Carbon degradation and mobilisation potentials of thawing permafrost peatlands in northern Norway inferred from laboratory incubations

Abstract

Permafrost soils are undergoing rapid thawing due to climate change and global warming. Permafrost peatlands are especially vulnerable, as they are located near the southern margin of the permafrost domain in the zones of discontinuous and sporadic permafrost. They store large quantities of carbon (C) which, upon thawing, may be decomposed and released as carbon dioxide (CO2), methane (CH4) and dissolved organic carbon (DOC). This study characterises patterns of potential C degradation and mobilisation within an area with sporadic permafrost by evaluating C degradation in three permafrost peatland ecosystems in Finnmark, Norway, under laboratory conditions. Active-layer, transition zone and permafrost samples from distinct cores were thawed under controlled conditions and incubated for up to 350 d under initially oxic or anoxic conditions while measuring CO2, CH4 and DOC production. Carbon degradation varied among the three peat plateaus but showed a similar trend over depth, with the largest CO2 production rates in the upper active layer and the top of the permafrost. Despite marked differences in peat chemistry between the layers, post-thaw CO2 production of permafrost peat throughout the first 350 d reached 67 %–125 % of that observed in samples from the top of the active layer. De novo CH4 production occurred after prolonged anoxic incubation in samples from the transition zone and permafrost, but it was not found in active-layer samples. CH4 production was highest in incubations from thermokarst peat sampled next to decaying peat plateaus. DOC production by active-layer samples throughout 350 d incubation exceeded gaseous C loss by up to 23-fold under anoxic conditions, whereas production by permafrost peat was small. Taken together, the results of our study suggest that permafrost peat in thawing Norwegian peat plateaus degrades at rates similar to those of active-layer peat, while the highest CH4 production can be expected after the inundation of thawed permafrost material in thermokarst ponds.