the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 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.
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Status: final response (author comments only)
- RC1: 'Comment on egusphere-2026-3054', Anonymous Referee #1, 18 Aug 2026
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RC2: 'Comment on egusphere-2026-3054', Anonymous Referee #2, 06 Oct 2026
This manuscript describes high rates of methane uptake in soil hummocks from Arctic tundra in Greenland. This important work reinforces the importance of microtopography in modeling greenhouse gas fluxes, especially because the high surface area and resulting gas exchange of the hummocks are not directly incorporated in two-dimensional models of the landscape.
Specific comments:
Section 2.2 Were CO2 flux measurements also collected during this campaign? If so, did the results indicate any trends in soil respiration or help to explain variability in CH4 fluxes?
Section 2.3.2. What were the results of these trace nutrient and metal analyses? Do they differentiate soil horizons or sampling sites or help interpret pH differences?
Section 2.4.2 Quantitative PCR requires careful description of experimental methods due to the potential for large errors in the exponential DNA amplification and interpretation. While the MIQE guidelines are a gold standard (DOI 10.1373/clinchem.2008.112797), critical information about standard templates and methods of data analysis should always be included. This is particularly important for estimates of microbial biomass using degenerate primers targeting the 16S rRNA gene, where differences in target amplification efficiency can create large biases, and variation in gene copy number per cell complicates interpretations. See for example, Callbeck et al. 2013. J. Microbiol. Methods. 93:148, http://dx.doi.org/10.1016/j.mimet.2013.03.010
Figure 4 & Table A1. Total C and water content correlate well in these samples (Pearson r-squared = 0.73, P=0.0002). This relationship is like other high-carbon soils in the Arctic. Can the authors shed light on whether this correlation is causal (e.g. water content preserves SOC or abundant SOC binds lots of water)?
Figure 5. Focusing only on qPCR results from single hummock and non-hummock plots gives a misleading perspective on the environmental variability of metabolic potential relative to the complete dataset in Appendix Figures B1-B3, particularly for non-hummock replicates. This variability should be addressed with appropriate statistical analysis in Section 3.2 rather than summarizing that all three NH plots exhibited similar patterns. Also, consider expressing gene copy numbers in log units if the data support a log-normal model.
Figure 7. Could temporal variation in soil moisture explain some of the high variance observed for hummock soil methane fluxes?
Section 4.2. How are the effects of soil water pH and oxygen availability on CH4 uptake and oxidation distinguished here?
Technical corrections:
Introduction (lines 34-69). This long paragraph should be broken up to better emphasize separate topics. For example, after the introduction to soil C storage a new paragraph starting on line 40 could describe cryoturbation, followed another paragraph on frost-derived microtopography (line 46). A final paragraph could revisit the SOM relocation due to cryoturbation (line 57).
Citation: https://doi.org/10.5194/egusphere-2026-3054-RC2 -
RC3: 'Comment on egusphere-2026-3054', Anonymous Referee #3, 10 Oct 2026
This manuscript investigates methane uptake associated with cryoturbated earth hummocks in West Greenland and its relationship with microbial community composition, using chamber-based CH₄ flux measurements, soil physicochemical characterisation, qPCR and 16S rRNA gene metabarcoding. The topic is timely and relevant, as spatial heterogeneity in Arctic landscapes remains an important source of uncertainty in regional greenhouse-gas budgets. I particularly appreciate the attempt to link methane flux observations with soil properties and microbial community composition. The microbial results also provide potentially valuable evidence that different microtopographic settings support distinct methanotrophic communities. The manuscript is generally well structured, and the integration of pedological, biogeochemical and microbial measurements is a clear strength.
Nevertheless, several associations observed in this case study are presented as evidence of underlying mechanisms. The manuscript would be considerably stronger if the authors distinguished more clearly between observations, mechanistic interpretations and hypotheses. At present, several conclusions rely on speculation and lack sufficient statistical support to establish robust relationships.
Main comments
Temporal alignment of measurements. Soil properties and CH₄ fluxes were measured at different times, which limits the interpretation of their relationships. Consequently, it remains unclear whether the greater methane uptake in hummocks reflects better-aerated conditions and enhanced gas diffusion, differences in methanotrophic abundance or activity, or a combination of these factors. The authors should acknowledge this limitation and qualify their mechanistic interpretations accordingly.
Spatial replication and statistical analysis. The statistical analysis should account for the spatial structure of the sampling design. A mixed-effects model would be appropriate if observations are nested within sites or sampling units, with repeated measurements accounted for where relevant.
Lines 330–340. This interpretation appears speculative, as no formal statistical analysis is presented to support it. Methane oxidation cannot be attributed to thermal effects alone, and its relationship with active-layer depth need not be linear. Please test the proposed relationships formally and distinguish statistically supported findings from possible explanations.
Lines 356–359. Are the reported differences statistically significant? Please provide the relevant statistical results and clarify whether these statements describe significant differences.
Lines 269–380. I appreciate this discussion, but it needs to be linked more directly to the measurements presented in the manuscript. The authors do not show analyses relating methane uptake to soil water content or water-table position. Please provide supporting analyses where the data allow, or frame these explanations explicitly as hypotheses.
Lines 402–405. What evidence supports the proposed causal relationship? Please clarify whether it is supported by the measurements, inferred from previous studies or offered as a possible explanation.
Line 500. The statement, “Further, our data also reveals a decoupling of methane uptake rates from seasonal soil warming”, requires stronger statistical support. Please present an analysis demonstrating this decoupling and clarify how variation in gas diffusion was considered. The absence of a clear seasonal relationship alone does not establish that temperature has no influence on methane uptake.
Minor and specific comments
Page 12, Figure 7: I suggest use cumulative methane uptake or cumulative methane-C uptake. Cumulative carbon uptake would make reader think CO2 is included.
Appendix, Figure B3: Please define the error bars and report the sample size associated with each estimate.
Page 27, Appendix, Figure B4: The caption contains an incomplete sentence. Please revise it for clarity and grammatical completeness.
Citation: https://doi.org/10.5194/egusphere-2026-3054-RC3
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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).