the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 10 Nov 2023
The Dynamic Effect of Root Exudates on Soil Structure: Aggregate Stability and Packing
Abstract. The importance of soil structure, packing and stability, cannot be overstated as it controls vital processes in the terrestrial environment. Physical, chemical and biological processes altogether affect the dynamics of soil structure with the biological driver being the most complex and least explored. We quantified, developing and applying advanced methods, the effect of mucilage (0.035 % w/w), the main substance in root exudates, on soil packing and stability, by micro-CT and laser granulometry (aggregate durability index), respectively. Upon mucilage addition to soils, or plant growth, soil aggregate size and aggregate stability both increased, however, the intensity varied between the soils, in the order of sandy-clay-loam > loamy-sand > clayey soils. Scanning electron microscope and X-ray diffraction measurements focusing on the smaller soil aggregates (<250 µm) and their mineralogy, bring forward their dominant role in aggregation and stabilization processes induced by mucilage. The complex effects of mucilage coupled with a physical driver, wetting and drying, on microorganism activity, were explored. Compensating microorganism activities, root mucilage consumption and self-mucilaginous polysaccharides production, most likely explain the stability steady state reached within three days. The presence of mucilage in sandy-clay-loam and clayey soils, intensified and overcame the aggregation and disaggregation induced by wetting and drying, respectively. Elucidating soil structure dynamics will enable better understanding of soil stability processes and thereby develop better strategies for soil erosion management.
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RC1: 'Comment on egusphere-2023-2501', Anonymous Referee #1, 01 Dec 2023
Dear authors,
I appreciate your efforts to elucidate the effect of mucilage on soil structure and stability. In its current state, this manuscript lacks essential methodological details, a comprehensive introduction, and a clear description of the objectives. The omission of a substantial body of literature and the general lack of context provided undermine the merit of the study presented, and the discussion and interpretation of the results remain superficial.
Please see below for more specific comments and suggestions.
Line 32: Could you elaborate on how water-stable aggregates improve water infiltration? With regard to splash erosion and erosion induced by runoff, the effect of stability appears plausible.
I suggest you extent the introduction with regarding processes affected by soil structure/aggregation.
Line 44: “Virtually all plants exude about 25% of their total photosynthetic output into the rhizosphere, of which approximately half is in the form of mucilage, a gelatinous high-molecular-weight substance consisting mainly of polysaccharides (Ahmed et al.,2014; Chaboud and Rougier, 1984; Walker et al., 2003).”
Ahmed et al. (2014) used an analogue for root mucilage (chia mucilage). This study does not support your statement.
Furthermore, your statement is an exaggeration given the limited number of studies dealing with plant mucilage.
Very few studies attempted to estimate or quantify mucilage exudation.
http://doi.wiley.com/10.1111/ejss.12487
https://www.frontiersin.org/article/10.3389/fenvs.2018.00087/full
http://doi.wiley.com/10.1002/jpln.201500511
Line 47: The cited study does not include results of soil packing.
Line 51-52: I recommend rephrasing: “…inter-aggregates pores within soil aggregates…”
Line 54-56: Unclear phrasing and insufficiently supported generalization.
The quality and scope of the introduction is not sufficient to provide a clear rationale for your study. Physical and chemical mechanisms are not introduced and explained, nor are potentially affected processes described. A mechanistic framework or context is not provided. It is also unclear why soil structure, aggregate stability, aggregate size distribution, etc. are important. What processes are affected?
What is the difference between packing and stability? How do you define packing? You studied changes in aggregate size and stability, how does this reflect conditions occurring in the rhizosphere? How would gradients in mucilage content from the root surface into the bulk soil affect changes in soil structure and stability?
Line 96: Carminati and Vetterlein, 2013 as well as Kroener et al. 2014 do not include any physical or chemical analysis of mucilage. Please, support your statement with appropriate references.
Line 103: “…suspension of dried mucilage in deionized water was prepared and applied to the soil samples…”. How was mucilage “applied” to the soil?
Line 106: You cite Zickenrott et al . 2016 here. The authors write the following: “The simple relationship shows that any choice of concentration between 0.05 and 50 mg DM g–1 sand can be justified.”, yet you claim 0.35 mg per g of soil is a typically found in the rhizosphere. To my knowledge, the only study which has attempted to measure the mucilage content is Holz et al. (2018). With regard to their study, 0.35 mg per g of soil would be very high.
Line 107-108: It is unclear what you mean by “initial” moisture content and “after each treatment”.
Line 109-113: The description of the method is incomplete.
How was the water applied to the soil?
At which rate?
Once you use WDC as an abbreviation for soil water content while later on, you refer to wetting drying cycles.
How do air dry conditions or the drying process reflect conditions in the rhizosphere?
Line 117-118: At which bulk density were the samples packed? Did you scan soil samples after the treatment? Did you scan aggregates only? Key information regarding the methodology is missing.
Line 163-164: The introduction does not contain any information on soil microbial activity. It is quite challenging to interpret your results without a minimum of background information. What are the processes controlling activity? What is your hypothesis regarding aggregation, soil stability and activity? Did you control the water content of soil samples or was the soil air dry?
Section 2.8: Did you control the soil water content? “High root density” is not a quantity. Did you quantify root biomass or the root adhering soil by any means?
Line 169: As per your description of soil, it does not solely consist of aggregates. Accordingly, you quantified soil aggregate stability, not “soil stability”. What does “in-situ” refer to? The soil was “collected” which indicates a destructive sampling. How did you “collect” the soil?
Line 185-186: Please, see my previous comments on mucilage content in the rhizosphere.
Line 186-192: It is unclear how you introduced mucilage to the soil (methodology). To refer to mucilage concentration in this context is inaccurate as the water content is very low (air dry) and the distribution of mucilage is apparently heterogeneous in space. I suggest you refer to mucilage content instead.
I suggest that you move your hypotheses to the end of the introduction. Furthermore, I recommend that you present and discuss your findings in the context of the published research in the field. A considerable fraction of relevant literature has been omitted. You may want to consider the following literature and references cited therein.
http://doi.wiley.com/10.2136/sssaj1991.03615995005500040030x
https://linkinghub.elsevier.com/retrieve/pii/001670619390106U
http://link.springer.com/10.1007/s11104-013-1910-y
http://doi.wiley.com/10.1111/ejss.12487
http://link.springer.com/10.1007/s11104-019-03939-9
http://link.springer.com/10.1007/s11104-017-3227-8
https://onlinelibrary.wiley.com/doi/10.1002/jpln.202000496
http://link.springer.com/10.1007/s10533-019-00626-w
https://onlinelibrary.wiley.com/doi/abs/10.2136/vzj2018.12.0211
https://onlinelibrary.wiley.com/doi/10.1002/jpln.201800042
http://doi.wiley.com/10.2136/vzj2017.01.0013
https://link.springer.com/article/10.1007/s11104-023-06353-4
https://onlinelibrary.wiley.com/doi/10.1002/jpln.201600453
Line 195-196: “In this context, aggregate size distribution encompasses both soil particles and aggregates, since they cannot be distinguished.” If your statement is true and you cannot distinguish between aggregated und unaggregated particles, how did you quantify aggregation?
Line 201-216: Description, discussion and interpretation of results is lacking.
Line 225-230: You state “Obviously, the degree of aggregation is affected not only by the percentage of small aggregates, but also by their mineralogy.”
I suggest that you base the interpretation and discussion of your results not only on the quantities derived in your study, but also in the context of the related literature and the physical and chemical mechanisms discussed therein.
Line 242-244: Do you suggest that the method is sensitive based on results obtained using the very same method?
Line 246-247: “The increase in stability indicates that mucilage not only acts as a glue, which binds the soil particles together, but also creates stronger interactions between the particles resulting in enhanced soil stability.” Please elaborate, what is the difference between the processes?
281-284: “…therefore the total effect of mucilage on soil stability may depend on the soil aggregates size distribution.” Soil stability has not been part of your study. You quantified the stability of aggregates.
Line 335+: Did you control the soil water content in this setup? If one soil was wetter than the other, then I would argue that this is the reason for the observed differences in respiration. With the same soil water content, the differences may disappear.
Line 340: It is unclear to what “trends” you refer to.
Line 373: “To increase the complexity of the study…”. This was unlikely the intended aim. Further, how is mucilage more of a biochemical than a physical driver?
Citation: https://doi.org/10.5194/egusphere-2023-2501-RC1 -
RC2: 'Comment on egusphere-2023-2501', Anonymous Referee #2, 12 Dec 2023
Dear authors,
I appreciate the novelty of your experiment which consisted in the combined use of X-ray and SEM analyses and ADI values to assess how chia seed mucilage affects soil microstructural stability in three different soils. The considerations on the changes of the aggregate diameter fractions with mucilage amendment is interesting. Still, there are several points which are strongly lacking in your study, and on which I strongly encourage you to work on to improve your manuscript.
First, do a thorough literature review, take into account the physical, chemical and soil microbiological state-of-the art results concerning soil amendment with mucilage and discuss your results in the light of these previous studies.
Second, re-think your research question and hypotheses considering your experiments.
Third, be more precise in your method and result part.
Finally, be honest with your experiments and the message they really deliver (in particularly 3.5).
Abstract
L14: do you mean upon mucilage addition on plant growths? This part of the sentence is not clear for me.
L17: Which role do smaller soil aggregates and their mineralogy have on aggregation and stabilisation processes induced by mucilage?
L20: What do you understand under the “stability steady state”?
L23-24: This lambda sentence does not really highlight your results and resituate them in the general context. You can spare it and rather clear some points in your abstract, as the ones L17 or L20.
Introduction
L32: what do you understand under a “well-structured” soil? Would you define it by its structure or by its withstanding against erosion? Please define, as this is the main focus of your paper. Are the water-stable aggregates only responsible for the improved soil properties? They rather contribute to these properties. Other processes in soil (and you study them in your paper!) contribute to these improved soil properties to resist erosion.
L43: I miss the highlight of the research gap on the effect of root exudated on soil structure. Please review what we know from the literature, what we don’t know and how your research question will contribute to better understand the contribution of root exudates to soil structure.
L43: You write yourself that root exudates are a biochemical driver. Please, define, with help of the literature, the biochemistry known of root exudates.
L45: There are state-of-the art studies on the physico-chemical properties of root mucilage. As these properties are responsible for the effect of mucilage on soil structure, the least would be to cite and shortly discuss them.
L47: This is quite an old study. Please cite state-of-the art studies and (again) draw shortly what we have learnt from them.
I don’t repeat myself anymore, but you keep neglecting the recent studies which have already led some light on processes initiated by mucilage and contributing to improve soil structure.
Also, mucilage is not the same as root exudates, as you should know. You cannot use them as synonym. Please review your text considering this.
L59: Please precise: which concentrations were taken in the studies you refer to, which concentration of mucilage do you expect to find in the rhizosphere?
L65: What do you understand behind “better imaging and analysis” of soil aggregate distribution with the ESEM? When you use comparative such as “better”, your sentence must include “than”. So, please, explain what has already been done in this field, and why your method is “better”.
L66: Hypotheses should follow the research question, which itself should follow the state of the art and highlight of the research gap. They are then followed by the methods you used to answer your research question and test them. Please review the structure of your introduction.
L66: Which mucilage are you talking about? Is this mucilage or root exudates you are meaning? It is known from the literature that some root exudates may weaken soil structure (Naveed et al 2017).
This hypothesis is not very new (cf previous recent literature on mucilage effect on soil structure!). Please rework your hypothesis considering what you did in your study (LL67-69).
Materials and Method
L76: Be more precise on the properties you determined for the soils you analysed.
L78: X-ray diffraction
L79: 2 and 3 in index
L79: You have just defined the abbreviation for XRD a line earlier. You don’t need to do it twice!
L85: specific
L86: Detail how you removed carbonate minerals and salts with buffered acetic acid. In general, please detail the method. It should be written, so that someone can follow what you did and repeat it.
L94: Citation style
L95: In the introduction you have written nothing about mucilage properties, although it is known that mucilage from various species has various properties. It has also been discussed in the literature, that chia seed mucilage is quite different from root mucilage. You should address this in your introduction and argue why you chose chia seed mucilage, and, considering this, how your results should be considered in contrast. Carminati and Vetterlein and Kroener et al didn’t investigate the chemical composition of mucilage. Please cite the right literature.
L99: According to my experience, an 800 μm sieve let damaged seeds through. Did you check, that you have only mucilage?
L103: Which concentration did the mucilage solution have? How did you apply it to the soil samples? Chia seed mucilage is actually quite viscous and mixing with soil is not trivial. How much soil did you take for each treatment?
L104: Please add after “0.35 mg dry mucilage per g of dry soil” the corresponding % concentration in soil.
L109: Why do you take WDC as abbreviation for soil water content? What do you mean under “factorial design”?
L110-111: Check grammar of the sentence.
L136: You destroy soil structure and packing changed by mucilage amendment after passing the samples through a 250 um sieve. It has been shown in the literature (Brax et al, 2020) with SEM that mucilage polymeric structures bridge particles. By sieving the particles, you destroy this structure, and by that the changed structure and packing from mucilage amendment.
Which instrument did you use? Same question for X-ray instrument.
2.7: How often did you do the titration? How many replicates did you have?
L 176: Section 2.3 is mucilage extraction. I believe you didn’t do this on this soil. Please correct.
In general, for this part: you describe many experiments. But I miss the red line between them, as research question and hypotheses, in response to which the experiments are tailored, absolutely need improvement.
Results and Discussion
L186: Before writing “to the best of our knowledge”, do first a thorough literature review and give which concentrations did the other studies consider!
L192: You can compare the strings and webs you observed with the SEM with the ones observed by Brax et al (2019, doi: 10.1016/j.geoderma.2019.01.013) as they are pretty similar.
L192: You hypothesize in the introduction, and with your results you answer your hypotheses. Your results can suggest explanations. But you don’t hypothesize in the result/discussion part.
L195: Explain what is area ratio and the difference with volume ratio. Explain more in detail how you calculated the volume and area ratio in the method part. Do you mean the pore volume or the soil particles and aggregates volume?
L199: After “respectively” cite the figure which you discuss.
L203: Which figure, which graphic?
L204: What about the variations from the replicates? How do they affect the “relatively small positive area”? The counts on x- and y-axis should be more precise to better follow you.
L205: Cite which graphic / result you refer to. In Fig S2, please detail the 6 graphics from a to f.
Generally, Figure S2 is often referred to and its results are used as argument for further speculations. Therefore, I encourage you to put Figure S2 in the main manuscript.
For Part 3.2: Please specify in the method part how you calculate ADI index.
In generally, please check grammar and structure of the sentences, particularly in part 3.
Figures: the format of the letter caption on the graphics (a-d) does not fit to the characters size and form on the figure. Add results of statistic tests on your figures (for example as letters on top of the bars) to emphasize the significance of the results.
When you discuss your results, add the errors when you cite the values (for example, L 302 29 +/- X %).
Figure 6: Check the spelling mistake in the caption.
In general, because each result is directly discussed in the result and discussion part, the overall discussion of the results is quite poor and the main message of the paper remains confusing. Consider separating the results and discussion part and bundling your discussion.
LL313-319: The discussion would really increase in interest if you could bring physico-chemical arguments explaining the decrease of ADI in Fig 6d and how this trend is reversed by mucilage.
L332: Check grammar (tense of verbs).
Ahmed in 2018 published two papers on the mineralization of mucilage by microorganisms. Please refer to these papers and discuss your results according to the state-of-the art knowledge.
LL333-334: Are there some data known from the literature concerning the production of EPS by bacteria in soil?
L335: Did you measure soil respiration each day? You cannot link the cumulative value in Fig. 7a with the daily Index values in Fig 7b and c. You can only make vague supposition concerning the EPS production by bacteria, as you haven’t measured or proven anything concerning this.
L448: You cannot understand the effects of two parameters (mucilage and microorganism) for which you have one cumulative value, by monitoring soil stability over time.
L352: Define steady state more precisely in the text and graphically. On the graphic, especially 7b, I don’t “see” a steady state trend.
In general, I miss the real added value of your investigations on what is already known in the literature. You have found quite interesting things concerning the interactions between structural stability, mineral composition and mucilage. According to me, these are the main findings of your paper. You should put the accent on it.
Mucilage has been investigated chemically and physically. You should take into account such result, as they serve you to explain and discuss your results. Of course, you use fancy techniques in your study, but I really miss a clear message on the originality of your results.
LL380-381: You cannot explain the “steady state” with the production of EPS polymers, you haven’t measured it!!!
Citation: https://doi.org/10.5194/egusphere-2023-2501-RC2 -
EC1: 'Comment on egusphere-2023-2501', Paul Hallett, 08 Jan 2024
Dear Authors,
You have received two comprehensive reviews, both requesting major revisions to the paper. They are in agreement that the literature review needs to be enhanced so that the novelty is clearer and the results are discussed more comprehensively. They have helpfully listed many specific comments as well, which will help to improve the manuscript.
Can you please submit your response by 27 January, after which a major revision will be requested?
Best regards,
Paul
Citation: https://doi.org/10.5194/egusphere-2023-2501-EC1
Interactive discussion
Status: closed
-
RC1: 'Comment on egusphere-2023-2501', Anonymous Referee #1, 01 Dec 2023
Dear authors,
I appreciate your efforts to elucidate the effect of mucilage on soil structure and stability. In its current state, this manuscript lacks essential methodological details, a comprehensive introduction, and a clear description of the objectives. The omission of a substantial body of literature and the general lack of context provided undermine the merit of the study presented, and the discussion and interpretation of the results remain superficial.
Please see below for more specific comments and suggestions.
Line 32: Could you elaborate on how water-stable aggregates improve water infiltration? With regard to splash erosion and erosion induced by runoff, the effect of stability appears plausible.
I suggest you extent the introduction with regarding processes affected by soil structure/aggregation.
Line 44: “Virtually all plants exude about 25% of their total photosynthetic output into the rhizosphere, of which approximately half is in the form of mucilage, a gelatinous high-molecular-weight substance consisting mainly of polysaccharides (Ahmed et al.,2014; Chaboud and Rougier, 1984; Walker et al., 2003).”
Ahmed et al. (2014) used an analogue for root mucilage (chia mucilage). This study does not support your statement.
Furthermore, your statement is an exaggeration given the limited number of studies dealing with plant mucilage.
Very few studies attempted to estimate or quantify mucilage exudation.
http://doi.wiley.com/10.1111/ejss.12487
https://www.frontiersin.org/article/10.3389/fenvs.2018.00087/full
http://doi.wiley.com/10.1002/jpln.201500511
Line 47: The cited study does not include results of soil packing.
Line 51-52: I recommend rephrasing: “…inter-aggregates pores within soil aggregates…”
Line 54-56: Unclear phrasing and insufficiently supported generalization.
The quality and scope of the introduction is not sufficient to provide a clear rationale for your study. Physical and chemical mechanisms are not introduced and explained, nor are potentially affected processes described. A mechanistic framework or context is not provided. It is also unclear why soil structure, aggregate stability, aggregate size distribution, etc. are important. What processes are affected?
What is the difference between packing and stability? How do you define packing? You studied changes in aggregate size and stability, how does this reflect conditions occurring in the rhizosphere? How would gradients in mucilage content from the root surface into the bulk soil affect changes in soil structure and stability?
Line 96: Carminati and Vetterlein, 2013 as well as Kroener et al. 2014 do not include any physical or chemical analysis of mucilage. Please, support your statement with appropriate references.
Line 103: “…suspension of dried mucilage in deionized water was prepared and applied to the soil samples…”. How was mucilage “applied” to the soil?
Line 106: You cite Zickenrott et al . 2016 here. The authors write the following: “The simple relationship shows that any choice of concentration between 0.05 and 50 mg DM g–1 sand can be justified.”, yet you claim 0.35 mg per g of soil is a typically found in the rhizosphere. To my knowledge, the only study which has attempted to measure the mucilage content is Holz et al. (2018). With regard to their study, 0.35 mg per g of soil would be very high.
Line 107-108: It is unclear what you mean by “initial” moisture content and “after each treatment”.
Line 109-113: The description of the method is incomplete.
How was the water applied to the soil?
At which rate?
Once you use WDC as an abbreviation for soil water content while later on, you refer to wetting drying cycles.
How do air dry conditions or the drying process reflect conditions in the rhizosphere?
Line 117-118: At which bulk density were the samples packed? Did you scan soil samples after the treatment? Did you scan aggregates only? Key information regarding the methodology is missing.
Line 163-164: The introduction does not contain any information on soil microbial activity. It is quite challenging to interpret your results without a minimum of background information. What are the processes controlling activity? What is your hypothesis regarding aggregation, soil stability and activity? Did you control the water content of soil samples or was the soil air dry?
Section 2.8: Did you control the soil water content? “High root density” is not a quantity. Did you quantify root biomass or the root adhering soil by any means?
Line 169: As per your description of soil, it does not solely consist of aggregates. Accordingly, you quantified soil aggregate stability, not “soil stability”. What does “in-situ” refer to? The soil was “collected” which indicates a destructive sampling. How did you “collect” the soil?
Line 185-186: Please, see my previous comments on mucilage content in the rhizosphere.
Line 186-192: It is unclear how you introduced mucilage to the soil (methodology). To refer to mucilage concentration in this context is inaccurate as the water content is very low (air dry) and the distribution of mucilage is apparently heterogeneous in space. I suggest you refer to mucilage content instead.
I suggest that you move your hypotheses to the end of the introduction. Furthermore, I recommend that you present and discuss your findings in the context of the published research in the field. A considerable fraction of relevant literature has been omitted. You may want to consider the following literature and references cited therein.
http://doi.wiley.com/10.2136/sssaj1991.03615995005500040030x
https://linkinghub.elsevier.com/retrieve/pii/001670619390106U
http://link.springer.com/10.1007/s11104-013-1910-y
http://doi.wiley.com/10.1111/ejss.12487
http://link.springer.com/10.1007/s11104-019-03939-9
http://link.springer.com/10.1007/s11104-017-3227-8
https://onlinelibrary.wiley.com/doi/10.1002/jpln.202000496
http://link.springer.com/10.1007/s10533-019-00626-w
https://onlinelibrary.wiley.com/doi/abs/10.2136/vzj2018.12.0211
https://onlinelibrary.wiley.com/doi/10.1002/jpln.201800042
http://doi.wiley.com/10.2136/vzj2017.01.0013
https://link.springer.com/article/10.1007/s11104-023-06353-4
https://onlinelibrary.wiley.com/doi/10.1002/jpln.201600453
Line 195-196: “In this context, aggregate size distribution encompasses both soil particles and aggregates, since they cannot be distinguished.” If your statement is true and you cannot distinguish between aggregated und unaggregated particles, how did you quantify aggregation?
Line 201-216: Description, discussion and interpretation of results is lacking.
Line 225-230: You state “Obviously, the degree of aggregation is affected not only by the percentage of small aggregates, but also by their mineralogy.”
I suggest that you base the interpretation and discussion of your results not only on the quantities derived in your study, but also in the context of the related literature and the physical and chemical mechanisms discussed therein.
Line 242-244: Do you suggest that the method is sensitive based on results obtained using the very same method?
Line 246-247: “The increase in stability indicates that mucilage not only acts as a glue, which binds the soil particles together, but also creates stronger interactions between the particles resulting in enhanced soil stability.” Please elaborate, what is the difference between the processes?
281-284: “…therefore the total effect of mucilage on soil stability may depend on the soil aggregates size distribution.” Soil stability has not been part of your study. You quantified the stability of aggregates.
Line 335+: Did you control the soil water content in this setup? If one soil was wetter than the other, then I would argue that this is the reason for the observed differences in respiration. With the same soil water content, the differences may disappear.
Line 340: It is unclear to what “trends” you refer to.
Line 373: “To increase the complexity of the study…”. This was unlikely the intended aim. Further, how is mucilage more of a biochemical than a physical driver?
Citation: https://doi.org/10.5194/egusphere-2023-2501-RC1 -
RC2: 'Comment on egusphere-2023-2501', Anonymous Referee #2, 12 Dec 2023
Dear authors,
I appreciate the novelty of your experiment which consisted in the combined use of X-ray and SEM analyses and ADI values to assess how chia seed mucilage affects soil microstructural stability in three different soils. The considerations on the changes of the aggregate diameter fractions with mucilage amendment is interesting. Still, there are several points which are strongly lacking in your study, and on which I strongly encourage you to work on to improve your manuscript.
First, do a thorough literature review, take into account the physical, chemical and soil microbiological state-of-the art results concerning soil amendment with mucilage and discuss your results in the light of these previous studies.
Second, re-think your research question and hypotheses considering your experiments.
Third, be more precise in your method and result part.
Finally, be honest with your experiments and the message they really deliver (in particularly 3.5).
Abstract
L14: do you mean upon mucilage addition on plant growths? This part of the sentence is not clear for me.
L17: Which role do smaller soil aggregates and their mineralogy have on aggregation and stabilisation processes induced by mucilage?
L20: What do you understand under the “stability steady state”?
L23-24: This lambda sentence does not really highlight your results and resituate them in the general context. You can spare it and rather clear some points in your abstract, as the ones L17 or L20.
Introduction
L32: what do you understand under a “well-structured” soil? Would you define it by its structure or by its withstanding against erosion? Please define, as this is the main focus of your paper. Are the water-stable aggregates only responsible for the improved soil properties? They rather contribute to these properties. Other processes in soil (and you study them in your paper!) contribute to these improved soil properties to resist erosion.
L43: I miss the highlight of the research gap on the effect of root exudated on soil structure. Please review what we know from the literature, what we don’t know and how your research question will contribute to better understand the contribution of root exudates to soil structure.
L43: You write yourself that root exudates are a biochemical driver. Please, define, with help of the literature, the biochemistry known of root exudates.
L45: There are state-of-the art studies on the physico-chemical properties of root mucilage. As these properties are responsible for the effect of mucilage on soil structure, the least would be to cite and shortly discuss them.
L47: This is quite an old study. Please cite state-of-the art studies and (again) draw shortly what we have learnt from them.
I don’t repeat myself anymore, but you keep neglecting the recent studies which have already led some light on processes initiated by mucilage and contributing to improve soil structure.
Also, mucilage is not the same as root exudates, as you should know. You cannot use them as synonym. Please review your text considering this.
L59: Please precise: which concentrations were taken in the studies you refer to, which concentration of mucilage do you expect to find in the rhizosphere?
L65: What do you understand behind “better imaging and analysis” of soil aggregate distribution with the ESEM? When you use comparative such as “better”, your sentence must include “than”. So, please, explain what has already been done in this field, and why your method is “better”.
L66: Hypotheses should follow the research question, which itself should follow the state of the art and highlight of the research gap. They are then followed by the methods you used to answer your research question and test them. Please review the structure of your introduction.
L66: Which mucilage are you talking about? Is this mucilage or root exudates you are meaning? It is known from the literature that some root exudates may weaken soil structure (Naveed et al 2017).
This hypothesis is not very new (cf previous recent literature on mucilage effect on soil structure!). Please rework your hypothesis considering what you did in your study (LL67-69).
Materials and Method
L76: Be more precise on the properties you determined for the soils you analysed.
L78: X-ray diffraction
L79: 2 and 3 in index
L79: You have just defined the abbreviation for XRD a line earlier. You don’t need to do it twice!
L85: specific
L86: Detail how you removed carbonate minerals and salts with buffered acetic acid. In general, please detail the method. It should be written, so that someone can follow what you did and repeat it.
L94: Citation style
L95: In the introduction you have written nothing about mucilage properties, although it is known that mucilage from various species has various properties. It has also been discussed in the literature, that chia seed mucilage is quite different from root mucilage. You should address this in your introduction and argue why you chose chia seed mucilage, and, considering this, how your results should be considered in contrast. Carminati and Vetterlein and Kroener et al didn’t investigate the chemical composition of mucilage. Please cite the right literature.
L99: According to my experience, an 800 μm sieve let damaged seeds through. Did you check, that you have only mucilage?
L103: Which concentration did the mucilage solution have? How did you apply it to the soil samples? Chia seed mucilage is actually quite viscous and mixing with soil is not trivial. How much soil did you take for each treatment?
L104: Please add after “0.35 mg dry mucilage per g of dry soil” the corresponding % concentration in soil.
L109: Why do you take WDC as abbreviation for soil water content? What do you mean under “factorial design”?
L110-111: Check grammar of the sentence.
L136: You destroy soil structure and packing changed by mucilage amendment after passing the samples through a 250 um sieve. It has been shown in the literature (Brax et al, 2020) with SEM that mucilage polymeric structures bridge particles. By sieving the particles, you destroy this structure, and by that the changed structure and packing from mucilage amendment.
Which instrument did you use? Same question for X-ray instrument.
2.7: How often did you do the titration? How many replicates did you have?
L 176: Section 2.3 is mucilage extraction. I believe you didn’t do this on this soil. Please correct.
In general, for this part: you describe many experiments. But I miss the red line between them, as research question and hypotheses, in response to which the experiments are tailored, absolutely need improvement.
Results and Discussion
L186: Before writing “to the best of our knowledge”, do first a thorough literature review and give which concentrations did the other studies consider!
L192: You can compare the strings and webs you observed with the SEM with the ones observed by Brax et al (2019, doi: 10.1016/j.geoderma.2019.01.013) as they are pretty similar.
L192: You hypothesize in the introduction, and with your results you answer your hypotheses. Your results can suggest explanations. But you don’t hypothesize in the result/discussion part.
L195: Explain what is area ratio and the difference with volume ratio. Explain more in detail how you calculated the volume and area ratio in the method part. Do you mean the pore volume or the soil particles and aggregates volume?
L199: After “respectively” cite the figure which you discuss.
L203: Which figure, which graphic?
L204: What about the variations from the replicates? How do they affect the “relatively small positive area”? The counts on x- and y-axis should be more precise to better follow you.
L205: Cite which graphic / result you refer to. In Fig S2, please detail the 6 graphics from a to f.
Generally, Figure S2 is often referred to and its results are used as argument for further speculations. Therefore, I encourage you to put Figure S2 in the main manuscript.
For Part 3.2: Please specify in the method part how you calculate ADI index.
In generally, please check grammar and structure of the sentences, particularly in part 3.
Figures: the format of the letter caption on the graphics (a-d) does not fit to the characters size and form on the figure. Add results of statistic tests on your figures (for example as letters on top of the bars) to emphasize the significance of the results.
When you discuss your results, add the errors when you cite the values (for example, L 302 29 +/- X %).
Figure 6: Check the spelling mistake in the caption.
In general, because each result is directly discussed in the result and discussion part, the overall discussion of the results is quite poor and the main message of the paper remains confusing. Consider separating the results and discussion part and bundling your discussion.
LL313-319: The discussion would really increase in interest if you could bring physico-chemical arguments explaining the decrease of ADI in Fig 6d and how this trend is reversed by mucilage.
L332: Check grammar (tense of verbs).
Ahmed in 2018 published two papers on the mineralization of mucilage by microorganisms. Please refer to these papers and discuss your results according to the state-of-the art knowledge.
LL333-334: Are there some data known from the literature concerning the production of EPS by bacteria in soil?
L335: Did you measure soil respiration each day? You cannot link the cumulative value in Fig. 7a with the daily Index values in Fig 7b and c. You can only make vague supposition concerning the EPS production by bacteria, as you haven’t measured or proven anything concerning this.
L448: You cannot understand the effects of two parameters (mucilage and microorganism) for which you have one cumulative value, by monitoring soil stability over time.
L352: Define steady state more precisely in the text and graphically. On the graphic, especially 7b, I don’t “see” a steady state trend.
In general, I miss the real added value of your investigations on what is already known in the literature. You have found quite interesting things concerning the interactions between structural stability, mineral composition and mucilage. According to me, these are the main findings of your paper. You should put the accent on it.
Mucilage has been investigated chemically and physically. You should take into account such result, as they serve you to explain and discuss your results. Of course, you use fancy techniques in your study, but I really miss a clear message on the originality of your results.
LL380-381: You cannot explain the “steady state” with the production of EPS polymers, you haven’t measured it!!!
Citation: https://doi.org/10.5194/egusphere-2023-2501-RC2 -
EC1: 'Comment on egusphere-2023-2501', Paul Hallett, 08 Jan 2024
Dear Authors,
You have received two comprehensive reviews, both requesting major revisions to the paper. They are in agreement that the literature review needs to be enhanced so that the novelty is clearer and the results are discussed more comprehensively. They have helpfully listed many specific comments as well, which will help to improve the manuscript.
Can you please submit your response by 27 January, after which a major revision will be requested?
Best regards,
Paul
Citation: https://doi.org/10.5194/egusphere-2023-2501-EC1
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Maoz Dor
Itamar Assa
Yael Mishael
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