the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 06 Sep 2024
Simulating soil atmosphere exchanges and CO2 fluxes for a restored peatland
Abstract. Restoration of drained and extracted peatlands can potentially return them to carbon (C) sinks, thus acting as significant climate change mitigation. However, whether the restored sites will remain C sinks or switch to sources with a changing climate is unknown. Therefore, we adapted the CoupModel to simulate soil atmosphere exchanges and the associated ecosystem CO2 fluxes of a restored bog. The study site was a peatland in eastern Canada that was extracted for eight years before restoration. The model outputs were first evaluated against three years (representing 14–16 years post restoration) of eddy covariance measurements of net ecosystem exchange (NEE), surface energy fluxes, soil temperature profiles, and water table depth data. A sensitivity analysis was conducted to evaluate the response of the simulated CO2 fluxes to the thickness of the newly grown mosses. The validated model was then used to assess the sensitivity of changes in climate forcing. CoupModel reproduced the measured surface energy fluxes and showed high agreement with the observed soil temperature, water table depth, and NEE data. The simulated NEE varied slightly when changing the thickness of newly grown mosses and acrotelm from 0.2 to 0.4 m but showed significantly less uptake for a 1 m thickness. The simulated NEE was -95 ± 19 g C m-2 yr-1 over the three evaluation years, and -101 ± 64 g C m-2 yr-1, ranging from -219 to +54 g C m-2 yr-1 with an extended 28-year climate data. After 14 years of restoration, the peatland has a mean C uptake rate similar to pristine sites, but with a much larger interannual variability, and under dry years, the restored peatland can switch back to a temporary C source. The model predicts a moderate reduction of CO2 uptake, but still a reasonable sink under future climate change conditions if the peatland is ecologically and hydrologically restored. The ability of CoupModel to simulate the CO2 dynamics and its thermal-hydro drivers for restored peatlands has important implications for emission accounting and climate-smart management of drained peatlands.
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CC1: 'Comment on egusphere-2024-2679', Aldis Butlers, 18 Sep 2024
The article generally focuses on presenting the model outputs. It would be valuable to add more information how the model overfitting was avoided and how the model was validated. Many assumptions on model inputs are made, therefore it would beneficial to evaluate sensitivity of the assumed parameters variation on the model outputs. It should be ensured that the results acquired on multiple times higher C removals compared to direct measurement results is a valid model outcome. Without throught documentation of validation procedure using data outside dataset used for model parameterization it may be premature to conclude that the model can simulate the coupled hydrology, heat, and CO2 fluxes of a restored peatland and can be used for exploring land-use change issues, suggesting climate-smart management practices, and Tier-3 reporting. The scarcity of ground-truth NEE measurements at rewetted sites also indicates challenges of proper validation of the models. Rushed conclusions should be avoided.
It should be more clearly reported what is the criteria applied to define the peatland as restored. It was mentioned that natural succession of introduction of trees can be noticeable in the study site, indicating that the site conditions are probably not suitable for maintaining typical bog characteristics. Is that safe to assume that the site is oligotrophic? After peat layer removal groundwater may reach upper layers of peat altering nutrient conditions.
Citation: https://doi.org/10.5194/egusphere-2024-2679-CC1 -
CC2: 'Reply on CC1', Hongxing He, 23 Sep 2024
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-2679/egusphere-2024-2679-CC2-supplement.pdf
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CC2: 'Reply on CC1', Hongxing He, 23 Sep 2024
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RC1: 'Comment on egusphere-2024-2679', Katharina Jentzsch, 30 Sep 2024
General comments
In their manuscript titled “Simulating soil atmosphere exchanges and CO2 fluxes for a restored peatland”, the authors used the CoupModel to simulate soil atmosphere exchanges, with a focus on CO2 fluxes, for a peatland in Quebec, Canada that was restored 14 to 16 years prior to the study period. To evaluate the model performance, the authors compared the simulated energy fluxes, soil temperatures, water table depth and NEE to in-situ measurements performed at the site during the study years. To simulate and test the effect on NEE of further time passing after restoration, the authors tested the sensitivity of the simulated NEE to changes in acrotelm thickness. They concluded that the model is well-suited to simulate soil atmosphere exchanges of a restored peatland. To assess the effect of interannual climate variability on CO2 uptake by the restored peatland, the authors quantified the variation in annual NEE using meteorological data from 28 years as climate forcing for the model. Comparing the results to the CO2 fluxes measured over 15 years at a nearby pristine peatland showed that at the restored site CO2 uptake was more strongly reduced by dry conditions. In an additional simulation the authors tested different climate change scenarios affecting air temperature and/or precipitation and showed that restored peatlands will likely maintain their function of taking up CO2 under a changing climate.
As explained by the authors, restoration of peatlands can return them to carbon sinks and thereby counter climate change. Considering the large areas of drained and extracted peatlands worldwide, investigating the current and predicting the future C uptake function of restored peatlands is a highly relevant scientific question with important implications for future management of restored peatlands and that is well within the scope of BG.
Besides simulating the CO2 exchange for a specific restored peatland, the authors increase the relevance of their study by evaluating the general applicability of the CoupModel to restored peatlands and by providing the model code for use by other researchers at other peatland sites. The scientific methods applied in the study are clearly outlined either within the manuscript or in earlier publications that the authors refer to. The authors thus ensure that their study results are reproducible.
The model used in the study has already been developed and published before. The novel part is that the authors showed that the model is also applicable to restored in addition to drained and pristine peatlands and can therefore now be used to simulate the full succession of land use change in peatlands.
The second main conclusion of the manuscript is that restored peatlands are more vulnerable to dry conditions than pristine sites and confirms the findings from earlier studies that are cited in the introduction of the manuscript.
The conclusions drawn from the study results are relatively broad considering that the results were derived using a model setup for one restored peatland and one point in time after restoration only. While the technical evaluation of CoupModel for restored peatlands is sufficiently supported by the results, the content-related conclusions on the resilience of restored peatlands to climate variability and on the emission factors of restored peatlands should be verified in future studies based on data sets from more study sites and years. Is it valid to call it a “long-term model simulation” if you use climate forcing for 28 years but assume that the peatland remains in a state of 14 to 16 years after restoration?
The overall presentation of the study is very clear and easy to follow as it uses a clear structure defined by the research objectives as well as fluent and precise language. The figures are very clear and easy to understand and therefore nicely support the study results.
Specific comments
Title:
ll. 1-2 Please consider revising the title to make it more precise. Which “soil atmosphere exchanges” do you mean? CO2 fluxes are a “soil atmosphere exchange” too.
Introduction:
In line 264 of the results section you briefly mention that the “thickness of the newly grown mosses […] partly represents the time since restoration”. To motivate your research objective 2) (testing the model sensitivity to changes in acrotelm thickness) it would be helpful if you could elaborate the structural changes in a peatland with time after restoration in one paragraph in the introduction.
Methods:
You state in your study objectives (l. 123) that one of your aims is to adapt the CoupModel for restored peatlands. Please expand a bit on lines 155-157 to elaborate the changes you made to CoupModel compared to the model setups for pristine and extracted sites. A schematic showing the layers of the soil profile that were considered in the CoupModel for pristine, extracted, and restored peatlands, respectively, would be helpful.
Results:
At the end of each paragraph of the model evaluation in section 3.1 you speculate about the reasons for deviations of model results from observations. As this is already an interpretation of your results, I think that it would be good to move combine the respective sentences to form an additional paragraph in the discussion section of your manuscript. I think that your finding of model deficiencies in simulating the time of phase changes in spring is a relevant discussion point. Maybe you could even add a sentence on how the model performance for this time of phase changes could be improved in the future?
l. 267 How did you choose the new acrotelm thicknesses of 20 cm, 40 cm, and 100 cm? At what times after restoration would you expect to find such acrotelm thicknesses? Please elaborate.
l. 271 In the introduction you mention that the main difference between pristine and restored peatlands that also leads to the higher vulnerability of restored peatlands to dry conditions is the missing mesotelm collapse layer in restored peatlands. If I understand correctly, by testing the sensitivity of modelled CO2 fluxes to the acrotelm thickness you aim to estimate how much the CO2 fluxes in a restored peatland can be expected to change with time after restoration. Do you think that your assumption that a mesotelm collapse layer does not develop in a restored peatland with increasing moss layer thickness is valid? Or could it be that with this model setup you artificially introduce a difference between pristine and restored peatlands that in reality disappears with time after restoration?
l. 278 Is this something that you would expect to happen in reality? Does this also happen in pristine peatlands?
l. 301 In the introduction and in the methods section you mention that the CoupModel has previously been used to simulate CO2 fluxes at Mer Bleue. Could you potentially add these simulated fluxes to the comparison of CO2 fluxes between the pristine Mer Bleue and the restored BDB site? I am wondering how comparable the model results from BDB and the observations from Mer Bleue are. Comparing model results from both BDB and Mer Bleue could help to rule out the possibility that model deficiencies introduce a higher sensitivity of CO2 fluxes to climate variability. This could be especially beneficial as the model results were evaluated against EC measurements only for years where climate conditions did not deviate much from the long-term mean (l. 289).
l. 305 Maybe mention that the average rate of CO2 uptake reported here is the same as the emission factor for BDB that you refer to in the discussion to avoid confusion about the terms.
Discussion:
l. 380 I think that this is an important conclusion that should be supported by data added to the results section. Can you estimate around what time after restoration a new acrotelm thickness of 40 cm could be reached?
Do you think that the remnant ditches will have a significant effect on the C balance of restored peatlands over long time periods after restoration? Should their effect be considered in future studies when evaluating the differences in the C uptake function between pristine and restored peatlands?
Conclusion:
You could consider adding your finding that restored peatlands will likely maintain their C uptake function under future climate change.
Figure 3:
Would it be more correct to display GPP and NPP with a negative sign similar to NEE? Consider defining the abbreviation NPP, as NEE, GPP and ER have also been explained in the main text.
Figure 7:
Would it be more correct to display GPP with a negative sign similar to NEE?
Technical corrections
Please note that I am not an English native speaker - therefore please consider the following as "suggestions" rather than "corrections".
l. 24 “to” instead of “of”?
l. 31 “in dry years” or “under dry conditions”?
l. 122 The word “fluxes” can be left out as CO2 is also exchanged between soil and atmosphere.
l. 183 Do you mean “C dynamics”?
l. 278 “cannot” instead of “can’t”
ll. 280, 315, 316 To me these sentences are all missing an “influence/impace/effect of” e.g. l. 280 “Effect of interannual climate variability on …”
l. 341 “… shows that CoupModel …”
ll. 382 – 383 Is the unit correct or should it be -100 mb as in line 265?
Citation: https://doi.org/10.5194/egusphere-2024-2679-RC1 - RC2: 'Comment on egusphere-2024-2679', Anonymous Referee #2, 13 Nov 2024
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