the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 03 Jun 2025
Enhanced CO2 Emissions Driven by Flooding in a Simulation of Palsa Degradation
Abstract. Climate change is predicted to put most of the permafrost habitats in the discontinuous zone, at risk of disappearing within the next few decades. On a decadal scale, abrupt permafrost thaw may result in larger C losses than gradual permafrost thaw, but drivers of C emissions are poorly understood. To investigate this, we measured C emissions from a palsa under simulated abrupt and gradual thaw scenarios. We continuously measured CO2 and CH4 emissions while deepening the permafrost table under flooded (abrupt) and non-flooded (gradual) conditions. Higher soil-moisture during permafrost thaw is commonly associated with decreasing CO2 and increasing CH4 emissions. Interestingly, our results showed consistent CH4 uptake across all the cores from the palsa and a twofold increase in CO2 emissions under abrupt thaw (flooded conditions). Peat quality analysis (FTIR) showed a higher degradation of C compounds at the permafrost table, likely due to the physical disruption of soil organic matter and the redox changes in the active layer caused by flooding. Averaged CO2 emissions were significantly higher under abrupt thaw (150 mg-CO2. m-2.h-1) compared to gradual thaw (70 mg-CO2. m-2.h-1), with limited permafrost peat contribution. Conversely, permafrost thaw under gradual thaw contributed to a twofold increase in CO2 emissions (57 to 98 mg-CO2. m-3.h-1). Finally, CO2 emissions increased with depth in saturated fens, suggesting that deep-rooted vegetation could be a transport pathway for CO2 outside the growing season. Our findings underline the potential for increased CO2 emissions during the transition to fen conditions under abrupt thaw scenarios and therefore the need for in-situ measurements.
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Status: final response (author comments only)
- RC1: 'Comment on egusphere-2025-1792', Anonymous Referee #1, 18 Jun 2025
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RC2: 'Comment on egusphere-2025-1792', Stanislav Chuvanov, 21 Sep 2025
General comments:
The presented study is both relevant and of interest to the scientific community, and it fits well within the scope of the journal BG. The mesocosm method is widely used in many studies and is well justified in the framework of this research; detailed experimental schemes and photographs will allow other researchers to reproduce the experiment. It is worth emphasizing the complexity of conducting such experiments and, more generally, the insufficient experimental investigation of the effects of permafrost thaw and soil moisture on CO₂ emissions from palsas (this issue is most often studied through computer modeling, with a focus on air or soil temperature). In this article, the authors attempt to fill this knowledge gap, which is undoubtedly important. The language is clear and precise, and the article is well structured and easy to follow. There are some questions regarding the conclusions, which could perhaps be made more specific, as well as some questions concerning the methodology, which will be discussed further. Overall, with minor revisions, I recommend the article for publication.
Specific comments:
- Please clarify how the authors distinguished the effect of soil temperature from (i) the direct physical process of thawing and (ii) the effect of flooded. If the intention is to assess specifically the influence of flooded (or soil moisture), this effect should be explicitly separated from the permafrost-thawing process. In particular, please state in Section 2.3.3 the time interval between the thawing stage and the addition of water. If this interval was short, CO₂ emission may have been driven predominantly by the rise in soil temperature and by the physical release of CO₂ previously trapped in permafrost, which would complicate attribution of the observed CO₂ emissions to soil flooding.
- In the context of your study, it would be valuable to measure CO₂ efflux (soil respiration) in situ at both the fen and the palsa to determine whether the differences observed in the laboratory mesocosms are reproduced under field conditions. The literature reports diverse findings: some studies find higher CO₂ efflux from palsas than from fens due to drier conditions in palsas and the suppressive effect of anaerobic conditions in fens, whereas other studies report equal or greater CO₂ emissions from fens. Because mesocosm experiments that closely approximate natural conditions are relatively scarce, it would be particularly informative to assess the correspondence between your laboratory mesocosm results and field measurements.
- Why were samples collected in winter? In this case, how long does it take for CO₂ efflux to stabilize after thawing the samples? The thawing process strongly affects CO₂ emissions; although a 12-week stabilization period is likely sufficient in many cases, did you test stabilization time experimentally?
- The manuscript would benefit from a clearer discussion of the temporal distinction between abrupt thaw and gradual thaw.
- Why was a peat sample from a fen affected by gradual thaw used as the control (Figure 2)? It seems more appropriate to use a palsa sample unaffected by thaw as the control, and then to compare it with the effects of gradual and abrupt thaw. Please explain the choice of control.
- The conclusion implies CO₂ transport via plant aerenchyma; however, aerenchymatous tissues primarily serve as conduits for CH₄ transport from anoxic peat to the atmosphere, bypassing aerobic, methane-oxidizing layers. This process can lead to reduced CO₂ emissions in fens with vascular plants, since it limits the oxidation of CH₄ to CO₂ (Lai, 2009).
I thank the authors for this interesting article, which has also provided me with valuable insights relevant to my own field of research :)
Citation: https://doi.org/10.5194/egusphere-2025-1792-RC2
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General comments:
The study presented here provides meaningful insights in the role of water and peat quality upon permafrost thaw, which is I highly relevant and recent research topic. BG seems to be an appropriate journal for the publication of this study.
Mesocosm incubations are a robust and established method, although the simulation of permafrost thaw via partial freeing and thawing is technically demanding and therefore only few studies exist, which the authors also correctly emphasize. The study is therefore addressing a recent and relevant research gap. The authors describe their experiments clear and concise, although open questions regarding the choice of water level, experimental time frame and data filtering remain, which are more specifically addressed in the specific comments. Briefly, the assumption of a certain water level that stays constant over the chosen time period is realistic but just one of many scenarios. Therefore, the manuscript would profit from a better justification of why these parameters were chosen the way they are. An important issue is the filtering of data: The authors state that due to saturation of the sensor, no CO2 values greater than 5.000 ppm could be accurately measured and were filtered out. However, the presentation of the results does not allow to understand the extent of this filtering and how it affects the overall results and statistics. Furthermore, the manuscript lacks any hypotheses and only a general aim is stated, which should be clarified. The conclusion section mentions an initial hypothesis which is contradicted by the results but there is no such hypothesis stated in the introduction. Generally, the objectives are rather short and it seems like this study was performed more explorative rather than having clear expectations or hypotheses. The actual difference between “abrupt” and “gradual” thaw needs also more explanation, since it is not very clear how fast abrupt thawing is defined in this study.
Overall, the paper needs some clarification and justifications but seems to be suitable for publication after addressing the concerns mentioned above and in the specific comments. The title and abstract are appropriate and the overall language and presentation are well chosen. The authors take the recent literature into account and summarize it sufficiently to understand the research gaps and limitations of the methodology.
Specific comments:
L 29f: Please specify the kinds of changes (e.g. how will the vegetation change, will it become wetter or drier, etc.)
L 42ff: What is meant by “C production”? Shouldn’t it be gas production? Also, consider changing “C decomposition” to “OM decomposition”
L 100: What was sampled in October? Or was it just an exploratory visit in order to map vegetation and active layer depth?
L 110: Please specify how the corer was modified.
L 131ff: How fast was that abrupt thaw?
L 173ff: Fluxes will be underestimated when all flux data > 5000 ppm CO2 is filtered out. With this in mind, results can still be interpreted in some way but it would be helpful to have information about the timing of these extreme values. Where they equally distributed throughout the experiment or did this problem occur only during a specific time frame? This information could be included in a graphic like figure 5 or figure S14 – S18
L 240ff: That was already explained in section 2.5.1.
L 249ff: Did you check beforehand whether the criteria for the tests (normality, homogeneity in variance, etc.) were fulfilled?
L 347f: Can you provide a rough estimate of how much higher your emissions are compared to other studies?
L 361: It would be good to have the measured and typical pH values stated here.
L 364: Did you also measure DOC after your incubation experiment? It would be interesting to see some kind of mass balance of OC over the incubation to get an idea of decomposition pathways, e.g. to see how much solid OC and DOC are transferred to gases and vice-versa.
L 381: Reads like there is a big bias caused by the dimension of the samples? Would a real-world scenario then maybe never reach anaerobic conditions because the soil dries or refreezes before?
L 391ff: I agree with the general concept of carbon release upon Fe(III) reduction and that this mechanisms can (partly) explain the results found here. However, I question that palsas are always Fe-rich. The cited work was a case-study and it would be good to see some kind of comparison of the both sites in terms of palsa formation, underlaying geology, etc. The addition of water indeed hampers oxygen availability but before Fe is reduced, other TEAs (NO3, MnO2) are used, which also needs some time. Since this study did not find anaerobic conditions immediately, couldn’t it be that Fe is still not reduced?
L 484: How realistic is this abrupt thaw scenario? Since it was not stated how long it takes under that scenario, it is hard to estimate whether this is just a theoretical scenario or realistic in permafrost regions.
Technical corrections:
L 1: Remove comma
L 44: Is Baysinger still in prep? This study is cited quite often here, which is a bit unfortune when it is still in preparation .
L 247: Parenthesis before the phrase is not necessary
L 256: R Core team not in literature list
L 260: Check the wording. “Deepest value” is slightly confusing since it also seems to be the lowest value? Do you mean the sample at the bottom of the core?
L 478: Missing space between “C” and “transport”