Impacts of Fresh Litter Inputs on Microbially Mediated C Fluxes Across an Arctic Permafrost Thaw Gradient

Thawing arctic permafrost contains 30-50% of global soil carbon and is expected to drive substantial alterations to carbon (C) cycling that will accelerate climate change. As permafrost thaws, old C may decompose and be released as carbon dioxide (CO₂) and methane (CH₄, a more potent greenhouse gas), but thawing soil can also increase plant-derived C inputs as perennial shrub communities transition to faster-growing annual wetland plants. Hence, the net effect of plant community changes on the C cycle trajectory is not yet well understood. To quantify patterns of microbial decomposition of fresh plant litter and the associated C gas emissions, we incubated ¹³C-enriched plant material from sedge and moss plants (Eriophorum and Sphagnum) with arctic peats from pre- and post-permafrost thaw areas under near-in situ conditions. CO₂ and CH₄ fluxes were highly ¹³C enriched allowing partitioning between unlabeled soil and labeled litter-induced decomposition. This partitioning showed positive and negative effects of litter addition on decomposition of soil, depending on thaw phase and gas species (CO₂ vs CH₄) and also showed three distinct phases of flux after litter addition which were not seen in incubations with no litter added. To characterize the microbiota responsible for these differing patterns of gas production, microbial DNA from incubations was density fractionated, revealing DNA enriched in ¹³C, due to uptake of C from the labeled plant litter. DNA profiles are currently being analyzed to reveal the individual lineages involved. Surprisingly, concomitant with the second peak in CH₄ flux there was a spike in low GC-content organisms in non-enriched incubations, seeming to indicate rapid growth of methanogens on fresh litter inputs. We conclude that the quantity and quality of litter inputs to thawing permafrost-associated soils will heavily impact the fate of C stored in these soils through influence on microbial activity.