Long term effects of permafrost thaw and wildfire on peatland carbon cycling

Boreal peatland complexes in the discontinuous permafrost zone of western Canada are mosaics of treed, permafrost affected peat plateaus interspersed with Sphagnum dominated thermokarst bogs where permafrost is absent. In a series of studies, we have assessed the implications and interactions of wildfire and permafrost thaw for peatland C cycling in this region. Carbon cycling in recently formed thermokarst bogs is of particular interest, as development of thermokarst bogs have been suggested to both cause substantial losses and gains in C storage. In order to reconcile disparate findings, we combined the monitoring of greenhouse gas fluxes with the study of peat cores in recently formed and mature thermokarst bogs. Preliminary results suggest that the mature thermokarst bogs are greater C sinks than recently formed thermokarst bogs during summer. Increased heat conduction in the wetter recently formed bogs caused substantially higher soil temperatures even at 3 m depth. Anaerobic laboratory incubations of deep peat showed that even moderate changes in temperature increased mineralization rates substantially. Furthermore, we found that mineralization rates of dissolved organic carbon (DOC) in near surface pore water was nearly ten times faster in recent than in mature thermokarst bogs. To assess to what degree wildfire accelerates permafrost thaw and thermokarst bog exansion, we combined a remote sensing landscape classification with in-situ monitoring of soil thermal regimes in sites that varied with regards to time since fire. Soil temperatures and active layer measurements indicated that the effect of wildfire on peat plateaus was most pronounced 10-15 years following the fire, and that it takes 30 to 40 years before soil thermal regimes of burned peat plateaus fully recover. In accordance, we found that peatland complexes affected by wildfire 20 to 30 years ago had twice as fast expansion rates of thermokarst bogs than nearby unburned peatland complexes. Our studies show that it is vital to account for the effects and interactions of wildfire and permafrost thaw in order to understand C cycling at the landscape scale.