Short-term Impacts from the 2015 Yukon-Kuskokwim River Delta Tundra Fire on Permafrost Vulnerability and C Loss

Fire frequency and areal extent have been increasing for the past several decades in carbon-rich boreal and tundra ecosystems. The summer of 2015 was the second worst fire year on record in Alaska. One of these 2015 Alaskan fires burned within the Yukon-Kuskokwim Delta (YK Delta), where more area was burned in 2015 than in the previous 74 years combined. The goal of this project was to examine the immediate effects of fire on carbon storage and export from the YK Delta and, by extension, to gain critical insights into how the carbon balance of arctic deltas will change over the coming decades. One year after the fire, in June 2016, we measured terrestrial and aquatic carbon outputs associated with the burn event, from three areas within the burn scar, and three non-burnt areas nearby. Here we present ecosystem respiration (Reco) partitioning results from this campaign, which was done using a dual-isotope approach to determine how the fire has altered the contribution of autotrophic and heterotrophic respiration sources from the active layer, and heterotrophic respiration sources from permafrost. Reco gas samples were collected from static chambers, and Reco sources (organic and mineral layer soils, above and belowground vegetation, and permafrost soil) were incubated and sampled for their isotopic composition (both δ13C and Δ14C). Removal of insulating soil organic layers by the fire significantly increased permafrost thaw in the burn scar compared to control areas, and thus, we expect that this will increase the contribution of "old" permafrost carbon to Reco. Ultimately, these results will inform long-term trajectories of the vulnerability and fate of delta carbon pools.