Arctic Tundra Fire Promotes Increased Greenhouse Gas Emissions from Freshwaters and Changes Methane Hotspot Locations across the Landscape in the Yukon-Kuskokwim Delta

Fire frequency and severity are increasing in high latitudes as a result of global climate change. The last decade has seen unprecedented tundra fires, such as the 2015 fires that burned >700 km² in the Yukon-Kuskokwim Delta (YKD), an area that is more than the previous 50 years combined. The YKD is located in subarctic tundra with discontinuous carbon-rich permafrost. Wetlands dominate the YKD and atmospheric models suggest the region may be a CH₄ hotspot. A lack of ground-based measurements limits our current understanding of the influence of fires on YKD ecosystem processes. We surveyed dissolved organic matter (DOM) and greenhouse gas concentrations and emissions across the landscape to address the question: How do landscape connectivity and fire affect the production, transport, and emission of greenhouse gases?

We measured gas concentrations and fluxes from 2015 to 2018. In unburned ecosystems, small ponds and pore water on elevated peat plateaus had the highest CH₄ and CO₂ concentrations, followed by fens, which border the peat plateaus, and lakes which the fens drain into, demonstrating loss as water moved downslope. Plateau ponds and lakes within burned regions had lower dissolved CH₄ and CO₂ than unburned, whereas burned fens had more dissolved CH₄ and CO₂ than unburned. Lakes had the highest rate of CH₄ flux compared to other water bodies in unburned ecosystems, averaging 70 mg C-CH₄/m²/d. The burned lakes had 50% lower CH₄ fluxes than the unburned. CH₄ fluxes from burned fens were an order of magnitude higher than the unburned fens, making fens the largest source of CH₄ across burned ecosystems. Across all landscape types, CO₂ fluxes were higher in burned than in unburned areas. The burn effect was greatest in fens, doubling flux rates to 1500 mg C-CO₂/m²/d. We found the highest CO₂ fluxes in unburned fens, followed by peat plateau surface water and ponds, and lowest in lakes. The change in CH₄ hotspots after the fire from lakes to fens was driven by changes in the composition of DOM. In contrast, CO₂ flux correlated with pH, temperature, and dissolved oxygen. Our results highlight the importance of terrestrial-aquatic transitions for regional carbon emissions, and demonstrate the need for a mechanistic understanding of the drivers of greenhouse gas emissions after fires in the context of landscape connectivity.