The stakes are enormous. More than 40 percent of all the carbon stored in terrestrial soils on Earth sits in permafrost-affected ground, a vast frozen reservoir accumulated over tens of thousands of years. When permafrost thaws, microbes gain access to this ancient organic matter and can convert it into carbon dioxide and methane, creating a positive feedback loop in which warming drives thaw, thaw releases greenhouse gases, and those gases drive further warming. Yet the review, led by Rebecca Hewitt of Amherst College together with an international team of plant ecologists, soil scientists and modellers, shows that the trajectory of this feedback depends critically on what happens in the narrow zone of soil surrounding plant roots, the rhizosphere.

Arctic greening, the climate-driven increase in vegetation productivity visible from orbit, has long been treated as a story about leaves. Satellites record the expanding canopies of shrubs and the lengthening growing season, and models translate that greenness into carbon uptake. But the authors emphasize that aboveground observations tell only half the story. In Arctic ecosystems, a disproportionate share of plant biomass lies below the surface, and root dynamics are changing in ways that remote sensing cannot detect: productivity is increasing, rooting depth is growing, phenology is shifting and functional traits are transforming. These changes determine not only how much carbon plants capture, but what happens to carbon already stored in the soil.

One of the most striking mechanisms the review highlights is rhizosphere priming. When roots release sugars, organic acids and amino acids into the soil, a category of inputs known as rhizodeposits, they can stimulate microbes to decompose soil organic matter far faster than they otherwise would. In essence, fresh plant carbon acts as fuel that lets microbial communities mine older, more recalcitrant carbon reserves. Experimental work cited in the review shows that plant-derived compounds can accelerate the decomposition of organic matter in Arctic permafrost soils, and that positive priming can speed carbon release from thawed permafrost. Intriguingly, the picture is not uniform: circum-Arctic peat soils appear to resist priming by plant-derived compounds, suggesting that hydrology, mineralogy and soil chemistry strongly modulate the effect.

Rooting depth emerges as another decisive variable. As the seasonally thawed active layer deepens, plants are following the thaw front downward, tapping nitrogen and other nutrients released from newly unfrozen soil. Studies of subarctic peatlands show strongly increased root growth and rooting depth that enhance plant interactions with thawing permafrost, and mycorrhizal fungi associated with tundra plants have been shown to contribute to plant acquisition of permafrost-derived nitrogen. This creates a biological feedback that cuts both ways. Deeper roots mean more nutrients, more plant growth and potentially more carbon stored in new biomass and root-derived soil organic matter. But they also mean that roots and their fungal partners deliver fresh carbon and oxygen into soil horizons where ancient carbon has been locked away, potentially accelerating its loss.

The review also draws attention to the role of mycorrhizal fungi, the symbiotic organisms that colonize plant roots and mediate nutrient exchange. Different fungal types occupy different positions on what researchers call the fungal collaboration gradient, and these associations shape whether carbon entering the soil is rapidly respired or stabilized. Recent analyses suggest that fungal residues contribute more to mineral-associated organic matter pools than plant residues, and that fungal necromass carbon dominates global soil organic carbon storage. In the Arctic, where ectomycorrhizal fungi partner with shrubs such as willows, birch and alder, and ericoid fungi partner with heath plants, shifts in vegetation composition driven by greening will therefore rewire the underground networks that govern carbon persistence.

Phenology, the timing of biological events, adds yet another layer of complexity. Research along Arctic elevation gradients has revealed that the belowground growing season can be up to 50 percent longer than the aboveground one, a hidden season of root activity that occurs beneath the snow. Moreover, root and leaf phenology are often asynchronous, and vegetation community type rather than microclimate appears to control this asynchrony. Soil microorganisms and plants also show divergent phenological responses to warming. These temporal mismatches mean that carbon and nutrient fluxes may occur at times of year when no aboveground measurements are being made, complicating efforts to build accurate annual carbon budgets.

The consequences ripple outward to Earth system models, the numerical engines behind climate projections. The review notes that Arctic root traits are poorly represented in global plant trait databases, and that rooting depth distributions influence modelled carbon emissions but cannot be reliably inferred from aboveground vegetation type. Models that fail to capture root dynamics, mycorrhizal trade-offs and rhizosphere priming may misjudge both the magnitude and even the direction of Arctic carbon feedbacks. Some studies indicate that microbiome adaptation could amplify modelled projections of global soil carbon loss under warming, while others show that plant uptake of carbon dioxide can outpace losses from permafrost in some regions. The uncertainty is not a footnote; it is central to the projection.

What emerges from the synthesis is a call for integration. The authors argue that linking molecular tools, such as metabolomics and metagenomics, with biogeochemical measurements and ecosystem-scale observations is essential to identify which mechanisms most strongly regulate carbon and nutrient fluxes. Techniques like minirhizotrons, buried transparent tubes that allow non-destructive imaging of roots and fungi in situ, are opening windows into a world that has been largely inaccessible. The review also points to gaps in field sampling across the Arctic, where patchy coverage biases understanding of climate change impacts, and to the value of representativeness-based network design for eddy covariance and other measurement infrastructure.

Ultimately, the review frames the Arctic rhizosphere as a biogeochemical frontier: a zone where shifting vegetation and newly thawed soils are creating interactions that have never existed before in the current climate era. Whether the greening Arctic stores more carbon or releases more of it may hinge on microscopic negotiations between roots, fungi and microbes in soil horizons that most models still treat crudely. The authors close with a plea for stronger collaboration among plant and microbial ecologists, soil scientists and ecosystem modellers, because the fate of a carbon pool larger than the atmosphere’s, and the pace of global warming itself, may be decided underground, one root tip at a time.

Subject of Research: The role of root and rhizosphere processes in the Arctic carbon cycle under permafrost thaw and vegetation greening

Article Title: The Arctic carbon cycle response to changing root and rhizosphere processes amid greening and permafrost thaw

Article References: Hewitt, R. E., Blume-Werry, G., Parker, T. C., Myers-Smith, I. H., Monteux, S., Salmon, V. G., Iversen, C. M., Street, L. E., Wookey, P. A., & Mack, M. C. (2026). The Arctic carbon cycle response to changing root and rhizosphere processes amid greening and permafrost thaw. Nature Reviews Earth & Environment. https://doi.org/10.1038/s43017-026-00832-9

Image Credits: AI Generated

DOI: 10.1038/s43017-026-00832-9

Keywords: Arctic, permafrost, carbon cycle, rhizosphere, roots, Arctic greening, mycorrhizal fungi, rhizosphere priming, soil organic matter, tundra, climate feedback, terrestrial biosphere models