Microbial communities and greenhouse gas production from a thermokarst bog chronosequence: Mechanisms of rapid carbon loss

Abstract:

Climate change in northern latitudes is expected to cause widespread permafrost thaw in Interior Alaska over the 21st century. Permafrost thaw may result in land subsidence and the formation of thermokarst bogs. In decades following thaw, previously forest floor (silvic) carbon (C) may be rapidly decomposed, likely due to accelerated microbial activities in young bog environments, resulting in a decadal to century scale positive feedback to climate warming. We examined rates and mechanisms of C loss from a thermokarst bog chronosequence (0-500 ybp) at the Alaska Peatland Experiment (APEX), part of the Bonanza Creek LTER near Fairbanks, AK. Silvic C losses were within ranges observed at other thermokarst chronosequence studies. Incubation studies and modeling results indicate that there are accelerated rates of microbial activity within the deeper silvic and humic soil horizons of the youngest bog. We hypothesized two potential mechanisms of rapid C loss and higher microbial activity in young thermokarst bogs: 1) higher availability of electron acceptors from thawed permafrost that fuel microbial activity, and 2) increased availability of labile C from both soil organic matter and dissolved organic matter in young bogs fuel microbial activity. We tested these hypotheses using anaerobic soil incubations and assays of sulfate reduction, Fe reduction, humic substance (HS) reduction, and nitrate reduction, combined with quantitative PCR of microbial functional groups associated with those processes. Assay results indicated that although sulfate reduction and denitrification were detectable in several of the bogs, only HS reduction was unique to the deep layers of the young thermokarst bog. The most striking difference among different aged bogs was dissolved organic matter, which was elevated in the youngest bogs. These results support both of our hypotheses: microbial activity is stimulated by the availability of labile C in the young bog as both a source of C for decomposition and electron accepting materials. As thermokarst bogs age, the labile fraction of OM is consumed and silvic C decomposition slows. These data highlight the importance of microbial ecology and the biogeochemical processes they mediate to understand decadal scale changes in C storage in thawing permafrost environments.