Does soil fauna prime microbial respiration in permafrost?

Permafrost soils are devoid of fauna while frozen, but little is known about how fauna affects their biogeochemistry upon thawing. Most soil fauna resides in the uppermost layers of the soil, and are therefore unlikely to colonize deep, often anoxic, soils at the bottom of the active layer where newly-thawed permafrost is found. However, abrupt thaw events can result in newly-thawed permafrost being exposed to the surface, and such events are both common throughout the circum-Arctic and an important uncertainty in permafrost biogeochemistry. While the exact faunal dispersal mechanisms remain unexplored, literature suggests that surrounding soil fauna can migrate into newly-thawed permafrost within a year after an abrupt thaw event.

To date, we have no information on whether the introduction of soil fauna alters the biogeochemical functioning of permafrost soils: most mechanistic studies are carried out with permafrost soil thawed in vitro, into which fauna has no chance of dispersing, while plot- or ecosystem-level measurement do not distinguish between faunal and microbial activity. Simple questions, such as whether the presence of soil fauna alters the microbial production of greenhouse gases, remain untested, in part due to a lack of appropriate methodology. Here we introduce isotopic partitioning of faunal (model Collembola Folsomia candida) and microbial CO₂ production in permafrost and active layer soil from a sub-arctic palsa peatland. Whole-organism isotopic enrichment coupled with ¹³C-CO₂ measurement allows us to test whether faunal presence primes microbial respiration. This method can be expanded both to other soil organisms and greenhouse gases, and thus represents a promising avenue towards a quantitative understanding of biotic interactions in newly-thawed permafrost soils.