The release of carbon from newly thawed permafrost is estimated to add between 0.05 and 0.39 °C to the simulated global mean surface air temperature by the year 2300. On decadal time-scales, the contribution of CH4 to this permafrost carbon feedback (PCF) can be equally important to that of CO2 but uncertainties arising from batch incubations that feed into projections are large. Here we sought what constraints CH4 production following permafrost thaw. We analyzed six permafrost cores from the high Canadian Arctic characterized by different land cover types, sediment properties, age and history of deposition for methanogenic abundance, activity and predictors of both. We found that the size of the active methanogenic population is a primary constraint on CH4 production following permafrost thaw and suggest that the proportion of methanogens getting active directly after thaw is only about 10% of the total methanogenic gene pool. The total methanogenic population size in turn varied by five orders of magnitude thus being highly heterogeneous and appeared to be a function of carbon density. In excess of methanogenic substrate carbon density further served as a good predictor for the potential production of CH4 after thaw. Finally, we propose that the millennial exposure to permafrost conditions bred high affinity methanogenic communities since we observed their inability to exhaust large concentrations of substrates and to substantially increase in population size after almost one year of incubation at moderate temperatures of 15°C in most of the incubations. Our study further supports related work that on the long-term methane production can contribute substantially to the permafrost carbon feedback since random incubations showed a pronounced evolution of methane and increase in methanogenic population size, respectively.

Supplement to: Liebner, Susanne; Lehr, Christian; Holm, Stine; Obu, Jaroslav; Knoblauch, Christian; Lantuit, Hugues; Fritz, Michael (in prep.): Methane production in thawing permafrost is constrained by methanogenic population size and carbon density.