Impacts of thermokarst formation on soil carbon dynamics on the North Slope of Alaska

Permafrost in the arctic contains more than twice as much carbon as currently exists in the atmosphere. As temperature increases, this soil carbon pool will become more vulnerable to release through two major pathways: gradual deepening of the active layer, or through catastrophic subsidence (thermokarst). We investigated the impacts of three modes of thermokarst formation on the gaseous and hydrologic flux of soil carbon on the North Slope of Alaska. The goals of our study were (a) to investigate the effects of thermokarst formation on soil moisture, temperature, and nutrient cycling, and (b) to compare gaseous and hydrologic carbon release between gradually thawed permafrost and permafrost disrupted by thermokarst formation. Eleven thermokarst sites were selected for study near the Toolik Field Station and the Kelly River Ranger Station, both on the North Slope of Alaska. During the summers of 2009 and 2010 we measured physical soil parameters (temperature, volumetric soil water content, depth of thaw, and elemental composition) to understand their role as potential drivers of carbon and nutrient export from soils affected by thermokarst formation. Due to disruption of insulating vegetation, average soil temperature at 10 cm increased from 6.33° C outside the features to 9.88° C within the features. Volumetric soil moisture content was impacted differentially depending on location within the thermokarst, with increased drainage at the margin of collapses and increased moisture near channels and depressions within features. Respiration increased 10-200 % depending on ground cover type and soil moisture and temperature conditions. The sites with highest respiration had elevated temperature and soil moisture (relative to the undisturbed tundra), suggesting that respiration was enhanced by increased water availability. We also observed increased hydrologic export of NH4, potentially due to increased decomposition, release from thawing permafrost, and movement of flowpaths from organic to mineral soil horizons. Though thermokarst currently affects only a small portion of the landscape, it may play a disproportionately large role in the carbon and nutrient dynamics of a changing arctic. Should thermokarst become more prevalent on a landscape scale, it could result in amplified nutrient cycling and net carbon release. Retrogressive thaw slump on the banks of a lake in the foothills of the Brooks range on the North Slope of Alaska