the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 02 Dec 2025
Soil depth and nutrient status are stronger drivers for short-term production and decomposition in temperate fens than water regime in a climatically dry year
Abstract. This study investigated whether rewetting of drained temperate, groundwater-fed fens results in consistent shifts of above- and belowground production and litter mass loss of vascular plants, as compared to undrained fens, and how these processes relate to abiotic and biotic factors. We tested the hypotheses that (1) due to higher availability nutrient rewetted and drained fens exhibit higher production, (2) due to higher decomposability of plant biomass rewetted fens show higher plant biomass decomposition than undrained, and (3) in rewetted fens, less peat is potentially formed than in undrained fens. We analysed the effects of hydrological status (undrained, drained, rewetted) on plant production and litter mass loss during one year in 39 peatland sites across temperate Europe. Above- and belowground productivity, as well as mass loss of autochthonous vascular plant litter and Rooibos tea were measured above- and belowground.
Aboveground vascular plant production was higher in rewetted fens than in undrained fens, in line with our first hypothesis. This difference was linked to the proportional graminoid cover and N content in aboveground biomass. In contrast to our first hypothesis, belowground production did not differ between fens of different hydrological status. It was highest in the 0–5 cm horizon, where soil temperature, and microbial biomass were also highest. Likewise, and in contrast to our second hypothesis, decomposition of above- and belowground vascular plant biomass was not affected by hydrological status. Only Rooibos tea mass loss showed small, ecologically irrelevant differences between differing hydrological statuses. Decomposition of aboveground biomass at soil surface was higher with higher nitrogen and phosphor content of the biomass, higher soil temperature, and higher cover proportion of herbs in the vegetation. Belowground biomass loss was positively correlated with phosphor content in soil porewater and aboveground biomass. One-third of the overall belowground biomass production took place in the 0–5 cm horizon, while decomposition in this horizon was lower than in the subsoil, irrespective of hydrological status. Our third hypothesis was also not confirmed because belowground production, decomposition and peat formation potential did not differ between the hydrological status. Although aboveground vascular plant production was higher in rewetted fens higher than in undrained, this difference will not result in a higher peat formation potential in rewetted fens due to the strong decomposition in of the easily degradable aboveground vascular plant biomass on a long time. Rather than hydrological status, soil depth, and nitrogen and phosphor availability had stronger impact on short-term vascular plant production and mass loss in the temperate fens studied. All these results were obtained during an extremely dry and warm growing season throughout Europe and causing deep drop downs of the water level at most sites. These meteorological conditions may have impacted the observed pattern of productivity and decomposition in unknown way.
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Status: closed (peer review stopped)
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RC1: 'Comment on egusphere-2025-5352', Anonymous Referee #1, 16 Dec 2025
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AC1: 'Reply on RC1', Camiel Aggenbach, 27 Feb 2026
Dear editor/ reviewer,
We have processed the comments of reviewer #1 and are happy to submit a new version of our manuscript. We experienced the comments as very constructive, and these helped us to improve the manuscript. For this we thank the reviewer. All comments we could deal with, and lead to modifications in the manuscript. In the document “20260227_Respond to comment on egusphere-2025-5352 Anonymous Referee 1 16 Dec 2025.pdf” we answered the reviewer’s comments.
Kind regards
Camiel Aggenbach
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AC1: 'Reply on RC1', Camiel Aggenbach, 27 Feb 2026
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RC2: 'Comment on egusphere-2025-5352', Anonymous Referee #2, 14 May 2026
General
I was excited to review a manuscript on this important topic that has thus far received too little attention. Also, I was delighted to see the number of sites in the material.
An acknowledged complication of the one-year study was that the year happened to be exceptionally dry. In peatlands, where the water level is a critical process regulator, this likely affected the results. The title framed this nicely, the practical issues such as, e.g., the decomposition frame upheaval were described in Methods, and the authors discuss the potential impacts at the end of Discussion. However, I found the discussion part to remain quite disconnected, and suggest that the potential impacts of drought are already concisely discussed in the previous sections of Discussion along with the results of the study. The subject matter in the specific section (lines 539-556) can easily be split to be presented with the general discussion on biomass production, decomposition, and – ultimately – peat formation potential.
Also, I was wondering if the drought could be brought up in the Introduction, including the hypotheses. It seems to me that the hypotheses remained on a general level, not considering the drought, even though the title would allow, and even suggest, drought effects to be incorporated in this part of the manuscript, too, even though they cannot be directly estimated based on the one-year data with no “regular year” as comparison.
The Material and Methods section would benefit from some added information; see detailed comments. Also, please describe on which grounds you have estimated that the undrained, drained, and rewetted sites represent originally similar fen types to the extent that comparing them is relevant.
It is good that pairwise comparisons of the sites were not focused on in the analysis, but a more general evaluation of patterns between the hydrological status groups was done. The extensive data were rigorously analyzed with RF. Yet, it occurred to me that the use of some further analytical method, such as structural equation modeling, to analyze the direct and indirect effects of your impressive explanatory variable set on production and decomposition, and, ultimately, the organic matter accumulation potential, might give further insights into the functioning of the fens. With such an analysis the authors could evaluate a conceptual model on how the different variables are linked to each other, formulated based on the their ecological knowledge of the systems. Not sure how well the data would facilitate the use of that method, but it might be worth exploring (if not tested yet).
Concerning presentation, the language needs some polishing: for instance, word order is not always the clearest for easy understanding the statements, words or punctuation are missing in some sentences, and the spelling of some words, such as phosphorus, need checking. I will not be detailing these, since easy tools for language check are available. Restructuring some paragraphs would improve the flow of the text and understanding the progress, both in Methods and in Results.
Detailed
Line 50: “…this difference will not…” – I would tune this down a bit (“may not”) as the decomposition rates measured were likely affected by the drought.
Lines 53-55: rather than the bit vague last sentence, you could simply state something like “This should be borne in mind when interpreting the results”.
Line 60: perhaps specify “carbon sink function through peat accumulation”, as in some cases, drained peatland forests may remain a carbon sink at the ecosystem level and even show small soil C sink.
Lines 69-70: The phrase “Whether rewetting…” repeats the end of the previous paragraph.
Lines 78-81: While these statements are still generally valid, there is literature on these topics that you seem to have missed both here and in Discussion. The reference from 26 years ago paints a somewhat gloomier picture than what is currently available. I am listing here some papers that might be of interest, but please note that I am not suggesting that you necessarily need to cite any of these:
https://doi.org/10.1139/b96-116
https://doi.org/10.1139/b03-043
https://doi.org/10.1111/gcb.13934
https://doi.org/10.1016/j.scitotenv.2022.159683
https://doi.org/10.1007/s10021-005-0070-1
https://doi.org/10.1890/0012-9658(2000)081[3464:ROBAFP]2.0.CO;2
https://doi.org/10.1111/jvs.12912Line 93: I think that you are not able to examine hypothesis 3 as currently formulated, since one year is too short for examining peat formation, and also your results will only reflect drought conditions. Please revise.
Line 97: Instead of “in the top, as well in the subsoil” you could define the actual depth range.
Lines 103-104: Please refer to Table S1. Also, I suggest adding a figure with a map and a schematic presentation of the study design, including the different measurement points. The latter would help following the method description, which is a bit complex simply because you measured so many parameters – which is of course excellent as such.
Line 108: Classification into hydrological states may be of less concern than including sites that are actually comparable (originally the same ecosystem type), so please add here also a description of how you ensured that was the case.
Lines 123-124: Mowing is likely to impact the ecosystem structure, including plant biomass production, so please tell some more (frequency, timing) about that, in the Supplement if it cannot be done concisely. Mowing of undrained sites means that they are not in a fully natural state, which is important to bear in mind when comparing them to unmanaged fens.
Line 127: the subheading remains a bit uninformative – could you rephrase to exclude the vague “other variables”?
Line 129: Are your functional plant groups the same as the more commonly used plant functional types? In any case, please give some more information on these, e.g., in a supplementary table. Could you actually list the contents of these for all sites? The information that you currently give on vegetation composition is very scarce, just the dominant plant species in Table S1.
Line 144: Perhaps a discussion matter: Laiho et al. (2014; https://doi.org/10.1007/s11104-014-2225-3) compared peat versus inorganic substrates in ingrowth cores in boreal sedge fens, finding that under dry conditions inorganic substrates yielded lower belowground production estimates than peat. This suggests that you may have somewhat underestimated belowground production.
Line 147: The pooling of the two layers probably means that you cannot visualize the depth distribution of belowground production? That would have been interesting to see as a supplementary figure.
Line 155 onwards, section 2.4: This section, especially, could become easier to follow through some reorganization. Also, could you please describe what the “representative plant materials” were for each site, and provide a schematic figure of the frames, in the supplement? On line 183, please specify the length of the incubation period(s). Perhaps note in the Discussion how the exclusion of meso- and macrofauna may have affected the mass loss rates. Also please discuss a bit the limitations of one-year decomposition data, which does not necessarily correlate with longer-term mass loss dynamics (e.g., Latter et al. 1998: https://doi.org/10.1007/s004420050357 ; Moore et al. 2017: https://doi.org/10.1007/s11104-017-3228-7).
Line 204 ,“we used the averaged…”: please explain this specifically for the two different depth segments. Or did you combine these two (which would make no sense)?
Line 261: can you give a reference for TURBOVEG?
Line 263: What was the division of nutrient-rich versus nutrient-poor conditions based on? I mostly agree, but some species were categorized differently from what I would expect.
Line 266, “feather mosses”: earlier, you call these “brown mosses”. I would stick to that term, since feather mosses commonly refer to species like Pleurozium schreberi and Hylocomium splendens.
Lines 279-282: one-way ANOVA – so there was no other hierarchy in the data that would have required the use of mixed model even in the simpler case? Can you present the two-factor model structure somewhere?
Results, section 3.1: I am not certain that the first paragraph is needed at all: the detailed results follow in the next paragraphs, and those suffice. For any non-significant patterns that you wish to mention (e.g., lack of interactions), you could add the information after presenting the significant patterns. If the readers need a reminder of which variables were measured, they can check Fig. 1. However, if you choose to keep it, please restructure it so that it simply states for which variables significant patterns were, or were not, found, putting the lists in the actual results statements, not in separate phrases.
I would suggest restructuring the section on decomposition. Start with aboveground decomposition, stating whether there were or were not significant patterns. Then tell the same for belowground. That is easier for the reader, and follows the structure of the production paragraph above. It is good to maintain consistent structure where possible. Also, I suggest presenting all production-related results, including the depth patterns of belowground production, before moving to decomposition and ultimately, the accumulation potential.
Lines 329-330: PFP from aboveground plant biomass would be a better phrasing: similarly, PFP from belowground biomass. Please present the accumulation potential results separately, after first presenting also the decomposition results, as those are also needed for the calculation, and understanding of, the accumulation values. Please provide also a total PFP value from both aboveground and belowground biomass.
Line 346: The paragraph starting here currently seems to hold all depth-related results mixed together. While that is one option, I would quite strongly recommend presenting the results rather as per variable type: production, decomposition, accumulation potential. These are the factors of interest, and depth-related patterns are just detailed features of those. Mention if the hydrological status affected the depth-related patterns or not.
Line 360, caption of Fig. 2: b) plant biomass, or plant biomass production? e) and f) Did you pool these that across hydrological status because hydrological status did not significantly affect the patterns, or for what reason? Please tell; do not make the reader to guess. Please rather use an abbreviation that more intuitively makes sense in the English languate. A quick reader will think that RT means Rooibos tea. Also, as the end part of the term is the same for both aboveground and belowground, make their abbreviations similarly corresponding as well. On the other hand, I do not see a strong reason for using the abbreviations at all, as it seems that Aboveground and Belowground, defined in the caption to relate to vascular plant biomass, are just as clear without the abbreviations added.
Figures 2 and 3: I am quite puzzled why you present the production and accumulation results always together, even though accumulation also depends fundamentally of decomposition. Another option for organizing the figures would be putting all depth-related patterns in a specific figure, including current panels 2 e) 2 f), 3 e) and 3 f). That would be visually more pleasing, and present accumulation in the context of both production and decomposition. While the current structuring would align better with my suggestion of presenting the results in the text as per variable type (and not combining all depth-related patterns together), that is not decisive as you can refer to multiple figures from one result statement, as also from new statements to figures already mentioned.
Line 384: “Belowground plant production” – “additionally”, as you have already described the depth patterns in the previous sentence? “Positively linked” or “negatively linked” – I would see it as the latter?
Line 385: Rather than “exceeded”, it would be clearer to state whether the water levels were higher than, or lower than, 10 cm above surface. Further, 10% of what? Bulk density is often presented as kg/m3, or some other mass-per-volume unit, so % may not be intuitively understandable for all readers. You actually have kg per liter as the unit in the figure.
Discussion, general: Please bear in mind all through the Discussion that you are dealing with an exceptionally dry year, and not necessarily general patterns. Otherwise, you may be unintentionally a bit misleading. I would suggest that you add in each section a concise consideration how the drought summer may have affected the results, and which patterns may be considered more generally valid. You have a lot of knowledge of these systems in the author team and that, with support from earlier published research it should be doable. Whereever you really do not feel that you can estimate the directions of the drought effects, please tell that, too. As all the sites were mowed or grazed, please also mention whether you think that may have affected plant biomass production and if so, how.
Lines 441-446: Could these patterns be caused by the dry year? Please refer to earlier published results on the relationship between biomass production and WL for fens for a bit more insight on this.
Lines 451-452: How was the depth distribution compared to other studies in fens?
Lines 464-468: This should rather be in Results than Discussion.
Line 482: “overrules” - Could this be because the water levels were generally low? How does this relate to other studies that examined the effects of WL or hydrological status on decomposition? E.g., https://doi.org/10.4141/cjss88-073 ; https://doi.org/10.1111/j.1365-2486.2011.02503.x
Line 483: What might these ”other factors” be?
Line 487: Based on other studies utilizing rooibos, what does rooibos mass loss generally describe? How does it generally relate to native litter mass loss?
Lines 494-495: This is quite surprising, and may suggest that the surface was too dry for efficient decomposition because of the drought. Can you please check other studies on belowground decomposition in peatlands and discuss a bit further?
Line 501, heading: Please rephrase to peat formation potential
Lines 519-520: Is this in Results? If not, please add; even more generally, the belowground to aboveground ratios are of interest.
Line 521: Rather “likely contributes”, bearing in mind that your year was exceptional.
Lines 524-525: Again, what might they be?
Lines 528-529: How about the potential impact of the drought?
Lines 545-550: Perhaps condense this to a simple statement that respiration-based results have varied, since you did not measure respiration; rather focus on discussing factors included in your data.
Citation: https://doi.org/10.5194/egusphere-2025-5352-RC2
Status: closed (peer review stopped)
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RC1: 'Comment on egusphere-2025-5352', Anonymous Referee #1, 16 Dec 2025
General comments
The manuscript by Aggenbach et al. addresses a timely and important topic: the effects of drainage and rewetting on biomass production and decomposition, key processes underlying peat formation and carbon storage in temperate fens. The study is thorough, with a large number of sites and well-executed methods. The finding that nutrient availability and soil depth had a stronger influence on productivity than hydrological status, particularly during an unusually dry and warm season, is insightful and highly relevant given the increasing frequency of European heatwaves. This result has important implications for peatland restoration, suggesting that rewetting alone may not be sufficient to restore carbon sequestration potential and that nutrient regimes and peat depth should also be considered.
A clearer introduction and a more concise, confident conclusion would strengthen the abstract and better communicate the significance of the study. The methods are generally sound, though a justification of the ingrowth core substrate for belowground production and clarification of how the small number of replicates per site were incorporated into statistical analyses would strengthen the manuscript.
Overall, the study provides valuable insights into the drivers of fen ecosystem functioning under changing climate conditions and will be of interest to the peatland and carbon cycling research community.
Specific comments
Abstract:
The abstract would benefit from a clearer introduction to the broader context and importance of the research topic. It currently opens with the study hypotheses but does not provide sufficient background on why the topic matters. I recommend beginning with a few sentences that highlight the role of peatlands as major carbon sinks, the existing gaps in our understanding of carbon cycling in these systems, and how this study aims to address those gaps. For instance, in-situ measurements of biomass production and decomposition, particularly belowground, are still underrepresented in fen peatland studies, even though they are crucial for understanding the effects of drainage and rewetting on biogeochemical cycling and carbon storage in these managed ecosystems.
The results section of the abstract is detailed, but the emphasis on hypotheses could be reduced in favor of clearly presenting the most important findings and their implications. The final sentences currently sound overly cautious and may unintentionally undermine confidence in the results. Rather than framing the extremely dry and warm growing season as a limitation, I encourage the authors to consider discussing the relevance of these conditions, given that many peatlands have been experiencing similarly warm and dry summers over the past decade(s). These observations are not simply anomalies but reflect ongoing climate change, which makes your results particularly timely and valuable.
Introduction:
L59–60: Please clarify whether this statement refers to a global context.
L72–73: Reference is missing for this statement.
L72–73: The end of this sentence is unclear. I suggest rephrasing as: “In fens, peat accumulation is strongly influenced by belowground biomass from vascular plants, especially sedges (Carex spp.), which often provide the largest share of organic inputs.”
L76–77: End the sentence after “rare” and start a new sentence afterwards.
L76–80: Some redundancy exists regarding the scarcity of belowground data. I suggest merging these lines into one sentence: “In situ observations of belowground biomass production and root responses in fens are generally rare…”
L83: Reference is missing.
L88–89: Consider naming some of the abiotic factors you investigated, providing examples, or simply referring to Fig. 1.
L93–94: Rather than “displacement peat,” you might say “less peat is formed by root ingrowth”, which is more straightforward and emphasizes the relevance of your study.
Methods:
The large number of sites and the broad geographic and climatic coverage are a major strength of the study. A map showing the spatial distribution of the 39 sites across Europe would greatly help readers visualize this gradient. I suggest citing Table S1 of the supplements already in the opening sentence of the site description, so readers immediately know where to find detailed metadata on site characteristics and history. This would facilitate understanding of the regional grouping and site categorization.
Further, the description of rewetted sites would benefit from more explicit information on management history, such as the approximate time since rewetting for individual sites or site groups. While the authors note that rewetting occurred “a few years to several decades” (L116-117) prior to sampling, a more constrained range or categorical grouping would improve transparency and allow readers to better assess potential legacy effects of drainage and rewetting.
L144: I acknowledge the substantial logistical effort involved in implementing ingrowth cores across such a large number of sites. I also understand that using autochthonous peat material is often not feasible on this scale (three replicates per site across 39 sites makes n=117 cores in total?; please confirm). However, the sand–nylon substrate differs markedly from peat in key properties relevant for root growth, despite the authors' attempts to resemble the average peat bulk density of all sites. Variables such as water-holding capacity, nutrient availability, oxygen conditions and microbial activity must differ substantially between peat and sand substrates. Justifying this choice and discussing how these differences may influence root and rhizome ingrowth relative to the surrounding peat would help readers to better interpret the belowground production estimates.
L145: Since the dominant species are sedges (L258-269), please explain how vertically installed ingrowth cores capture realistic belowground production in species that have horizontal rhizome growth but also, to a great extent, vertical root growth. Could that have led to an underestimation of root compared to rhizome growth or even an underestimation of root production in deeper soil depth (L450-452). If yes, was this accounted for?
L160-161: Please also indicate the mesh size of the Rooibos bags then.
L200-206: Please add a formula to this description of calculating the peat formation potential. Is this method based on any reference?
L275-302: The authors state that replicate measurements were averaged per site and that these site means were used for subsequent statistical analyses. This approach is appropriate for avoiding pseudoreplication, however, it would be helpful to clarify consistently which explanations apply to which analyses. In particular, for the random forest analyses it remains unclear whether site-averaged values or individual replicate measurements were used, and how within-site dependence was handled in this context. Given the relatively small number of replicates per site and the high spatial heterogeneity typical of fens, a brief clarification of the unit of replication for each analysis would strengthen confidence in the robustness of the results.
Just a site note, why was the ANOVA performed with a different R version (4.2.) than the random forest analysis (4.3.1)?
Results:
L325: “mass loss of local/root belowground biomass”
Figure 2 and 3: Several figures in the manuscript use color schemes that may be difficult to distinguish for readers with color vision deficiencies. Ensuring figures are colorblind-friendly is important because it improves accessibility, allows all readers to accurately interpret the data, and avoids misinterpretation of patterns or trends. I recommend using color palettes that are designed for colorblind accessibility (e.g., viridis, RColorBrewer, scico or colorspaces are some examples for R packages I can recommend) or adding distinct symbols/patterns in addition to color to convey information. Very helpful tool: https://www.color-blindness.com/coblis-color-blindness-simulator/
Discussion:
L539-556: I assume the conditions observed during the study year were extreme compared to long-term averages calculated over several decades. It would be helpful to put these conditions into context relative to the last decade as well. Were such warm and dry conditions still very unusual, or have similar events become more frequent? Providing this context would help readers interpret the observed patterns of productivity and decomposition in light of ongoing climate trends.
L502: A reference is missing.
Technical corrections
in-situ in italics throughout the text
Please use spp. instead of spec., as it is the correct and widely accepted abbreviation for multiple species within a genus.
L106: Delete one “a” or move the first “a” in brackets.
L271-272: Please introduce abbreviations of carbon and nitrogen and phosphorous, potassium, FAAS as well as CFA. Check the document for more of these unexplained abbreviations.
Citation: https://doi.org/10.5194/egusphere-2025-5352-RC1 -
AC1: 'Reply on RC1', Camiel Aggenbach, 27 Feb 2026
Dear editor/ reviewer,
We have processed the comments of reviewer #1 and are happy to submit a new version of our manuscript. We experienced the comments as very constructive, and these helped us to improve the manuscript. For this we thank the reviewer. All comments we could deal with, and lead to modifications in the manuscript. In the document “20260227_Respond to comment on egusphere-2025-5352 Anonymous Referee 1 16 Dec 2025.pdf” we answered the reviewer’s comments.
Kind regards
Camiel Aggenbach
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AC1: 'Reply on RC1', Camiel Aggenbach, 27 Feb 2026
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RC2: 'Comment on egusphere-2025-5352', Anonymous Referee #2, 14 May 2026
General
I was excited to review a manuscript on this important topic that has thus far received too little attention. Also, I was delighted to see the number of sites in the material.
An acknowledged complication of the one-year study was that the year happened to be exceptionally dry. In peatlands, where the water level is a critical process regulator, this likely affected the results. The title framed this nicely, the practical issues such as, e.g., the decomposition frame upheaval were described in Methods, and the authors discuss the potential impacts at the end of Discussion. However, I found the discussion part to remain quite disconnected, and suggest that the potential impacts of drought are already concisely discussed in the previous sections of Discussion along with the results of the study. The subject matter in the specific section (lines 539-556) can easily be split to be presented with the general discussion on biomass production, decomposition, and – ultimately – peat formation potential.
Also, I was wondering if the drought could be brought up in the Introduction, including the hypotheses. It seems to me that the hypotheses remained on a general level, not considering the drought, even though the title would allow, and even suggest, drought effects to be incorporated in this part of the manuscript, too, even though they cannot be directly estimated based on the one-year data with no “regular year” as comparison.
The Material and Methods section would benefit from some added information; see detailed comments. Also, please describe on which grounds you have estimated that the undrained, drained, and rewetted sites represent originally similar fen types to the extent that comparing them is relevant.
It is good that pairwise comparisons of the sites were not focused on in the analysis, but a more general evaluation of patterns between the hydrological status groups was done. The extensive data were rigorously analyzed with RF. Yet, it occurred to me that the use of some further analytical method, such as structural equation modeling, to analyze the direct and indirect effects of your impressive explanatory variable set on production and decomposition, and, ultimately, the organic matter accumulation potential, might give further insights into the functioning of the fens. With such an analysis the authors could evaluate a conceptual model on how the different variables are linked to each other, formulated based on the their ecological knowledge of the systems. Not sure how well the data would facilitate the use of that method, but it might be worth exploring (if not tested yet).
Concerning presentation, the language needs some polishing: for instance, word order is not always the clearest for easy understanding the statements, words or punctuation are missing in some sentences, and the spelling of some words, such as phosphorus, need checking. I will not be detailing these, since easy tools for language check are available. Restructuring some paragraphs would improve the flow of the text and understanding the progress, both in Methods and in Results.
Detailed
Line 50: “…this difference will not…” – I would tune this down a bit (“may not”) as the decomposition rates measured were likely affected by the drought.
Lines 53-55: rather than the bit vague last sentence, you could simply state something like “This should be borne in mind when interpreting the results”.
Line 60: perhaps specify “carbon sink function through peat accumulation”, as in some cases, drained peatland forests may remain a carbon sink at the ecosystem level and even show small soil C sink.
Lines 69-70: The phrase “Whether rewetting…” repeats the end of the previous paragraph.
Lines 78-81: While these statements are still generally valid, there is literature on these topics that you seem to have missed both here and in Discussion. The reference from 26 years ago paints a somewhat gloomier picture than what is currently available. I am listing here some papers that might be of interest, but please note that I am not suggesting that you necessarily need to cite any of these:
https://doi.org/10.1139/b96-116
https://doi.org/10.1139/b03-043
https://doi.org/10.1111/gcb.13934
https://doi.org/10.1016/j.scitotenv.2022.159683
https://doi.org/10.1007/s10021-005-0070-1
https://doi.org/10.1890/0012-9658(2000)081[3464:ROBAFP]2.0.CO;2
https://doi.org/10.1111/jvs.12912Line 93: I think that you are not able to examine hypothesis 3 as currently formulated, since one year is too short for examining peat formation, and also your results will only reflect drought conditions. Please revise.
Line 97: Instead of “in the top, as well in the subsoil” you could define the actual depth range.
Lines 103-104: Please refer to Table S1. Also, I suggest adding a figure with a map and a schematic presentation of the study design, including the different measurement points. The latter would help following the method description, which is a bit complex simply because you measured so many parameters – which is of course excellent as such.
Line 108: Classification into hydrological states may be of less concern than including sites that are actually comparable (originally the same ecosystem type), so please add here also a description of how you ensured that was the case.
Lines 123-124: Mowing is likely to impact the ecosystem structure, including plant biomass production, so please tell some more (frequency, timing) about that, in the Supplement if it cannot be done concisely. Mowing of undrained sites means that they are not in a fully natural state, which is important to bear in mind when comparing them to unmanaged fens.
Line 127: the subheading remains a bit uninformative – could you rephrase to exclude the vague “other variables”?
Line 129: Are your functional plant groups the same as the more commonly used plant functional types? In any case, please give some more information on these, e.g., in a supplementary table. Could you actually list the contents of these for all sites? The information that you currently give on vegetation composition is very scarce, just the dominant plant species in Table S1.
Line 144: Perhaps a discussion matter: Laiho et al. (2014; https://doi.org/10.1007/s11104-014-2225-3) compared peat versus inorganic substrates in ingrowth cores in boreal sedge fens, finding that under dry conditions inorganic substrates yielded lower belowground production estimates than peat. This suggests that you may have somewhat underestimated belowground production.
Line 147: The pooling of the two layers probably means that you cannot visualize the depth distribution of belowground production? That would have been interesting to see as a supplementary figure.
Line 155 onwards, section 2.4: This section, especially, could become easier to follow through some reorganization. Also, could you please describe what the “representative plant materials” were for each site, and provide a schematic figure of the frames, in the supplement? On line 183, please specify the length of the incubation period(s). Perhaps note in the Discussion how the exclusion of meso- and macrofauna may have affected the mass loss rates. Also please discuss a bit the limitations of one-year decomposition data, which does not necessarily correlate with longer-term mass loss dynamics (e.g., Latter et al. 1998: https://doi.org/10.1007/s004420050357 ; Moore et al. 2017: https://doi.org/10.1007/s11104-017-3228-7).
Line 204 ,“we used the averaged…”: please explain this specifically for the two different depth segments. Or did you combine these two (which would make no sense)?
Line 261: can you give a reference for TURBOVEG?
Line 263: What was the division of nutrient-rich versus nutrient-poor conditions based on? I mostly agree, but some species were categorized differently from what I would expect.
Line 266, “feather mosses”: earlier, you call these “brown mosses”. I would stick to that term, since feather mosses commonly refer to species like Pleurozium schreberi and Hylocomium splendens.
Lines 279-282: one-way ANOVA – so there was no other hierarchy in the data that would have required the use of mixed model even in the simpler case? Can you present the two-factor model structure somewhere?
Results, section 3.1: I am not certain that the first paragraph is needed at all: the detailed results follow in the next paragraphs, and those suffice. For any non-significant patterns that you wish to mention (e.g., lack of interactions), you could add the information after presenting the significant patterns. If the readers need a reminder of which variables were measured, they can check Fig. 1. However, if you choose to keep it, please restructure it so that it simply states for which variables significant patterns were, or were not, found, putting the lists in the actual results statements, not in separate phrases.
I would suggest restructuring the section on decomposition. Start with aboveground decomposition, stating whether there were or were not significant patterns. Then tell the same for belowground. That is easier for the reader, and follows the structure of the production paragraph above. It is good to maintain consistent structure where possible. Also, I suggest presenting all production-related results, including the depth patterns of belowground production, before moving to decomposition and ultimately, the accumulation potential.
Lines 329-330: PFP from aboveground plant biomass would be a better phrasing: similarly, PFP from belowground biomass. Please present the accumulation potential results separately, after first presenting also the decomposition results, as those are also needed for the calculation, and understanding of, the accumulation values. Please provide also a total PFP value from both aboveground and belowground biomass.
Line 346: The paragraph starting here currently seems to hold all depth-related results mixed together. While that is one option, I would quite strongly recommend presenting the results rather as per variable type: production, decomposition, accumulation potential. These are the factors of interest, and depth-related patterns are just detailed features of those. Mention if the hydrological status affected the depth-related patterns or not.
Line 360, caption of Fig. 2: b) plant biomass, or plant biomass production? e) and f) Did you pool these that across hydrological status because hydrological status did not significantly affect the patterns, or for what reason? Please tell; do not make the reader to guess. Please rather use an abbreviation that more intuitively makes sense in the English languate. A quick reader will think that RT means Rooibos tea. Also, as the end part of the term is the same for both aboveground and belowground, make their abbreviations similarly corresponding as well. On the other hand, I do not see a strong reason for using the abbreviations at all, as it seems that Aboveground and Belowground, defined in the caption to relate to vascular plant biomass, are just as clear without the abbreviations added.
Figures 2 and 3: I am quite puzzled why you present the production and accumulation results always together, even though accumulation also depends fundamentally of decomposition. Another option for organizing the figures would be putting all depth-related patterns in a specific figure, including current panels 2 e) 2 f), 3 e) and 3 f). That would be visually more pleasing, and present accumulation in the context of both production and decomposition. While the current structuring would align better with my suggestion of presenting the results in the text as per variable type (and not combining all depth-related patterns together), that is not decisive as you can refer to multiple figures from one result statement, as also from new statements to figures already mentioned.
Line 384: “Belowground plant production” – “additionally”, as you have already described the depth patterns in the previous sentence? “Positively linked” or “negatively linked” – I would see it as the latter?
Line 385: Rather than “exceeded”, it would be clearer to state whether the water levels were higher than, or lower than, 10 cm above surface. Further, 10% of what? Bulk density is often presented as kg/m3, or some other mass-per-volume unit, so % may not be intuitively understandable for all readers. You actually have kg per liter as the unit in the figure.
Discussion, general: Please bear in mind all through the Discussion that you are dealing with an exceptionally dry year, and not necessarily general patterns. Otherwise, you may be unintentionally a bit misleading. I would suggest that you add in each section a concise consideration how the drought summer may have affected the results, and which patterns may be considered more generally valid. You have a lot of knowledge of these systems in the author team and that, with support from earlier published research it should be doable. Whereever you really do not feel that you can estimate the directions of the drought effects, please tell that, too. As all the sites were mowed or grazed, please also mention whether you think that may have affected plant biomass production and if so, how.
Lines 441-446: Could these patterns be caused by the dry year? Please refer to earlier published results on the relationship between biomass production and WL for fens for a bit more insight on this.
Lines 451-452: How was the depth distribution compared to other studies in fens?
Lines 464-468: This should rather be in Results than Discussion.
Line 482: “overrules” - Could this be because the water levels were generally low? How does this relate to other studies that examined the effects of WL or hydrological status on decomposition? E.g., https://doi.org/10.4141/cjss88-073 ; https://doi.org/10.1111/j.1365-2486.2011.02503.x
Line 483: What might these ”other factors” be?
Line 487: Based on other studies utilizing rooibos, what does rooibos mass loss generally describe? How does it generally relate to native litter mass loss?
Lines 494-495: This is quite surprising, and may suggest that the surface was too dry for efficient decomposition because of the drought. Can you please check other studies on belowground decomposition in peatlands and discuss a bit further?
Line 501, heading: Please rephrase to peat formation potential
Lines 519-520: Is this in Results? If not, please add; even more generally, the belowground to aboveground ratios are of interest.
Line 521: Rather “likely contributes”, bearing in mind that your year was exceptional.
Lines 524-525: Again, what might they be?
Lines 528-529: How about the potential impact of the drought?
Lines 545-550: Perhaps condense this to a simple statement that respiration-based results have varied, since you did not measure respiration; rather focus on discussing factors included in your data.
Citation: https://doi.org/10.5194/egusphere-2025-5352-RC2
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General comments
The manuscript by Aggenbach et al. addresses a timely and important topic: the effects of drainage and rewetting on biomass production and decomposition, key processes underlying peat formation and carbon storage in temperate fens. The study is thorough, with a large number of sites and well-executed methods. The finding that nutrient availability and soil depth had a stronger influence on productivity than hydrological status, particularly during an unusually dry and warm season, is insightful and highly relevant given the increasing frequency of European heatwaves. This result has important implications for peatland restoration, suggesting that rewetting alone may not be sufficient to restore carbon sequestration potential and that nutrient regimes and peat depth should also be considered.
A clearer introduction and a more concise, confident conclusion would strengthen the abstract and better communicate the significance of the study. The methods are generally sound, though a justification of the ingrowth core substrate for belowground production and clarification of how the small number of replicates per site were incorporated into statistical analyses would strengthen the manuscript.
Overall, the study provides valuable insights into the drivers of fen ecosystem functioning under changing climate conditions and will be of interest to the peatland and carbon cycling research community.
Specific comments
Abstract:
The abstract would benefit from a clearer introduction to the broader context and importance of the research topic. It currently opens with the study hypotheses but does not provide sufficient background on why the topic matters. I recommend beginning with a few sentences that highlight the role of peatlands as major carbon sinks, the existing gaps in our understanding of carbon cycling in these systems, and how this study aims to address those gaps. For instance, in-situ measurements of biomass production and decomposition, particularly belowground, are still underrepresented in fen peatland studies, even though they are crucial for understanding the effects of drainage and rewetting on biogeochemical cycling and carbon storage in these managed ecosystems.
The results section of the abstract is detailed, but the emphasis on hypotheses could be reduced in favor of clearly presenting the most important findings and their implications. The final sentences currently sound overly cautious and may unintentionally undermine confidence in the results. Rather than framing the extremely dry and warm growing season as a limitation, I encourage the authors to consider discussing the relevance of these conditions, given that many peatlands have been experiencing similarly warm and dry summers over the past decade(s). These observations are not simply anomalies but reflect ongoing climate change, which makes your results particularly timely and valuable.
Introduction:
L59–60: Please clarify whether this statement refers to a global context.
L72–73: Reference is missing for this statement.
L72–73: The end of this sentence is unclear. I suggest rephrasing as: “In fens, peat accumulation is strongly influenced by belowground biomass from vascular plants, especially sedges (Carex spp.), which often provide the largest share of organic inputs.”
L76–77: End the sentence after “rare” and start a new sentence afterwards.
L76–80: Some redundancy exists regarding the scarcity of belowground data. I suggest merging these lines into one sentence: “In situ observations of belowground biomass production and root responses in fens are generally rare…”
L83: Reference is missing.
L88–89: Consider naming some of the abiotic factors you investigated, providing examples, or simply referring to Fig. 1.
L93–94: Rather than “displacement peat,” you might say “less peat is formed by root ingrowth”, which is more straightforward and emphasizes the relevance of your study.
Methods:
The large number of sites and the broad geographic and climatic coverage are a major strength of the study. A map showing the spatial distribution of the 39 sites across Europe would greatly help readers visualize this gradient. I suggest citing Table S1 of the supplements already in the opening sentence of the site description, so readers immediately know where to find detailed metadata on site characteristics and history. This would facilitate understanding of the regional grouping and site categorization.
Further, the description of rewetted sites would benefit from more explicit information on management history, such as the approximate time since rewetting for individual sites or site groups. While the authors note that rewetting occurred “a few years to several decades” (L116-117) prior to sampling, a more constrained range or categorical grouping would improve transparency and allow readers to better assess potential legacy effects of drainage and rewetting.
L144: I acknowledge the substantial logistical effort involved in implementing ingrowth cores across such a large number of sites. I also understand that using autochthonous peat material is often not feasible on this scale (three replicates per site across 39 sites makes n=117 cores in total?; please confirm). However, the sand–nylon substrate differs markedly from peat in key properties relevant for root growth, despite the authors' attempts to resemble the average peat bulk density of all sites. Variables such as water-holding capacity, nutrient availability, oxygen conditions and microbial activity must differ substantially between peat and sand substrates. Justifying this choice and discussing how these differences may influence root and rhizome ingrowth relative to the surrounding peat would help readers to better interpret the belowground production estimates.
L145: Since the dominant species are sedges (L258-269), please explain how vertically installed ingrowth cores capture realistic belowground production in species that have horizontal rhizome growth but also, to a great extent, vertical root growth. Could that have led to an underestimation of root compared to rhizome growth or even an underestimation of root production in deeper soil depth (L450-452). If yes, was this accounted for?
L160-161: Please also indicate the mesh size of the Rooibos bags then.
L200-206: Please add a formula to this description of calculating the peat formation potential. Is this method based on any reference?
L275-302: The authors state that replicate measurements were averaged per site and that these site means were used for subsequent statistical analyses. This approach is appropriate for avoiding pseudoreplication, however, it would be helpful to clarify consistently which explanations apply to which analyses. In particular, for the random forest analyses it remains unclear whether site-averaged values or individual replicate measurements were used, and how within-site dependence was handled in this context. Given the relatively small number of replicates per site and the high spatial heterogeneity typical of fens, a brief clarification of the unit of replication for each analysis would strengthen confidence in the robustness of the results.
Just a site note, why was the ANOVA performed with a different R version (4.2.) than the random forest analysis (4.3.1)?
Results:
L325: “mass loss of local/root belowground biomass”
Figure 2 and 3: Several figures in the manuscript use color schemes that may be difficult to distinguish for readers with color vision deficiencies. Ensuring figures are colorblind-friendly is important because it improves accessibility, allows all readers to accurately interpret the data, and avoids misinterpretation of patterns or trends. I recommend using color palettes that are designed for colorblind accessibility (e.g., viridis, RColorBrewer, scico or colorspaces are some examples for R packages I can recommend) or adding distinct symbols/patterns in addition to color to convey information. Very helpful tool: https://www.color-blindness.com/coblis-color-blindness-simulator/
Discussion:
L539-556: I assume the conditions observed during the study year were extreme compared to long-term averages calculated over several decades. It would be helpful to put these conditions into context relative to the last decade as well. Were such warm and dry conditions still very unusual, or have similar events become more frequent? Providing this context would help readers interpret the observed patterns of productivity and decomposition in light of ongoing climate trends.
L502: A reference is missing.
Technical corrections
in-situ in italics throughout the text
Please use spp. instead of spec., as it is the correct and widely accepted abbreviation for multiple species within a genus.
L106: Delete one “a” or move the first “a” in brackets.
L271-272: Please introduce abbreviations of carbon and nitrogen and phosphorous, potassium, FAAS as well as CFA. Check the document for more of these unexplained abbreviations.