The research team, led by Sylvain Monteux of the Swedish University of Agricultural Sciences and UiT The Arctic University of Norway, set out to test a deceptively simple question: are permafrost microbes capable of doing everything the thawed soil demands of them? For decades, scientists studying the permafrost carbon feedback have focused on how much carbon is stored, how fast it will thaw, and how warm the soils will get. Most laboratory incubation studies, which measure greenhouse gas production from soil samples over months or years, implicitly assume that the microbial communities present in the permafrost contain all the functional machinery needed to decompose the organic matter. The new work challenges that assumption directly.

The researchers collected permafrost soils from four widespread types across the Arctic: a Sphagnum-dominated palsa peat from sub-Arctic Sweden, a sedge peat from tussock tundra on the Alaskan North Slope, a cryoturbated mineral soil from Franklin Bluffs in Alaska, and a Pleistocene-old Yedoma sediment from the CRREL Permafrost Tunnel near Fox, Alaska. These soil types differ enormously in age and origin. Yedoma deposits formed in unglaciated regions during the late Pleistocene and store 297 petagrams of carbon and 37 petagrams of nitrogen, roughly 29 percent of all carbon and 38 percent of all nitrogen in permafrost. Cryoturbated soils, where frost churns organic matter deep into the ground, hold up to 46 percent of permafrost carbon. Peat permafrost soils account for another 18 percent of permafrost carbon.

Because permafrost remains frozen, microbial dispersal is essentially blocked, and local extinctions of microbial species cannot be counteracted by new arrivals from the overlying active layer, the soil that thaws and refreezes each year. Over thousands of years, this isolation is thought to erode diversity and functional redundancy, leaving permafrost communities deprived of certain capabilities. Earlier work had demonstrated such functional limitations in Yedoma permafrost, but it remained unclear whether younger Holocene deposits, frozen for only a few thousand years, suffered the same deficits, and whether the limitations applied to different greenhouse gases.

To find out, the team ran two incubation experiments lasting up to 389 days. In the first, permafrost soils were incubated under oxic conditions at 10 degrees Celsius and inoculated with one of three microbial communities: a slurry made from their own overlying active layer, a functionally rich community from a temperate French grassland serving as a positive control, or sterile water as a control. In the second experiment, palsa permafrost was incubated under anoxic conditions for 175 days with and without its own active layer inoculum, simulating the waterlogged conditions that follow palsa thaw. The researchers tracked carbon dioxide, methane, and nitrous oxide production, mineral nitrogen pools, bacterial community composition through 16S rRNA gene sequencing, and the abundance of functional genes involved in nitrogen cycling and methanogenesis.

The results were striking. Adding the multifunctional grassland community increased cumulative carbon dioxide production in all four permafrost soils, with gains ranging from 20.1 to 59.6 percent. This demonstrated that functional limitations are widespread, not confined to ancient Yedoma deposits. The palsa permafrost also responded to its own active layer inoculum, with carbon dioxide production rising 15.4 percent, but the tussock and cryoturbated soils did not respond to their local inocula. In the Yedoma sediment, all three active layer inocula significantly increased carbon dioxide production, though less than the multifunctional community did, and the order of effectiveness was reversed: the cryoturbated active layer inoculum caused the greatest increase at 41.5 percent, while the palsa inoculum caused the smallest at 9.1 percent.

Nitrogen dynamics provided further evidence of missing functions. Ammonium declined in all soils over the incubation, and nitrate and nitrite accumulated to very high levels in soils receiving the multifunctional inoculum, indicating that nitrification, the oxidation of ammonium to nitrate, was largely absent from the uninoculated permafrost. Nitrous oxide emissions, normally below detection, appeared in soils inoculated with the multifunctional community and, in the palsa soil, with its own active layer. Gene analysis showed that archaeal amoA genes, which code for ammonia oxidation, were prevalent only in the cryoturbated active layer soil and were not detected in the permafrost unless introduced with the multifunctional inoculum. By contrast, nir and nosZ genes for denitrification were present in all soils by the end of the incubation, suggesting that nitrous oxide production is limited mainly by the supply of nitrate rather than by the absence of denitrifying organisms.

The anoxic experiment delivered perhaps the most consequential result. When palsa permafrost was inoculated with its own active layer community, cumulative methane production over six months quintupled, a 395 percent increase, while carbon dioxide production actually decreased. Because methane is a far more potent greenhouse gas than carbon dioxide, the estimated total warming potential of the emissions rose by 35 percent even after accounting for the carbon dioxide reduction. Notably, the abundance of mcrA genes, which code for the key enzyme of methanogenesis, did not increase with inoculation, and most mcrA copies in the pre-incubation permafrost appear to belong to relic DNA from dormant or dead cells. This suggests that the permafrost harbored methanogens in an inactive state, and that the active layer community, or interactions with it, revived or complemented methane production rather than simply adding more methanogens.

The bacterial sequencing data tied the biogeochemical changes to community dynamics. Changes in greenhouse gas production or nitrogen pools were detected only in cases where inoculation significantly altered bacterial community composition, a pattern the researchers interpret as microbial community coalescence, the interchange of entire communities when one invades another. Strong, long-lasting shifts in community composition, systematically linked to increased bacterial richness, indicated successful coalescence in most soils. Among the bacterial classes associated with increased carbon dioxide production were several poorly characterized groups, including Holophagae, which contained sequences identified as Geothrix fermentans, an iron-reducing bacterium, and Nitrososphaeria, archaeal ammonia oxidizers including Nitrosocosmicus species. The researchers speculate that the impact of coalescence depends on complementarity between the functions present in the permafrost and those in the arriving community, and that some functions may be missing from both layers, differing across soil types and space.

The implications for climate projections are significant. If permafrost microbial communities routinely lack functions that arriving active layer microbes can restore, then greenhouse gas estimates based on permafrost-only incubations may substantially underestimate future emissions. The researchers argue that incubation studies should explicitly compare the presence and absence of an active layer inoculum, and that field experiments across the permafrost region are needed to untangle the drivers of coalescence and quantify its biogeochemical consequences. Disruptive thaw events such as thaw slumps and thermokarst, which mix soil layers, may accelerate microbial migration more effectively than gradual percolation of water, and growing human pressure in the Arctic through tourism and mining could further disperse microorganisms. As the Arctic warms, understanding which microbes are missing, which are arriving, and what happens when they meet may prove essential to predicting the permafrost climate feedback and incorporating it into climate change mitigation strategies.

Subject of Research: Microbial functional limitations and greenhouse gas production in thawing permafrost soils

Article Title: Active layer microbial inocula restore missing functions across thawed permafrost soils

Article References: Monteux, S., Dorrepaal, E., Fontaine, S., Gavazov, K., Hallin, S., Juhanson, J., Keuper, F., Tveit, A. T., Walz, J., Wegner, R., Wild, B., & Krab, E. J. (2026). Active layer microbial inocula restore missing functions across thawed permafrost soils. Nature Communications, 17(1), Article 10230. https://doi.org/10.1038/s41467-026-78060-4

Image Credits: AI Generated

DOI: 10.1038/s41467-026-78060-4

Keywords: permafrost, microbial ecology, greenhouse gases, carbon dioxide, methane, nitrous oxide, active layer, community coalescence, climate feedback, Arctic soils, nitrification, Yedoma