Active layer microbial inocula restore missing functions across thawed permafrost soils.

Microbial dynamics in thawing permafrost induce a climate feedback of uncertain magnitude. Upon thaw, active layer microorganisms may establish in formerly frozen layers and introduce missing functions. Here we test the prevalence of functional limitations and how active layer microorganisms can mitigate these across four widespread permafrost types over 389 days. By restoring functions with a multifunctional microbial community, we demonstrate widespread functional limitations, as increased CO2 emissions across four permafrost types, and onset N2O emissions in two of those. However, realistic active layer inocula increased CO2 or N2O production less than the multifunctional community, due to unsuccessful coalescence or limitations in the inocula. Under anoxia, active layer inocula quintupled CH4 while decreasing CO2 production, resulting in a 35% increase in CO2-eq over six months. Understanding the functional limitations of permafrost microbial communities will be key to predicting their substantial, yet unaccounted, consequences on the biogeochemistry of the permafrost region.