| 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.
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.