the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 01 Aug 2024
Dynamics of CO2 and CH4 fluxes in Red Sea mangrove soils
Abstract. Red Sea mangroves have a lower carbon burial rate than the global average, whereby small greenhouse gas fluxes may offset a large proportion of carbon burial. Monthly soil core sampling was conducted across 2 years at two sites within a central Eastern Red Sea mangrove stand to examine carbon dioxide (CO2) and methane (CH4) fluxes under dry and inundated conditions. Fluxes were highly variable, characterized by a prevalence of low emissions punctuated by bursts of high emissions. At the landward site, average ± SE (median) flux from the soil-air interface was 3111 ± 929 (811) µmol CO2 m−2 d−1 and 1.68 ± 0.63 (0.26) µmol CH4 m−2 d−1 under light conditions, and 8657 ± 2269 (1615) µmol CO2 m−2 d−1 and 0.84 ± 0.79 (0.59) µmol CH4 m−2 d−1 under dark conditions. Average ± SE (median) sea-air fluxes were –55 ± 165 (-79) µmol CO2 m−2 d−1 and 0.12 ± 0.23 (0.08) µmol CH4 m−2 d−1 under light conditions, and 27 ± 48 (53) µmol CO2 m−2 d−1 and 0.16 ± 0.13 (0.09) µmol CH4 m−2 d−1 in dark conditions. The seaward site recorded higher CH4 flux, averaging 18.7 ± 8.18 (1.7) and 17.1 ± 4.55 (7.7) µmol CH4 m−2 d−1 in light and dark conditions. Mean fluxes offset 94.5 % of carbon burial, with a median of 4.9 % skewed by extreme variability. However, reported CO2 removal by total alkalinity emission from carbonate dissolution greatly exceeded both processes and drives the role of these ecosystems as intense CO2 sinks.
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RC1: 'Comment on egusphere-2024-1831', Anonymous Referee #2, 15 Aug 2024
General comments
This manuscript investigates CO2 and CH4 fluxes in arid mangroves along the Red Sea. The main findings are GHG fluxes offsets 95% of soil carbon burial in seaward mangrove sites and become net sources during high emission events. However, when total alkalinity enhancement is incorporated, < 4% of carbon sequestration potential is offset by the GHG fluxes. The study also finds that temperature is the most important single variable in predicting CO2 flux under light conditions, second only to the year of sampling due to temporal interannual variability. This study also looked at the relationship between isotopic signature and found a negative correlation between δ13C-CH4, and CO2 flux in both dark and light conditions, which offer insights into how microbial processing are affecting resulting GHG fluxes. Overall, It’s a very well written and novel piece of research. The main recommendation I have is for authors to include a couple of sentences to acknowledge limitations related to how incubation technique used in this study could have affected GHG gases relative to the field-based observation such as static chambers and continuous eddy covariance. Other than that, I outlined below a few minor points to help authors improving on some grammar and syntax edits that may be necessary. Congrats, great paper, I enjoyed reading it and look forward to seeing it in print soon!
Specific comments
Ln 43. ‘physiological’ or ‘ecophysiological’ instead?
Ln 104. ‘cores’ instead of ‘scores’
Tables 1 and 2. Consider adding significance test results to this table, e.g., compact letter display.
Figure 4. I’d remove the left panel. I didn’t find this zoomed in graph helpful to visualize and understand your results.
Ln 268. Remove ‘good’ or replace it by ‘high’.
Ln 313. below ‘the’ salinity or below ‘salinities’
Ln 317. ‘plotsseaaaaaaaaaaaa’?
Ln 317. Remove the first ‘is’ from ‘this is method is’
Ln 320. ‘physico-chemical’ instead?
Ln 322-330. I agree with you. No one study will ever account for all possible drivers of GHG fluxes. And you are right, lots of these variables can be autocorrelated or have multicollinearity issues. But the relative importance you found could have been very different had you included, say, ammonium or Fe2 in your analyses, right? With that in mind, I think you could offer a sentence or two on this limitation and implications for follow up studies.
Citation: https://doi.org/10.5194/egusphere-2024-1831-RC1 -
AC2: 'Reply on RC1', Jessica Breavington, 22 Sep 2024
We would like to thank reviewer 2 for taking the time to give their thorough feedback and useful comments on our paper. We elaborated on the limitations of incubation technique over other methods, and implemented the helpful suggestions regarding the figures and tables. Please find our detailed responses and actions taken to address all comments in the uploaded pdf file.
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AC2: 'Reply on RC1', Jessica Breavington, 22 Sep 2024
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RC2: 'Comment on egusphere-2024-1831', Anonymous Referee #1, 23 Aug 2024
This is a very interesting, nicely conducted study that deserves publication. The experiment has been conducted with care and the research question is clear. However, some methods need to be described in more detail, some results should be reported in more detail and some sections of the discussion should be expanded to exploit the scientific benefit that the data harbours.
General: Were there no roots in the soil/sediment cores? If mangrove soils store carbon sequestered by the trees, this carbon must get into the soil somehow.
L 40-43: Maybe add a sentence elaborating how this aridity influences mangrove growth. Does it also play a role in selecting the dominant mangrove species?
L 44: Add “also” following “…mangroves is…” as the small tidal range is probably not the only constraint on mangrove distribution.
L 59: What did Sea et al. (2018) find out? It is only one study, but probably one with some results.
L 115-116: I have a hard time imagining that 12h of equilibration time are sufficient. Diyd you test whether these 12 h suffice indeed?
L 117-118: What is “light”? Laboratory light intensity? Was there a climate chamber to provide the high light intensity prevailing in the region? Or were the cores outside the lab for the “light” phase?
L 149-150: How was bulk density determined?
L 185-194: It would be nice to see, somewhere in this passage, the %Corg, which is a useful parameter in soil science.
L 310-311: Why is this sentence underlined?
L 338-339: Yes, these differences are not fully resolved. But one reason could be different kinds of transport processes inducing CH4 release (ebullitions vas. Diffusive flux). I suggest elaborating this issue a little.
L 366-384: This issue should be better exploited, also in the results section. Which soil parameters influence acetoclastic and hydrogenotrophic methanogenesis? When do mangroves grow most strongly in the Red sea? This should be the time of a higher rate of acetoclastic CH4 production. Does d13C efflux change with season?
Figs 2, 3: I have the impression that most high CO2 and CH4 fluxes occur between March and May, in both years. Is this mirrored by the d13C signature of the fluxes? Is this the season of highest plant growth?
Figs 5, 6: The predictive power of the year of sampling is interesting. This should be discussed some more.
Citation: https://doi.org/10.5194/egusphere-2024-1831-RC2 -
AC1: 'Reply on RC2', Jessica Breavington, 22 Sep 2024
We would like to thank reviewer 1 for taking the time to give their thorough feedback and useful comments on our paper. We expanded the different sections as requested to explain the methodology and results in more detail, and also expanded on the discussion. For in depth replies, actions taken, and changes in the manuscript please find the document attached. Also uploaded within the same pdf is the correlation matrix figure used for the additional analysis referenced in our response.
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AC1: 'Reply on RC2', Jessica Breavington, 22 Sep 2024
Status: closed
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RC1: 'Comment on egusphere-2024-1831', Anonymous Referee #2, 15 Aug 2024
General comments
This manuscript investigates CO2 and CH4 fluxes in arid mangroves along the Red Sea. The main findings are GHG fluxes offsets 95% of soil carbon burial in seaward mangrove sites and become net sources during high emission events. However, when total alkalinity enhancement is incorporated, < 4% of carbon sequestration potential is offset by the GHG fluxes. The study also finds that temperature is the most important single variable in predicting CO2 flux under light conditions, second only to the year of sampling due to temporal interannual variability. This study also looked at the relationship between isotopic signature and found a negative correlation between δ13C-CH4, and CO2 flux in both dark and light conditions, which offer insights into how microbial processing are affecting resulting GHG fluxes. Overall, It’s a very well written and novel piece of research. The main recommendation I have is for authors to include a couple of sentences to acknowledge limitations related to how incubation technique used in this study could have affected GHG gases relative to the field-based observation such as static chambers and continuous eddy covariance. Other than that, I outlined below a few minor points to help authors improving on some grammar and syntax edits that may be necessary. Congrats, great paper, I enjoyed reading it and look forward to seeing it in print soon!
Specific comments
Ln 43. ‘physiological’ or ‘ecophysiological’ instead?
Ln 104. ‘cores’ instead of ‘scores’
Tables 1 and 2. Consider adding significance test results to this table, e.g., compact letter display.
Figure 4. I’d remove the left panel. I didn’t find this zoomed in graph helpful to visualize and understand your results.
Ln 268. Remove ‘good’ or replace it by ‘high’.
Ln 313. below ‘the’ salinity or below ‘salinities’
Ln 317. ‘plotsseaaaaaaaaaaaa’?
Ln 317. Remove the first ‘is’ from ‘this is method is’
Ln 320. ‘physico-chemical’ instead?
Ln 322-330. I agree with you. No one study will ever account for all possible drivers of GHG fluxes. And you are right, lots of these variables can be autocorrelated or have multicollinearity issues. But the relative importance you found could have been very different had you included, say, ammonium or Fe2 in your analyses, right? With that in mind, I think you could offer a sentence or two on this limitation and implications for follow up studies.
Citation: https://doi.org/10.5194/egusphere-2024-1831-RC1 -
AC2: 'Reply on RC1', Jessica Breavington, 22 Sep 2024
We would like to thank reviewer 2 for taking the time to give their thorough feedback and useful comments on our paper. We elaborated on the limitations of incubation technique over other methods, and implemented the helpful suggestions regarding the figures and tables. Please find our detailed responses and actions taken to address all comments in the uploaded pdf file.
-
AC2: 'Reply on RC1', Jessica Breavington, 22 Sep 2024
-
RC2: 'Comment on egusphere-2024-1831', Anonymous Referee #1, 23 Aug 2024
This is a very interesting, nicely conducted study that deserves publication. The experiment has been conducted with care and the research question is clear. However, some methods need to be described in more detail, some results should be reported in more detail and some sections of the discussion should be expanded to exploit the scientific benefit that the data harbours.
General: Were there no roots in the soil/sediment cores? If mangrove soils store carbon sequestered by the trees, this carbon must get into the soil somehow.
L 40-43: Maybe add a sentence elaborating how this aridity influences mangrove growth. Does it also play a role in selecting the dominant mangrove species?
L 44: Add “also” following “…mangroves is…” as the small tidal range is probably not the only constraint on mangrove distribution.
L 59: What did Sea et al. (2018) find out? It is only one study, but probably one with some results.
L 115-116: I have a hard time imagining that 12h of equilibration time are sufficient. Diyd you test whether these 12 h suffice indeed?
L 117-118: What is “light”? Laboratory light intensity? Was there a climate chamber to provide the high light intensity prevailing in the region? Or were the cores outside the lab for the “light” phase?
L 149-150: How was bulk density determined?
L 185-194: It would be nice to see, somewhere in this passage, the %Corg, which is a useful parameter in soil science.
L 310-311: Why is this sentence underlined?
L 338-339: Yes, these differences are not fully resolved. But one reason could be different kinds of transport processes inducing CH4 release (ebullitions vas. Diffusive flux). I suggest elaborating this issue a little.
L 366-384: This issue should be better exploited, also in the results section. Which soil parameters influence acetoclastic and hydrogenotrophic methanogenesis? When do mangroves grow most strongly in the Red sea? This should be the time of a higher rate of acetoclastic CH4 production. Does d13C efflux change with season?
Figs 2, 3: I have the impression that most high CO2 and CH4 fluxes occur between March and May, in both years. Is this mirrored by the d13C signature of the fluxes? Is this the season of highest plant growth?
Figs 5, 6: The predictive power of the year of sampling is interesting. This should be discussed some more.
Citation: https://doi.org/10.5194/egusphere-2024-1831-RC2 -
AC1: 'Reply on RC2', Jessica Breavington, 22 Sep 2024
We would like to thank reviewer 1 for taking the time to give their thorough feedback and useful comments on our paper. We expanded the different sections as requested to explain the methodology and results in more detail, and also expanded on the discussion. For in depth replies, actions taken, and changes in the manuscript please find the document attached. Also uploaded within the same pdf is the correlation matrix figure used for the additional analysis referenced in our response.
-
AC1: 'Reply on RC2', Jessica Breavington, 22 Sep 2024
Data sets
Raw data Jessica Breavington, Alexandra Steckbauer, Chuancheng Fu, Mongi Ennasri, and Carlos M. Duarte https://doi.org/10.6084/m9.figshare.26085898
Landward and seaward fluxes and soil properties Jessica Breavington, Alexandra Steckbauer, Chuancheng Fu, Mongi Ennasri, and Carlos M. Duarte https://doi.org/10.6084/m9.figshare.26085928
Model code and software
Random forest variable importance Jessica Breavington, Alexandra Steckbauer, Chuancheng Fu, Mongi Ennasri, and Carlos M. Duarte https://doi.org/10.6084/m9.figshare.26085940
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