Active layer microbial inocula restore missing functions across thawed permafrost soils

Microbial dynamics in thawing permafrost induce a climate feedback of uncertain magnitude. Estimates rely largely on incubations of experimentally-thawed permafrost soils, which may have limited microbial functionality after millennia of frost. In nature, however, seasonally-thawed active layer microorganisms may enter the underlying permafrost soil and introduce missing functions. Here we test the prevalence of functional limitations and how introducing active layer microorganisms can mitigate these across four widespread permafrost soil types. We measure the production of CO2, CH4, N2O, nitrate and ammonium content, bacterial community composition and genes controlling nitrogen cycling. By restoring functions with a diverse microbial community from a temperate grassland, we demonstrate widespread functional limitations in permafrost soils, as CO2 and N2O emissions increased across permafrost types over 389 days. However, realistic active layer inocula increased CO2 or N2O production less than observed with the diverse microbial community, and not systematically, due to unsuccessful coalescence or limitations in the inoculating soil communities themselves. Under anoxia, active layer inocula quintupled CH4 while decreasing CO2 production, resulting in a 35% increase in CO2-eq over six months. Understanding the factors that will lift the functional limitations of permafrost microbial communities will be key to predicting their substantial, yet unaccounted, consequences on the biogeochemistry of the permafrost region.