Effects of Pneumatophore Density on Methane Emissions in Mangroves

Round 1

Reviewer 1 Report

The authors measured methane emissions from mangroves in Taiwan with the goal to relate the emissions to soil properties and to pneumatophore density. Their major findings were that methane emissions were greater for one mangrove species with pneumatophores over another mangrove species with prop roots. The authors also found a significant relationship between methane emissions and A. marina pneumatophore density.

Overall, the paper and the research presented is average and the novelty of the project is over emphasized. For example, there have been many studies before that have related methane emissions to pneumatophore density, including the ones referenced in the Figure 9, but none of these are cited in the introduction. The introduction makes it seem like this has not been done before. The authors also state that this study shows that it is important to include methane emissions in mangrove carbon budgets. This also is something that is not novel and methane emissions have in fact been included in recent blue carbon budgets. The manuscript can be greatly improved if the authors go into these details of how other studies have advanced our knowledge on methane emissions from mangroves and how this study will grow our knowledge on the subject.

Another major issue I have is the citing of the wrong papers/data. I did not go through each reference but I did find that one reference in the introduction, Rosentreter et al. (2018), was referred to as representing a study that demonstrates the importance of including methane emissions into carbon budgets globally. The study that is referenced is for local Australian estuaries fringed by mangroves as opposed to another study by Rosentetreter et al., published also that year, that actually did look at global carbon budgets and the importance of including methane emissions. Another study I found to be mis-referenced was Purvaja and Ramesh, 2001 in Figure 9. The authors here use that study to plot methane emissions against pneumatophore density in a tropical mangrove system; however, the paper that is referenced does not include data on pneumatophore density. This makes me question where they got the pneumatophore data from. I also then wonder whether any of the other references I did not check were wrongly cited.

I also find issue with how the methane emission data is reported differently between the text, tables, and figures. In the text, it's reported A. marina mangroves had methane emissions as high as 765.9 umol m-2 h-1 (reported in the introduction and in the discussion); however, I don't see where that is coming from if the reader looks at the data presented in the tables and figures. The max methane emission data presented in Figure 3 is between 300-400 umol m-1 h-1 and Figure 4 has a max between 120-150 umol m-1 h-1. Was this 765.9 umol m-2 h-1 an outlier, and if so, it needs to be shown. If it was removed, explain why it was removed from the figures. Also, why do the ranges in methane emissions for mangroves differ between Figures 3 and 4 differ? The data must not be representing the means in Figure 4 as the means reported in Table 2 also differ from the data presented in this figure. Another, minor issue is that the text describing the range of mangrove methane emissions (second line of the first paragraph) should be specified as reporting the range of the means.

The next major issue I have is that the authors are over emphasizing the importance of salinity and pH on the flux of methane emissions from K. obovata mangroves. The correlations seen are real; however, the methane emissions from these mangroves are very low and are driven by a high Eh. Even though there was no correlation found between Eh and methane emissions, doesn't mean Eh isn't important for controlling methane production and flux. In fact, the high Eh is the primary factor for having low methane emissions compared to A. marina. Similarly, a minor issue related to redox potential in this manuscript is that the authors use two acronyms (Eh and ORP) in the introduction. Stick with one acronym throughout the manuscript, don't use both.

My last major issue is that I'd like to see the authors devote more of their discussion to the importance of root structure on methane emissions. This is one of their main findings but they don't put much effort in describing why there were differences between the two species of mangroves (maybe only a couple of sentences placed sparingly throughout the manuscript).

Minor issues:

• Throughout the manuscript: Referencing to either species of mangrove by the genus name rather than the short species notation. The authors did not measure methane emissions from multiple species of either genus so the authors should not be using the genus name throughout the manuscript (starting in the abstract) rather than the species name.
• In 2.3., were soil samples collected using stainless cores or stainless steel cores?
• Also 2.3., if the syringes are only 5 cm in length, were two used to collect the top 10 cm?
• How were soil samples stored in the lab and how long was the interval between sampling and analyses?
• Tables 2/3 - No units for salinity?
• Figure 7 - Figure description says (a) is pH but the figure shows it as being ORP.
• Discussion, paragraph 3 - authors discuss soil OM had a significantly positive correlation with methane emissions but this was only shown for the mudflats, not the mangroves. Were there significant positive correlations with the mangroves, and if so, plot them in a figure and give the p-value.
• Discussion, paragraph 5 - should mention that there is a difference in seasonality between studies plotted in Figure 9. Some recorded year-long fluxes, which included seasons with low methane emissions, while this study only looked at pneumatophore fluxes in the two seasons that were predicted to give the highest methane emissions. Seasonality would not have as big of an effect as latitude demonstrated here; however, it is important to mention the difference in study intervals.

Author Response

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Author Response File: Author Response.pdf

Reviewer 2 Report

This study aims to quantify the CH₄ emissions from mangrove sediment surface (two mangrove species and mudflat) and to clarify their regulatory factors (especially the pneumatophore density). The data is clearly presented, and the text is generally well written. However, some unclear and inaccurate parts need to be significantly improved before publication. My comments and suggestions are follows.

<Introduction - Hypotheses>

I felt that the first hypothesis (CH4 emissions are regulated by soil properties) is trivial and then, the second hypothesis can be tested simply by studying in the mangrove forests where Avcennia marina, which has pneumatophores, predominates. So, please clarify whether this study interested in the inter-species difference in CH₄ emission or not.

<Materials and Methods>

Figure 1

Although authors represent their study sites as A, B, C…, these sites were represented in two-letters codes (e.g., WZ and XF) in the text and figures/tables, which would quite confusing for the readers. How about using A, B, C… also in the text and figures/tables. Alternatively, a more systematic site code indicating the vegetation type, such as Av-1, Av-2…, Ka-1, Ka-2…, may be more useful to the reader.

Is there any difference in soil texture among sites? (Authors state that the all soil textures were “silt”) More detail information on soil texture (e.g., particle size distribution data) would be useful in considering the difference in CH₄ emissions.

Table 1

What is “ratio of mangrove to mudflat area”? How did you determine it?

Reconsider the description of "soil texture" (see comments above)

Line 1 in subsection 2.2 (Methane Emissions Measurements)

Please clarify when the on-site measurements were taken. The term “a complete seasonal cycle” is inadequate.

Line 2 in subsection 2.2 (Methane Emissions Measurements)

“Table 1” here seems strange.

Add more explanation on “mudflats”. Were they located in the forests (canopy gap)? Or, located in the open space outside the forest?

Line 7 in subsection 2.2 (Methane Emissions Measurements)

At this time, did you consider the presence/absence of holes by benthos such as crabs?

The second paragraph in subsection 2.2 (Methane Emissions Measurements)

More detail timing of measurement during “emersion period” is needed since rising tide or falling tide may differently affect the gas dynamics through sediment.

In addition, please explain how you chose 3-5 points (replications) at each site.

Methane emission measurement

In particular, CH₄ emission from the sediment surface of mudflats seemed to be very low. So, did you obtain a clear and significant linear increase in CH₄ concentration in the chamber during all measurements? In addition, adding detectable limit of CH₄ emission in your measurements would be helpful.

Subsection 2.3 (Soil Sample Analyses)

How did you measure pH and ORP in the field? Did you insert electrodes directly into the sediment? What depth?

I could not understand how you obtained soil subsamples using stainless cores and syringes. You collected large sediment core (diameter: 7 cm and length: 80 cm), right? After that, I could not understand how you retrieved the soil subsample (0-10 cm) by small syringes (length: 5cm). Did you retrieve the sample in two parts (0-5 and 5-10 cm)? If you collected 2.9 cm * 10 cm subsamples, the volume of the subsamples should exceed 60 ml, but you used 50 ml tubes. Please clarify this point.

<Results>

Subsection 3.1 (CH₄ emission and soil properties)

Although you seemed to measure CH₄ flux in various seasons, there was no information or mention on seasonal changes/differences in CH₄ flux and soil properties. Do Figure 3 and 4 represent pooled data for all seasons?

Figure 3

Does this figure contain the mudflats data? Please clarify.

Add number of replicates in each data.

Figure 4

Add number of replicates in each data.

Table 3

Why are there only five site data? If for any reason you have measured the mudflats only in these sites, please indicate so in the Methods section.

Subsection 3.2 (Effects of Soil Properties…)

I am concerned about multicollinearity in multiple regressions. That is, influx of seawater, which has a relatively high pH, would raises both of the pH and salinity of sediment. Was there a significantly high positive correlation between salinity and pH in sediment?

Subsection 3.2, 2^nd paragraph

The first sentence of this paragraph should be moved to Methods section.

Figure 8

The authors stated that effect of pneumatophores density on CH₄ emissions was tested at BM and FY sites in spring and summer, however, the spring data for FY site was not shown in this figure.

<Discussion>

Lines 7-8 of the third paragraph

Why does high salinity shift microbial metabolism from methanogenesis to sulfate reduction? Rather, relative availability of sulfate ions in the sediment may be more important… Please refer to existing researches and add more explanation for this mechanism.

I agree with that ORP and organic matter content were major determinant for CH₄ emission in mudflats. In addition, I also have no objection to the author's claim that pneumatophores density was an important factor in CH₄ release in the Avicennia marina zone as shown in Figure 8. However, the CH₄ emission in Kandelia obovate was not sufficiently discussed in this paper. If CH₄ emission in Kandelia obovate was really very low compared to the Avicennia marina, what is the reason? What do you think about the differences in the regulators of CH₄ release between two different species and mudflats?

Figure 9 was very impressive, however, what do you think about the large difference in slope even in the same climate zone (tropical 1-3)? Also, it's trivial, but it's difficult to distinguish between Tropical 1 and 3 colors.

Author Response

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Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

I have read through the revised manuscript and found that the authors revised carefully following the comments and that the manuscript was highly improved.

Response No. 11
In general, soil pH is determined by suspending the soil sample with constant volume water (e.g., soil:water = 1:2.5). Is it possible to measure “soil pH” by directly inserting the electrode into the soil? Is this your original method? If not so, please add appropriate citation for this procedure.

Response No. 20:
I felt that relationship between salinity and sulfate ion concentration (Line 234) was still unclear in this manuscript. Explanation on why the soil with high salinity showed the higher availability of sulfate would helpful for the readers.

<Other point>
Throughout the manuscript, the full mangrove species name (Kandelia obovate and Avicennia marina) are used but abbreviations such as K. obovate and A. marina should be used, expect for the first mention.

Lines 130-132:
This statement can be combined with lines 126-128 for brevity.

Author Response

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Author Response File: Author Response.pdf