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

Permafrost thaw in northern peatlands often leads to increased methane (CH4) emissions, but the underlying controls responsible for increased emissions and the duration for which they persist have yet to be fully elucidated. We assessed how shifting environmental conditions affect microbial communities and the magnitude and stable isotopic signature (delta C-13) 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 (similar to 30 and similar to 200 years 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 high-throughput 16S rRNA gene sequencing, we show that microbial communities were distinct near the surface and converged with depth, but fewer differences remained down to the lowest depth (160 cm). Microbial community analysis and delta C-13 data from CH4 surface emissions and dissolved gas depth profiles show that hydrogenotrophic methanogenesis was the dominant pathway at both sites. However, mean delta C-13-CH4 signatures of both dissolved gas profiles and surface CH4 emissions were found to be iso-topically heavier in the young bog (-63 parts per thousand and -65 parts per thousand, respectively) compared to the mature bog (-69 parts per thousand and -75 parts per thousand, respectively), suggesting that acetoclastic methano-genesis was relatively more enhanced throughout the young bog peat profile. Furthermore, mean young bog CH4 emissions of 82 mg CH4 m(-2) d(-1) were similar to 3 times greater than the 32 mg CH4 m(-2) d(-1) observed in the mature bog. Our study suggests that interactions between the methano-genic community, hydrophilic vegetation, warmer temperatures, and saturated surface conditions enhance CH4 emissions in young thermokarst bogs but that these favourable conditions only persist for the initial decades after permafrost thaw.

Automated summary

Permafrost thaw in northern peatlands leads to increased methane emissions, and this study investigates the factors responsible and their duration. The researchers found that microbial communities, temperature, and saturated surface conditions enhance methane emissions in young thermokarst bogs. However, these favorable conditions only persist for the initial decades after permafrost thaw.