the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 24 Oct 2023
Sorption of Colored vs Noncolored Organic Matter by Tidal Marsh Soils
Abstract. Tidal marshes are significant sources of colored (or chromophoric) dissolved organic carbon (CDOC) to adjacent waters and, as a result, contribute substantially to their optical complexity making CDOC a good indicator of water quality. Despite this, our mechanistic understanding of the processes that regulate the exchange and transformation of CDOC at the tidal marsh-estuarine interface remains limited. We hypothesized that tidal marsh soils regulate this exchange and transformation subject to soil mineralogy and salinity environment. To test this hypothesis, we generated initial mass sorption isotherms of CDOC and noncolored dissolved organic carbon (NCDOC) using anaerobic batch incubations with four tidal wetland soils, representing a range of organic carbon content (1.77 ± 0.12 % to 36.2 ± 2.2 %) and across four salinity treatments (0, 10, 20, and 35). CDOC sorption followed Langmuir isotherms that were similar in shape to those of total DOC, but with greater maximum sorption capacity and lower binding affinity. Like isotherms of total DOC, CDOC maximum sorption capacity increased and binding affinity decreased with greater salinity. Initial natively adsorbed colored organic carbon was low and increased with soil organic content. In contrast, NCDOC desorbed under all conditions with desorption increasing linearly with initial CDOC concentration. This suggests that for our test solutions (made from Great Dismal Swamp DOC), CDOC displaced NCDOC on tidal marsh soils. Parallel factor analysis of 3-D excitation emission matrices and specific ultraviolet absorbance measurements suggested that CDOC sorption was driven primarily by the exchange of highly aromatic humic-like CDOC. Taken together, these results suggest that tidal marsh soils regulate export and composition of CDOC depending on the complex interplay between soil minerology, water salinity, and CDOC vs NCDOC composition.
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RC1: 'Comment on egusphere-2023-2329', Anonymous Referee #1, 31 Dec 2023
Overall I found the paper to be a nicely written and clear study. The experiment is interesting and novel, and the topic is an important one with major implications for marine biogeochemistry.
The one major nagging question that I had throughout the entire review was that the authors have not accounted for, or even discussed anywhere, the potential limitations of using absorbance alone to partition colored versus non-colored DOC concentrations. Specifically, can shifts in absorbance in response to salinization induce optical changes in the DOM pool that alter absorbance through concentration-independent processes? Salt additions that alter ion binding patterns in the soil matrix could liberate/precipitate stuff like iron that may impact absorbance, no? It could also alter the chelation of DOM and metals and other materials, altering optical conditions? I am not raising these potential mechanisms to say that they invalidate the results. It is likely that at these DOC concentrations, such effects would be less important. BUT, using knowledge from the literature, and any other information on hand, can the authors speak to what role, if any, these effects have on their interpretations and conclusions (even if only to caveat them)?
Specific comments:
L23- GDS DOC – mention what this is at the start of abst. It’s introduced in a strange spot.
L33 - Intro – Polydisp. – define what this is in brackets.
L58 – Run on sentence.
L96 – What is a “filter cale”? Can you reword with something more generic? I have never seen the word “cale” before like this.
L106- Is the NaN3 a preservative? Explain purpose briefly.
L123- Any details about this regression R2/strength criteria used? Were relationships scrutinized in any way?
L163- Fig 1 caption. Explain blue solid/dashed arrows so readers don’t need to sift through text to interpret figure.
L204 – Fig 3. I spent a bunch of time trying to figure this out, then realized you are using negative values to mean desorption. Please state this up front in the caption. Even better would be to add a dashed zero line to signify the difference.
L215- All of this isotherm theory and mathematical model interpretation would be far easier to follow as a section fully developed in the methods, perhaps with a conceptual diagram of the isotherm graph.
L240- Throughout results, the ability for readers to wade through this info would be improved with the use of sub-headings to group info by themes.
L251- Figure 4- Confusing. Is the y axis still initial minus final? If so, make that one clear.
L273- Again, a bunch of this background definition content would be nice to see in a dedicated methods section up front.
L281- Initial sentence not needed.
L293- “than less” – grammar.
L330- Do the authors find this odd that these components decrease? NCDOC increases, so should these components not also go up? I am not saying that they must, I am likely missing something here, but just want the authors to explain this to me/readers.
L345- This figure feels a bit unnecessary/of limited use in current format. I’d swap out the text ”dismalswamp” with something generic so that someone outside the region will understand is the stock solution without knowing the location it came from, and not having to consult the methods again. The grey/brown DOC could be defined in a legend so it is visually obvious what the color coding means. The figure could be redone to reflect what increasing salinity does to DOC sorption, which is a key question as I take it in the paper.
L362- Ref format in brackets, run on sentence as written.
L364- Why redefining GDS here?
L393- Photodegradation reference needed here.
L420- If the conceptual figure could link to this sentence it would be more useful. See my comment above about incorporating salinity.
L437- You come back to flood/ebb differences in DOM here. A few more sentences integrating the mechanisms exposed by the experiment, and how they potentially help to explain the empirical observations in the literature, would strengthen the discussion.
Citation: https://doi.org/10.5194/egusphere-2023-2329-RC1 - AC1: 'Reply on RC1', Patrick Neale, 20 Feb 2024
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RC2: 'Comment on egusphere-2023-2329', Anonymous Referee #2, 16 Jan 2024
“Sorption of Colored vs Noncolored Organic Matter by Tidal Marsh Soils”
The manuscript presents novel data on adsorption-desorption of DOM (as in DOC and CDOM) in tidal marsh wetland soils under anaerobic conditions, under different salinity regimes. The manuscript is well-structured and easy to read and contains valuable new insights, which have important implications for future changes in such vulnerable coastal ecosystems.
I recommend publication after the following comments have been sufficiently addressed:
- The methods lack information on precision and trueness of TOC, DOC and metals measurements. EEMs are introduced in the results only. I understand the reasoning, but it still comes as a bit of a surprise and lacks important information on measurements. This should be more detailed and go into the methods section. In my view the rationale can be mentioned without giving away too much of the results.
- The Instant Ocean salt origin (is it sea salt, or mineral salts mixed) and composition should be mentioned in the methods.
- pH measurements are mentioned, but I could not find the data in the results. Both, pH and Instant Ocean salt composition (specifically the divalent cations) should have an influence on adsorption-desorption properties of DOM. The data should be presented and discussed accordingly.
- Assuming that DOM only interacts with leachable (poorly crystalline) iron and aluminum, it should be easy to evaluate whether the amount of leachable metals suffices to adsorb these huge amounts of concentrated DOM. There are plenty of experimental estimates on metals:carbon ratios and it would be an interesting calculation exercise in my view. Again, additional factors may play a role like divalent salt cations, arsenic or perhaps DOM aromatic-aromatic interactions.
- Section 3.3, line 281 “the spectral characteristics of DOC are a robust proxy…” is a bold statement considering the EEMs results in this manuscript. On the contrary, it seems that optical properties are limited to assessing CDOC behavior including absorbance and fluorescence. Consider rephrasing this and including a few sentences in the discussion on why there is so little additional insight.
- Section 3.3, lines 292-293 “the average increase was slightly greater (…) than less for …” I think the “less” can be removed.
- Acknowledgements, lines 490-491 “We would also like to thank the staff of the United States Department of Energy Environmental Molecular Science Laboratory for conducting the FT-ICR MS measurements.” I’d be happy to see those data, but I guess the line is from another (future?) paper. Please conduct a final cross-check for typos and inconsistencies throughout the manuscript.
Citation: https://doi.org/10.5194/egusphere-2023-2329-RC2 - AC2: 'Reply on RC2', Patrick Neale, 20 Feb 2024
Status: closed
-
RC1: 'Comment on egusphere-2023-2329', Anonymous Referee #1, 31 Dec 2023
Overall I found the paper to be a nicely written and clear study. The experiment is interesting and novel, and the topic is an important one with major implications for marine biogeochemistry.
The one major nagging question that I had throughout the entire review was that the authors have not accounted for, or even discussed anywhere, the potential limitations of using absorbance alone to partition colored versus non-colored DOC concentrations. Specifically, can shifts in absorbance in response to salinization induce optical changes in the DOM pool that alter absorbance through concentration-independent processes? Salt additions that alter ion binding patterns in the soil matrix could liberate/precipitate stuff like iron that may impact absorbance, no? It could also alter the chelation of DOM and metals and other materials, altering optical conditions? I am not raising these potential mechanisms to say that they invalidate the results. It is likely that at these DOC concentrations, such effects would be less important. BUT, using knowledge from the literature, and any other information on hand, can the authors speak to what role, if any, these effects have on their interpretations and conclusions (even if only to caveat them)?
Specific comments:
L23- GDS DOC – mention what this is at the start of abst. It’s introduced in a strange spot.
L33 - Intro – Polydisp. – define what this is in brackets.
L58 – Run on sentence.
L96 – What is a “filter cale”? Can you reword with something more generic? I have never seen the word “cale” before like this.
L106- Is the NaN3 a preservative? Explain purpose briefly.
L123- Any details about this regression R2/strength criteria used? Were relationships scrutinized in any way?
L163- Fig 1 caption. Explain blue solid/dashed arrows so readers don’t need to sift through text to interpret figure.
L204 – Fig 3. I spent a bunch of time trying to figure this out, then realized you are using negative values to mean desorption. Please state this up front in the caption. Even better would be to add a dashed zero line to signify the difference.
L215- All of this isotherm theory and mathematical model interpretation would be far easier to follow as a section fully developed in the methods, perhaps with a conceptual diagram of the isotherm graph.
L240- Throughout results, the ability for readers to wade through this info would be improved with the use of sub-headings to group info by themes.
L251- Figure 4- Confusing. Is the y axis still initial minus final? If so, make that one clear.
L273- Again, a bunch of this background definition content would be nice to see in a dedicated methods section up front.
L281- Initial sentence not needed.
L293- “than less” – grammar.
L330- Do the authors find this odd that these components decrease? NCDOC increases, so should these components not also go up? I am not saying that they must, I am likely missing something here, but just want the authors to explain this to me/readers.
L345- This figure feels a bit unnecessary/of limited use in current format. I’d swap out the text ”dismalswamp” with something generic so that someone outside the region will understand is the stock solution without knowing the location it came from, and not having to consult the methods again. The grey/brown DOC could be defined in a legend so it is visually obvious what the color coding means. The figure could be redone to reflect what increasing salinity does to DOC sorption, which is a key question as I take it in the paper.
L362- Ref format in brackets, run on sentence as written.
L364- Why redefining GDS here?
L393- Photodegradation reference needed here.
L420- If the conceptual figure could link to this sentence it would be more useful. See my comment above about incorporating salinity.
L437- You come back to flood/ebb differences in DOM here. A few more sentences integrating the mechanisms exposed by the experiment, and how they potentially help to explain the empirical observations in the literature, would strengthen the discussion.
Citation: https://doi.org/10.5194/egusphere-2023-2329-RC1 - AC1: 'Reply on RC1', Patrick Neale, 20 Feb 2024
-
RC2: 'Comment on egusphere-2023-2329', Anonymous Referee #2, 16 Jan 2024
“Sorption of Colored vs Noncolored Organic Matter by Tidal Marsh Soils”
The manuscript presents novel data on adsorption-desorption of DOM (as in DOC and CDOM) in tidal marsh wetland soils under anaerobic conditions, under different salinity regimes. The manuscript is well-structured and easy to read and contains valuable new insights, which have important implications for future changes in such vulnerable coastal ecosystems.
I recommend publication after the following comments have been sufficiently addressed:
- The methods lack information on precision and trueness of TOC, DOC and metals measurements. EEMs are introduced in the results only. I understand the reasoning, but it still comes as a bit of a surprise and lacks important information on measurements. This should be more detailed and go into the methods section. In my view the rationale can be mentioned without giving away too much of the results.
- The Instant Ocean salt origin (is it sea salt, or mineral salts mixed) and composition should be mentioned in the methods.
- pH measurements are mentioned, but I could not find the data in the results. Both, pH and Instant Ocean salt composition (specifically the divalent cations) should have an influence on adsorption-desorption properties of DOM. The data should be presented and discussed accordingly.
- Assuming that DOM only interacts with leachable (poorly crystalline) iron and aluminum, it should be easy to evaluate whether the amount of leachable metals suffices to adsorb these huge amounts of concentrated DOM. There are plenty of experimental estimates on metals:carbon ratios and it would be an interesting calculation exercise in my view. Again, additional factors may play a role like divalent salt cations, arsenic or perhaps DOM aromatic-aromatic interactions.
- Section 3.3, line 281 “the spectral characteristics of DOC are a robust proxy…” is a bold statement considering the EEMs results in this manuscript. On the contrary, it seems that optical properties are limited to assessing CDOC behavior including absorbance and fluorescence. Consider rephrasing this and including a few sentences in the discussion on why there is so little additional insight.
- Section 3.3, lines 292-293 “the average increase was slightly greater (…) than less for …” I think the “less” can be removed.
- Acknowledgements, lines 490-491 “We would also like to thank the staff of the United States Department of Energy Environmental Molecular Science Laboratory for conducting the FT-ICR MS measurements.” I’d be happy to see those data, but I guess the line is from another (future?) paper. Please conduct a final cross-check for typos and inconsistencies throughout the manuscript.
Citation: https://doi.org/10.5194/egusphere-2023-2329-RC2 - AC2: 'Reply on RC2', Patrick Neale, 20 Feb 2024
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