Review of: Patterns and drivers of organic matter decomposition in peatland open-water pools

Julien Arsenault, Julie Talbot, Tim R. Moore, Klaus-Holger Knorr, Henning Teickner, Jean- François Lapierre

General Comments:

The paper outlines an interesting study of peatland pools in Canada. They combine a litterbag decomposition experiment with lab-based incubations to understand the drivers of OM decomposition and GHG emissions from peat sediments. Peatland pools are under-studied – this paper is a significant contribution to research on these interesting environments. The justification for the research is clear, and the methods are well described. The results are sometimes a bit unclear, but there are some really interesting results hidden in there. The implications for C emissions and climate need to be more explicit.

Abstract:

As it is a complex experiment, there is a lot of information in the abstract, which makes it a bit more difficult to read. However, it generally outlines the research well.

Introduction:

The introduction outlines existing research briefly and succinctly. There are a few papers on temperate peatland pools that missing from this summary of past research (relating to pools in the UK Forsinard Flows, and small ditches in Sweden) that could be included to support biogeochemistry findings (especially re: depth, DOC and dissolved CO₂ in pools).

Methods:

Figure 1 is good, it’s a clear and concise explanation of the experimental design.

Typha latifolia isn’t found at the site, why did you choose it for the litter bags? Was it because it isn’t found there naturally? It would be good to include a justification for this.

Do the measurements are line 104-105 mean that you did a site survey of 158 of the 600 pools?

Why were the litterbags collected multiple times during the experiment? The paper focusses in those collected at the end of the experiment – did the ones collected earlier not show any interesting results?

At line 130, do you mean ‘for up to 27 months’ here? Or were molecular composition analyses only done on litter from the final sampling occasion?

What does the ‘t’ stand for at line 172?

Results:

3.1.1. Figure 2 and Table 1 show similar results – does Table 1 add anything to the story that isn’t covered by Figure 2? The values for intercept, %MR and r2 aren’t discussed in the text, maybe they could go in SI?

3.1.2. At line 255, you say there is a distinctive pattern, and then say there is no detectable change – and then say there was 106% and 98% (which I would say is a change, even if it’s a small one). This gives mixed signals.

The results in this section are very interesting.

Line 267: ‘regardless of the pool’ – does this mean pool depth, or pool number/location?

The caption of Figure 3 was difficult to interpret, where you refer to pool depth category, and then have ‘regardless of depth of incubation’. I see why you have used that wording, but is there a way to make it clearer which ‘depth’ is which?

In Figure 3, can you include the number of samples that each box represents? (e.g. n=2)

Line 280: HI – can you remind what the different HI ratios mean here?

When you say ‘increased over time’, do you mean between initial and final weight, or for all litterbag retrievals between the beginning and end?

3.1.3. Are the results in Figure 4A the same as those in Figure 2? The k values at different depths? Figure 4 shows a lot of information very clearly.

3.1.4. The second sentence in this section is hard to understand. Can you re-word it?

3.2.1. The sentence at line 320 makes it sound like the P concentrations were higher in the pool rather than the sediments.

The lowest value of CH₄ production in Table 3 is -0.6, but in the sentence at line 323, you write the range as between ‘-0.03 and 123 ug CH₄’

Figure 5 caption – ‘…of the six studied pools (G1 to G5)…’ what about pool G6? Also please check you are referring to the correct graphs/gases in the text (especially at  line 331). I don’t think you refer to Figure 5A in the results text at all.

3.2.2. No attempt to explain the results or PCA here.

3.3. You write that ‘spatial patterns emerged’ but then end this section saying ‘there seemed to be little relationship between CO₂ and CH₄ production rates….’ – so what are the spatial patterns?

Discussion:

4.1. The sentence at line 407 needs a bit more explaining please.

4.2. This section is clear and concise.

4.3. The header of this section sounds the same as section 4.1.

In section 4.1. you state that O₂ concentration, light and temperature are drivers of litter decomposition in peat pools, whereas in this section you talk about P content of the sediment.

The result at line 465 is very interesting and wasn’t mentioned earlier in the study (or wasn’t highlighted as much as it could be).

Supplementary information:

Tables S1-S4 could be condensed into one table, just showing p values and post-hoc test results for each test? I don’t think that knowing the degrees of freedom, sum of squares and mean square of each test adds much.

Figure S1 repeats the C/N and N/P ratio graphs that are in Figure 3 – probably unnecessary.

Text edits with line numbers:

Line 15: The sentence is long and contains a lot of information, could it be rewritten as two sentences to make it clearer?

Line 71: hence and hence and hence – too many hences.

Line 224: ‘… was more degraded than…’

The study assesses spatial variation in organic matter (OM) decomposition rates in peatland pools at a temperate peatland site. The authors also evaluated the influence of OM chemistry on decomposition rates of fresh litter in situ and on sediments ex situ. The authors' finding that decomposition rates vary spatially in peatland pools is highly relevant for assessments of peatland greenhouse gas (GHG) emissions and the paper will be of interest to the readership of Biogeosciences.

The research approach and methods are well-designed overall, however the authors should justify the decision to oven-dry litter samples for the in situ decomposition experiment, and compare decomposition rates of litter to other studies in the discussion. The threshold for statistical significance should be clarified in the methods. 

The paper presents a comprehensive dataset and the interpretation and discussion of results needs to be re-examined and strengthened in some instances. A few issues stand out:

According to the introduction OM decomposition is generally faster in pools than in surrounding soils, but in this study k values tended to be much higher in the acrotelm compared to the pools for T. latifolia and k values appear to be similar for pools and the acrotelm in the case of S. capillifolium (Table 1). This should be discussed.

While the authors propose that decomposition rate in pools depends primarily on environmental conditions (i.e. depth), there was no difference in decomposition rates of S. capillifolium among depths (Table 1). It seems there are dual influences of environment and litter composition on decomposition rates in situ that the authors should consider more carefully.

Regarding the influence of OM chemistry on decomposition of sediments ex situ statements in the abstract and discussion contradict each other.  The abstract states that CO2 production by sediments decreases with increasing OM humification and at line 440 it is stated that "CO2 production was positively related to an increase in OM humification". The abstract states that CH4 production decreases with increasing N:P but this result is not significant at p < 0.05 (Table 4), though there is a significant correlation between CH4 and C content as well as Na content at p < 0.05 (see comment above about clarification of threshold for determination of statistical significance). This section of the discussion doesn't address the potential mechanisms explaining these correlations, though an explicit aim of the study is to "assess the role OM chemistry plays on decomposition rates for litter and sediments" and an implied aim is to increase knowledge of mechanisms controlling OM decomposition in peatland pools.

The abstract states that differences in fresh litter and pool sediment decomposability is a function of O2 concentrations, light, and temperature, which all decrease with increasing depth. Variation in these environmental parameters with depth are not presented in the present study but it seems they were included in a previous paper (Aresenault et al. 2018). The relationship between decomposition rates and O2, light, and temperature should be presented in the present study to substantiate this conclusion.

It's also not clear how sediment decomposition rate can vary with depth since the sediments were collected from the bottoms of the pools. Perhaps the authors mean overall pool depth, but the ex situ decomposition rates were highest at the intermediate pool depth, not the shallowest pools. It's interesting that the total ex situ production of CO2 corresponds to the sediments from the pools with the highest CO2 fluxes in situ. The authors have focused on the influence of physical factors but it seems that chemistry is also important.