Microbial methane production associated with permafrost soils in Yedoma (Alaska) upland forested and grassland hillslopes. We show that upland forested and grassland hillslopes with thermokarst features initiated in the last 50 to 70 years by thawing of ice-rich yedoma permafrost soils are an exceptionally large source of microbial 14C-depleted methane, especially in winter when surface soils freeze, limiting methanotrophy.

Microbial methane production associated with permafrost soils has been considered only in the seasonally thawed active layer and in inundated thermokarst lake and wetland sedimentsWalzEtAl2017,Turetsky2020NatGeo. Until now, dry, upland soils that dominate 87% of the ArcticHugeliusetal2014, are assumed to be an atmospheric methane sink that compensates for much of the northern wetland-derived feedbacks to climate warming this centuryOhEtAl2020. Contrary evidence about the role of uplands would require a paradigm shift leading to a new framework for understanding the permafrost carbon feedback. Here we used field-flux measurements, radiocarbon dating and borehole drilling to show that talik (thaw bulb) development in yedoma permafrost soils leads to large methane emissions in dry upland environments. Eddy covariance methane emissions from a graminoid-dominated hillslope with thermokarst-mounds were three times higher annually than wetland emissions from the same latitude. Seventy percent of upland emissions occurred in winter, when surface soils were frozen, shutting down the powerful capacity of methane oxidizing bacteria, but permitting escape of methane produced in underlying anoxic taliks. These findings require restructuring our framework of permafrost carbon feedback trajectories to climate change, and imply that methane emission from upland taliks has the potential to overwhelm methane consumption by surface soils.