Earth hummock soils as hot-spots of atmospheric methane uptake in Arctic tundra: a case study from Qeqertarsuaq, West Greenland

Abstract. Arctic permafrost-affected soils are expected to influence the global greenhouse gas (GHG) budget, although the magnitude of this impact remains uncertain. Methane (CH₄) and carbon dioxide (CO₂) cycling in these environments is largely mediated by soil microorganisms. Cryoturbated soil structures, leading to frost patterned ground phenomena such as unsorted circles, palsas, ice wedges and earth hummocks, are widespread in Arctic landscapes and have been linked to altered GHG fluxes. However, the role of cryoturbated earth hummocks and especially of their microbial community in regulating GHG fluxes remains unexplored. We present one of the first comprehensive assessments of GHG fluxes of cryoturbated earth hummocks in West Greenland, integrating gas flux measurements, soil chemistry and molecular biological analyses. In comparison to less cryoturbated tundra, all investigated earth hummocks exhibited higher CH₄ uptake. Molecular data further revealed an enhanced genetic potential for CH₄ oxidation in hummocks, characterized by a higher relative abundance of atmospheric (high affinity) methanotrophs, while the nearby non-hummocky tundra was dominated by low- and medium-affinity methanotrophs. Consistently, elevated copy numbers of the methanotrophy marker gene pmoA indicate that earth hummocks function as hotspots for bacterial methanotrophy. The relatively low pH in the upper horizons of the earth hummock suggests a hydrological decoupling from the minerogenic groundwater, which appears to create favorable conditions for high-affinity atmospheric methanotrophs. Overall, our results identify cryoturbated earth hummocks as strong localized methane sinks and highlight the importance of microtopography in shaping methanotrophic communities and GHG fluxes. These findings underscore the need for a better representation of Arctic tundra microtopography in upscaling GHG fluxes and for an improved mechanistic understanding of methane uptake in cryoturbated tundra soils, particularly about the coupling of hydrology, CH₄ supply, nutrient regime, and pH in regulating methanotrophic activity.