Controls on Carbon Release from Arctic Thermokarst Landscapes: Changing Redox Conditions Driven by Soil Saturation and Drainage
Erin Berns-Herrboldt1,4, Teri O’Meara2,3, Elizabeth Herndon3, Benjamin Sulman2,3, Baohua Gu3, David Graham1* (email@example.com), Colleen Iversen2
1Biosciences Division, Oak Ridge National Laboratory, Oak Ridge, TN; 2Climate Change Science Institute, Oak Ridge National Laboratory, Oak Ridge, TN; 3Environmental Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN; 4Natural and Applied Sciences Department, University of Wisconsin–Green Bay, WI
Thawing Arctic permafrost causes land surface subsidence and formation of thermokarst drainage channels and lakes, altering biogeochemical redox cycles and soil organic carbon (SOC) decomposition. Soil saturation promotes anaerobic microbial activity and changes the balance of carbon (C) released as dissolved organic carbon (DOC), carbon dioxide (CO2), and methane (CH4). There is uncertainty about the degree to which soil saturation and subsequent redox transitions will impact Arctic fluxes and how to represent connections with hydrology and microbial SOC decomposition in ecosystem-scale models. Researchers evaluated if soil saturation and drainage—and associated changes in oxygen (O2), iron cycling, and microbial communities—contributes to enhanced SOC decomposition. Two column experiments were conducted to simulate saturation and drainage in soils collected from an inundated thermokarst channel and the adjacent upland tundra near Council, Alaska. Over three months, continuous measurements of O2 and volumetric water content were paired with discrete measurements of porewater pH, DOC, and iron. Column headspace was sampled for CO2 and CH4. Results show that during draining the upland column had 70% higher CO2 fluxes than the thermokarst column, likely due to faster drainage and higher O2 concentrations; intermittent CH4 release was also observed for the thermokarst column. During saturation, ferrous total iron increased, and pH decreased for both columns. DOC increased (150 mg/L to over 500 mg/L) in the top portion of the upland column during saturation, which likely contributed to 50% higher CO2 fluxes during the second drainage. The thermokarst column did not release as much DOC, possibly due to previous release of labile under saturated conditions. Soil extractions indicate downward transport of organic-bound iron in the upland column. Soils with high water content had higher relative abundances of hydrogenotrophic and methyltrophic methanogens and higher ferrous iron supported putative iron and methane oxidizers. This study underscores the importance of redox cycling in Arctic SOC decomposition. Continued research targeting changes in Arctic soil redox gradients should be explored to inform model development for simulating future scenarios resulting from permafrost thaw.