the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Carbon balance and emissions of methane and nitrous oxide during four years of moderate rewetting of a cultivated peat soil site
Abstract. We experimented a gradual water table rise at a highly degraded agricultural peat soil site with plots of willow, forage and mixed vegetation (set-aside) in southern Finland. We measured the emissions of carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) for four years. The mean annual ground water table depth was about 80, 40, 40 and 30 cm in 2019–2022, respectively. The results indicated that a 10 cm raise in the water table depth was able to slow down annual CO2 emissions from soil respiration by 0.87 Mg CO2-C ha-1. CH4 fluxes changed from uptake to emissions with a raise in the water table depth, and the maximum mean annual emission rate was 11 kg CH4-C. Nitrous oxide emissions ranged from 2 to 33 kg N2O-N ha-1 year; they were high from bare soil in the beginning of the experiment but decreased towards the end of the experiment. Short rotation cropping of willow reached net sequestration of carbon before harvest, but all treatments and years showed net loss of carbon based on the net ecosystem carbon balance. Overall, the short rotation coppice of willow had the most favourable carbon and greenhouse gas balance over the years (10 Mg CO2 eq. on the average over four years). The total greenhouse gas balance of the forage and set-aside treatments did not go under 27 Mg CO2 eq. ha-1 year-1 highlighting the challenge in curbing peat decomposition in highly degraded cultivated peatlands.
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RC1: 'Comment on egusphere-2024-934', Anonymous Referee #1, 21 May 2024
General comments:
Kristiina Lång and co-authors present data from four years of measuring greenhouse gas emissions (CO2, N2O, CH4) of a (moderately) rewetted former agricultural field site in southern Finland. Three treatments were established (willow, forage, set-aside (bilberry was planted but didn’t grow)). The presented dataset comprises greenhouse gas flux data for four years. In addition, a full C balance including photosynthesis, ecosystem respiration, net ecosystem exchange, carbon in the plant biomass, and net ecosystem carbon balance was calculated. Overall, it is a very detailed dataset which will certainly be of interest for biogeochemical modeling of greenhouse gas emissions from rewetted peatlands. The paper is well written and easy to read. My main remarks are:
- A knowledge gap / description of the relevance of the study is missing.
- Methodology is not always clear (see specific comments).
- The authors did a lot of modeling on measured fluxes, but it remains unclear why and what the additional knowledge gain is.
Specific comments:
The introduction nicely summarizes the current knowledge about greenhouse gas emissions from peat soils and the effects of rewetting and changing water table depths. The introduction is followed by three hypotheses but misses a description of the research gap. After reading the introduction, it remains unclear why this study was necessary and what knowledge the authors attempt to gain. Similarly, results are mostly discussed separately in relation to other studies. There’s only little comparison between treatments and the overall significance of the (new?) findings remains unclear. The end of the abstract hints towards the general challenge, but also misses the description of a research gap/challenge.
The description of the greenhouse gas measurements in the methods section is not easy to follow. It would help to get an overview of the different measured parameter first and then a description of the sampling techniques and different chambers applied. It could also help to include a figure summarizing how GP, ER, NEE, and NECB relate to each other. This could also help to explain why some fluxes have negative signs. Which is not consistently used throughout the manuscript: sometimes GP fluxes/values are described as positive values, sometimes as negative values (compare Fig. 1, Tab. 3, and L294ff).
It is unclear why some parameters were not assessed for willow (see Tab. 3). This is not described in the methodology.
A lot of outlier removal was done. It is well described and mostly justified. However, if you have 9 high values in one plot (L236), there might be an explanation to that and it feels strange that you simply decided to exclude these values.
In addition to flux measurements, a lot of modeling was done. However, it’s neither part of the introduction, nor of the hypotheses. Also, the importance/relevance of the modeling approaches in addition to measuring is unclear as it is not described.
The description of the vegetation in the set-aside plots is not clear. It is written that ‘The number of species in the set-aside plots was determined once in the summer 2021’ (L80). While in L276ff there is a species number for summer 2022. It’s also unclear how there can be 19 different vascular plants in 18 different plant species.
Technical corrections:
Graphical abstract: shouldn’t that be < 30 in the figure?
L13 11kg CH4 à what area?
L43 not clear that paludiculture is always on peat soils
L46 regarding
L51 what is an oxidized layer? Please add an example
L114 reference for Canopeo app
L127 please add information why these different chambers were used
L148 what is the Vaisala GMP-343 probe measuring?
L151 why is this done? (the shading)
L178 equation 2 does not match description
L186 what about willow?
L235 leading to
L318 why are results not shown? Could be in the supplementary
L369 why are results not shown? Could be in the supplementary
L371ff headline numberings not consistent, headings in discussion are meaningless
L375 what crop was grown in the nearby field?
L421 comparable number
Tabe 2: please explain STD
Figure 1: panel letters missing, description of panels doesn’t match y-axes, please check if colors are color-blind-safe?
Figure 2: panel letters are huge in comparison to the font in the figure. Where do the fitted values come from? What’s their level of significance? Not clear from the description.
Figure 3: unclear is it important to show the variation in CH4 fluxes in the first two years (first panel a) but not in the remaining time (second panel a) or N2O? Broken y-axes might be a solution
Citation: https://doi.org/10.5194/egusphere-2024-934-RC1 -
RC2: 'Comment on egusphere-2024-934', Anonymous Referee #2, 07 Jun 2024
The authors have measured GHG fluxes and carbon balance in a Finnish peatland site that has previously been cultivated as agricultural land as has recently been somewhat rewetted by reducing drainage. The study design includes three different vegetation options and utilises several measurement techniques to determine fluxes in and out of the soil. Overall, the manuscript is well written and the study design interesting. However, I do think the manuscript would benefit from some improvements, mainly to the justification of the research topic and methodology.
General comments:
Abstract:
This is somewhat a question of preference but I would encourage writing at least something of the justification of this manuscript to the abstract as well. Half a sentence at the end of the abstract is quite little.
Introduction:
The general background of paludiculture in cultivated peatlands is well explained but the specific justification of this paper is somewhat missing. I would have wanted to know, here or in the methods, why you chose forage, willow and set-aside (although this was a little bit better explained) options.
L41: Furthermore, you state that CH4 emissions do not compromise the mitigation potential according to literature which makes it sound like there isn’t much to research here, yet it’s still a research question in this manuscript. I would expand a little bit on this topic, seen as it’s one of the research questions (and certainly an interesting one at that).
Materials and methods:
I had a little trouble keeping in mind which areas the terms “plot, site, block and experimental area” refer to. Firstly, in L104, I’d refer specifically to the site instead of experimental area if WTD was indeed only measured from the corners of the whole site, to keep it simpler. Secondly, I know the graphical abstract should be as simple as possible, but a graph showing the study design in more detail and explaining “plot”, “block” and “site” would really help.
L69: Considering how important rewetting is for mitigating the effects of cultivated peatlands, I would have liked to see a little more information on the rewetting method itself. Now, the only information is that a control well was installed but could you maybe explain this in a bit more detail.
L104-114: There were some changes made to the ancillary measurement techniques during the study campaign. This is understandable, but did you estimate if these, namely changing the WTD measurement location and swapping LAI to green canopy cover, have any effect on the results? It would also be nice to see, for example in an appendix, how the green canopy cover responds better to photosynthesis. Now this sentence was rather brushed aside.
I’m a little confused about the modelling work here. Could you at least explain why the flux modelling was done? I would also expect to see some estimation of how the modelled fluxes compared to measurements in addition to stating the modelled and measured maximum values.
Results:
Fig 1: Please increase the text size. For example, moving the y-axis text to each plot’s headline and keeping only the unit on the y-axis would already make this much easier to read. Furthermore, is there a good reason why all the plots don’t show all treatments? For temperature and PAR this makes sense, but why is there no black line for vegetation index and daily mean GP? I’m also not sure why you haven’t included the measurements for daily mean GP. For WTD, are these results taken from the four corners of the site for the whole period or are the plot measurements included in 21-22? If the latter, I’m not sure this is a good way to show the results as it seems like there is more variability in the later years. If the former, is there another reason for the increased variance?
Table 3: Is there a reason why table 3 shows only some of the results for willow? This was perhaps addressed somewhere but I couldn’t find it, and perhaps the reason could be reiterated in the table caption.
L350: I would really prefer to see similar figures to Fig 2 from both crop vs. CH4 and year vs. CH4 although this can easily be put to the appendix (and combined as well). With such a low n-value, I don’t think it’s sufficient to say that these had no effect on CH4 fluxes only based on the p-value, particularly when in table 3, there does seem to be a clear relationship between CH4 and year. There is such a strong change during the years from a sink to a source that a very simple statistics might not capture the relationship but it doesn’t mean it’s not worth showing. I would also like to see some other metrics for measuring the relationship between these variables besides p-value. The same applies to the N2O fluxes where there are relationships between the variables but only the p-value is given. Fig 3 does provide some of this information but it is very difficult to read and to distinguish differences particularly between the vegetation options.
Discussion:
I would somewhere like to see some discussion related to the rewetting. You state in the introduction that the target WTD was reached only periodically but could you explain why that is?
Conclusions:
The last sentence of the conclusions is a bit surprising as I’m not sure that this (while certainly true) is strongly related to or clearly visible from this work.
Specific comments:
Graphical abstract: Could you explain why the graphical abstract doesn’t show any CH4 emissions? According to figure 2 and section 3.3, there should be a relationship between WTD and CH4 .
L43: Adding “i.e.” before “crop production” might make the sentence a bit easier to read.
L46: “As regards to GHG mitigation”
L101-102: Could you add references to both the FMI station and the global radiation data product?
L108: Could you explain what “WTD was measured manually from monitoring pipes when possible” means?
L117: no green vegetation was present
L233: Fig 2 should probably be Fig 1.
L268: As snow is not an important variable in this manuscript, this is a bit of a nitpicky comment, but are the snow days calculated from a calendar year or from each snow season? To my mind, calculating calendar year snow days doesn’t make much sense as it gives very little information on the impact of snow cover on any ecosystem processes.
L379: probably should be “rised” and not “raised”
L467: “considerably"
L468: “WTD does not rise"
Citation: https://doi.org/10.5194/egusphere-2024-934-RC2
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