pH and dissolved species (DOC, iron, major anions, and organic acids) with depth through the soil profile. Upland and thermokarst soils were compared in terms of carbon release and relative importance of SOC respiration, iron reduction/oxidation, and methanogenesis. An improved understanding of dynamic water and oxygen availability on redox cycling and carbon release in Arctic soils could support development of process-based reactive transport and SOC degradation models that are being coupled to terrestrial components of Earth system models.