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 10oC 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