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.