| 15 Jun 2026
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.
This manuscript highlights some interesting (not studied previously, but perhaps not unexpected) results - that methane uptake in cryoturbated hummocks in Arctic tundra is higher than in the surrounding non-hummock areas. This makes sense as the soil is more aerated, and the authors have shown how the low pH, oligotrophic hummock creates a suitable environment for methanotrophy (specifically the genus Methylocapsa). The main findings are based on a detailed analysis of total abundance of methanogens and methanotrophs (quantitative PCR and 16S rRNA gene metabarcoding), together with flux measurements. The limitation in the study was that this was an analysis of just 1 hummock, and 3 non-hummocks over the 3 summer months. Scaling up over a wider spatial area or across the year is not really possible. But nevertheless it's important work, highlighting the variability in soil uptake in tundra areas and showing that hummocks may be a larger sink of methane than initially thought and that the cryotopography of the Arctic needs to be taken into account when working out methane fluxes, so it would be a valuable publication.
The manuscript was well written, the methodology appears sound, and figures showing the results were clear. I have listed below a few detailed points for consideration.
Please give some more details about the chamber flux measurements. The collar diameter was given, but not chamber volume or other details about the chamber. Were replicate measurements made, or just 1 per week? Were samples collected at a similar time of day each week?
In Figure 5, just one of the NH profiles was shown. Looking at the others, which are in the appendix there seems to be quite a lot of variability in the PCR analysis across the 3 NH profiles. Why was this particular one chosen for the main paper? I think the variability should be noted in the paper, and considered in the discussion.
Is the NH profile in figure 6 again just one of the 3 profiles measured? If so please note that, and again consider discussing variability between the 3 NH profiles.
For figure 7 it is notable that there is a lot of variability in the methane flux for the hummock. Could you look further into the reasons for the higher uptake on some days than others? Did you try replicate measurements on the same day, to see how much variation there is in repeat measurements?
Line 333 - mentions that uptake is not affected by meteorology, but please clarify that this is just for the 3 month period studied in the summer growing season. We might expect it to be more affected in the shoulder seasons that weren't studied. Which meteorological parameters were considered other than temperature? Was atmospheric pressure also considered when you looked at whether there was a meteorological influence?
References:
Generally good referencing with relevant publications included.
Line 46 - publication date for French is 2018 in the reference list.
Line 53 - add et al., after Kokelj
Line 66 - Angel et al. is 2012 in reference list.
Line 90 - is there a report for the IUSS Working Group WRB, 2022 that could be referenced and included in the reference list?
Line 183 - delete et al., after Knief (only 1 author according to the reference list).
I couldn't see Vekeman et al, 2016 or Awala et al., 2023 (lines 299/300), Dutaur & Verchot, 2007 (line 421), or Dedysh et al., 2005 (line 429) in the reference list.
Saunois et al., 2020 (line 394), should perhaps be replaced with the more recent Global Methane Budget (Saunois et al., 2024).