Aerobic Conditions are Required for Rapid Carbon Losses following Permafrost Thaw

Abstract:

Permafrost soils store between 1,330–1,580 billion tonnes of carbon (C), which may start to be released to the atmosphere if warming promotes widespread thaw. Considerable uncertainty remains regarding how rapidly this C may be released and what proportion will be released as the more powerful greenhouse gas, methane (CH₄), rather than carbon dioxide (CO₂). Central to answering these questions, is quantifying in situ rates of old, previously-frozen C release under aerobic conditions versus anaerobic conditions. To meet this aim, we investigated the amount and form (CH₄ versus CO₂) of previously-frozen C released in response to 1) forest fire-induced permafrost thaw (aerobic soil conditions) and 2) permafrost plateau collapse in peatlands which results in waterlogging (anaerobic conditions), in the Yukon and Northwest Territories of Canada. We quantified in situ rates of CO₂ and CH₄ release, using a novel collar design to either include or exclude C released from deep soil layers, and collected samples for ¹⁴CO₂ and ¹⁴CH₄ analysis. Our ¹⁴CO₂ results identified substantial rates of old C release from burnt forests where the organic horizon was thick and deep thaw was observed. In contrast, in the wetlands old C release was barely detectable as CO₂ (<1%), and, although high CH₄ fluxes were measured, a maximum of 1.5-2g m^-2 yr^-1 was derived from previously-frozen C. Furthermore, because peat accumulation increased substantially (>4 times), , thaw resulted in net C gain in the wetland. However, the high rate of CH₄ release, albeit derived mainly from contemporary sources, means that permafrost plateau collapse can still represent a positive feedback to climate change in terms of global warming potential. Overall, our results indicate a much greater release of permafrost C under aerobic conditions, with little release of previously-frozen C as either CO₂ or CH₄ under anaerobic conditions.