High peatland methane emissions following permafrost thaw: enhanced acetoclastic methanogenesis during early successional stages

Abstract. Permafrost thaw in northern peatlands often leads to increased methane (CH4) emissions, but gaps remain in our understanding of the underlying controls responsible for increased emissions and the duration for which they persist. We assessed how shifting ecological conditions affect microbial communities, and the magnitude and stable isotopic signature (δ13C) of CH4 emissions along a thermokarst bog transect in boreal western Canada. Thermokarst bogs develop following permafrost thaw when dry, elevated peat plateaus collapse and become saturated and dominated by Sphagnum mosses. We differentiated between a young and a mature thermokarst bog stage (~30 and years ~200 since thaw, respectively). The young bog located along the thermokarst edge, was wetter, warmer and dominated by hydrophilic vegetation compared to the mature bog. Using 16S rRNA gene high throughput sequencing, we show that microbial communities were distinct near the surface and converged with depth, but lesser differences remained down to the lowest depth (160 cm). Microbial community analysis and δ13C data from CH4 surface emissions and dissolved gas depth profiles show that hydrogenotrophic methanogenesis was the dominant pathway at both sites. However, the young bog was found to have isotopically heavier δ13C-CH4 in both dissolved gases profiles and surface CH4 emissions, suggesting that acetoclastic methanogenesis was relatively more enhanced throughout the young bog peat profile. Furthermore, young bog CH4 emissions were three times greater than the mature bog. Our study suggests that interactions between ecological conditions and methanogenic communities enhance CH4 emissions in young thermokarst bogs, but these favorable conditions only persist for the initial decades after permafrost thaw.