In Situ Contribution of Old Respired CO2 from Soils in Burnt and Collapsed Permafrost in Canada

Abstract:

Permafrost degradation is associated with an aggradation of the active layer thus exposing previously frozen soil carbon (C) to microbial activity. This may increase the generation of greenhouse gases and potentially increase rates of climate change. However, the rate of C release remains highly uncertain, not least because few in situ studies have measured the rate at which previously frozen C is released from the soil surface, post thaw. We quantified the contribution of this “old” C being released as CO₂ from permafrost degraded soils in sporadic and discontinuous permafrost in Yukon and Northwest Territories, Canada. Firstly, we studied the effect of fire on black spruce forests as the removal of vegetation, especially mosses, may play a key role on thaw depth. Secondly, we investigated the collapse of peatland plateau after permafrost thaw which resulted in the formation of wetlands. We combined radiocarbon measurements of respired CO₂ with a novel collar-design that either included or excluded CO₂ released from deeper soil horizons. Our results show that, while excluding deeper layers did reduce the average age of the C being released from the soil surface, more than 90% of the CO₂ came from contemporary sources, even after burnt and permafrost plateau collapse. Furthermore, soil cores dated using ²¹⁰Pb show that the rapid accumulation of sedge peat after plateau collapse may more than compensate for any C losses from depth. Our results from the Canadian boreal contrast strongly with findings from other geographical areas emphasising the complexities of predicting the impact of permafrost thaw on the carbon balance of northern ecosystems.