The long-term biogeochemical fate of C in Subarctic thawing peat plateaus

Global warming causes permafrost to thaw at an unprecedented rate. In Northern Scandinavia, permafrost peat plateaus have been found to decline rapidly during the last decades, releasing old organic carbon to decomposition and runoff. Thawing peat plateaus can partly turn into thermokarst ponds, with consequences for the biogeochemical fate of the released carbon. We investigated carbon degradation of thawing permafrost peat by incubating permafrost peat and thermokarst sediments from three peat plateaus in Northern Norway. The samples were incubated field moist at 10^oC for almost one year. Initial decomposition was dominated by CO₂ production which strongly responded to oxygen availability, while methane (CH₄) production was small. Methane production increased drastically after more than ten months, indicating that thawed permafrost peat has a considerable potential to produce CH₄ after a time lag. The cumulative CH₄ production of thawed permafrost peat after one year of incubation exceeded that of overlaying active layer peat by up to 641 times, illustrating the potential of thawing subarctic permafrost to act as an additional CH₄ source. Comparing laboratory thawed permafrost peat to thermokarst peat revealed remarkable differences in CH₄ production, with much higher CH₄ production potentials in thermokarst sediments during the first months of incubation and in some samples exceeding CH₄ production measured in permafrost peat after one year. This suggests that the potential to produce CH₄ increases dramatically with thermokarst formation. Interestingly, thawed permafrost peat produced more DOC over the period of one year than gaseous C (CO₂ and CH₄), which suggests that hydrological conditions are key to the understanding of the fate of C released from thawing peat plateaus.