Soil microbial communities play a critical role in regulating the Earth’s climate through the production and consumption of greenhouse gases. Warming, particularly of subarctic soils, is predicted to cause widespread permafrost thaw. The high carbon (C) content of these systems could release CH4 and CO2 to the atmosphere, transitioning them from a net sink to a net source of C to the atmosphere. What are the impacts of permafrost degradation on C storage and can these changes be explained by variations in microbially-driven biogeochemical fluxes? We examined changes in carbon storage, greenhouse gas production, and microbial community composition and activity along a lowland chronosequence of collapse-scar bog development following a natural thaw event at the Alaska Peatland Experiment (APEX) in interior Alaska. The young bog was formed approximately 20 years ago and the oldest bog formed approximately 70 years ago.
Using a mass balance approach combined with macrofossil, isotopic analyses, and radiocarbon dating of peat, we show that peat C contained in the forest floor and permafrost is rapidly lost post thaw. At the young bog site we have observed higher rates of CH4 flux and ecosystem respiration at the surface, higher potential for higher rates of CH4 production within the deepest soil horizons, and higher modeled rates of CO2 and CH4 production (based upon porewater concentrations and isotopes of CO2 and CH4) compared to the older bog. Higher rates of methanogenesis and methane flux in the young bog were correlated with both increased availability of labile C from dissolved organic matter and, a higher abundance of aerenchymatous sedges (e.g. Carex sp) at the surface. We suspect that increased occurrence of Carex sp. in the young bog allows for more rapid transport of CH4 from the deep within the peat profile thereby circumventing the potential for CH4 oxidation. Moreover, variation in the composition of the microbial community (measured using 16S rRNA gene sequencing and QPCR) was reflective of the processes and rates occurring at these sites. Our results highlight how variation in microbial activity, driven in part by temporal changes in C availability post-thaw, interact with plant-mediated transport processes to drive rapid C loss early in the transition from permafrost to bog environment.