Review Reports

Round  1

Reviewer 1 Report

General comments:

Wang and colleagues perfomed an study on the effect of forest type, using soil columns, to investigate how different soil properties can have an influence on the soil [CO2] in depth and efflux at the surface. In their experiments, they used undisturbed soil profiles in diferent tratments (upright, upside down, and mixed). The authors concluded that these treatments (soil column inversion) have an effect on the transport of CO2 in the soil profile. This study was performed 10 years ago (2008-2009).

On the overall this manuscript, on the side of statistical design and measurements, I found it well conceived and performed, however, the logic of the experiment is not interesting and not easily translatable to a real-world situation. The findings of the authors are kind of obvious (i.e., invertion of the profile, this is the effect of putting the soil solumn upside down in the laboratory), and there is not a real or interesting finding. This come together with a lack of a scientific question / hypothesis. The way it is not written just tell me that the authors did the experiment and saw what happened. Needed to say, there is an excesive number of Tables/Figures in proportion to the lenght of the manuscript (3 Figures and 2 Tables for a manuscript of 4 pages lenght).

L84. Xu and Qi [18] uses those estimates of representative times from other study (Larionova et al 1989). There are more recent studies on this topic (e.g., Cueva et al 2017 and Jian et al 2018).

Cueva et al. Potential bias of daily soil CO₂ efflux estimates due to sampling time. Sci. Rep. 2017

Jian et al. Measurement strategies to account for soil respiration temporal heterogeneity across diverse regions. Soil Biol. Biochem. 2018

Author Response

Response to Reviewer 1’ Comments

Point 1: Wang and colleagues performed a study on the effect of forest type, using soil columns, to investigate how different soil properties can have an influence on the soil [CO2] in depth and efflux at the surface. In their experiments, they used undisturbed soil profiles in different treatments (upright, upside down, and mixed). The authors concluded that these treatments (soil column inversion) have an effect on the transport of CO2 in the soil profile. This study was performed 10 years ago (2008-2009).

On the overall this manuscript, on the side of statistical design and measurements, I found it well conceived and performed, however, the logic of the experiment is not interesting and not easily translatable to a real-world situation. The findings of the authors are kind of obvious (i.e., inversion of the profile, this is the effect of putting the soil column upside down in the laboratory), and there is not a real or interesting finding. This comes together with a lack of a scientific question / hypothesis. The way it is not written just tells me that the authors did the experiment and saw what happened. Needed to say, there is an excessive number of Tables/Figures in proportion to the length of the manuscript (3 Figures and 2 Tables for a manuscript of 4 pages length).

Response 1: Thank the reviewer for these precious comments and suggestions. This study was performed 10 years ago (2008-2009) but the results were not submitted until now because the two major student researchers were busy  graduating at that time and paid little attention to these data. We revisited the dataset again and again trying to get a solid scientific story and in fact found the results were interesting and the basic knowledge in this study could help us to better understand the mechanism of the vertical stratification of soil profile CO₂. We agree with the reviewer that research data should be published timely though we did not do that.

We improved the introduction, discussion and conclusion sections as suggested and hope that the corrections will meet with approval.

Point 2: L84. Xu and Qi [18] use those estimates of representative times from other study (Larionova et al 1989). There are more recent studies on this topic (e.g., Cueva et al 2017 and Jian et al 2018).

Cueva et al. Potential bias of daily soil CO2 efflux estimates due to sampling time. Sci. Rep. 2017

Jian et al. Measurement strategies to account for soil respiration temporal heterogeneity across diverse regions. Soil Biol. Biochem. 2018

Response 2: We are extremely grateful to the reviewer for this suggestion and cited the references as suggested (See L105).

Larionova, A.A.; Rozonova, L.N.; Samoylov, T.I. Dynamics of gas exchange in the profile of a gray forest soil. Sov. Soil Sci. 1989, 24(7), 1359–1372.

Jian, J.S.; Steele, M.K.; Day, S.D.; Quinn, T.R.; Hodges, S.C. Measurement strategies to account for soil respiration temporal heterogeneity across diverse regions. Soil Biol. Biochem. 2018, 125, 167-177.

Reviewer 2 Report

Reviewer #

Forests-426168

Summary

The manuscript Forests-426168, entitled “Forest soil profile inversion intensifies the vertical stratification of soil CO₂ concentration without altering soil surface CO₂ flux” describes a soil column experiment examining changes in carbon dioxide under contrasted condition: no modification, “inversion” of the profile and “mixing” of the soil layers. The paper, in general, offers an important amount of experimental data of certain relevance regarding the experimental manipulation of soil columns to assess changes in soil respiration. A consequence of the experimental design is the decoupling of the soil organic matter from its original source of assimilation; the nature and fate of the inverted material is of great interest due to its potential influence of carbon dynamics. In this sense, the manuscript offers a good deal of interest as it focus on how concentration and fluxes of carbon dioxide change under particular conditions (i.e. “inversion” and “mixing” of the soil profile).

After a thorough inspection, several issues needing explanation/clarification have been detected, as explained in detail in the text following.

Remarks

First, the experimental section shows some potential caveats: (1) a thorough description (e.g. differences in physical properties or clay type) and characterisation of the (presumably) contrasted soils involved in the experiment is of importance; (2) a more in-depth assessment of the “mixing” treatment would be helpful: what information is obtained by attending at this treatment? What is the hypothesis a priory justifying the inclusion of the “mixing” as treatment?; (3) a clear-cut description of the number of replicates used (6 replications vs 4 measurements; see details below) is paramount; (4) following (2), it is important that the statistical analyses applied are explained in greater detail and compiled e.g. as a new sub-section (”statistical analyses”).

Second, as commented above, the description of the statistical methods used in this work is key to understand the full data analysis and interpret results completely. Statistical results must then be included in figures and tables where possible to help readers to understand the differences found (if any) as depending on the treatments considered in the experiment.

Third, as consequence of previous paragraphs and the details following, the conclusions in the study are not fully supported by the information available in the current version of the study. This fact may constrain the actual relevance of this work. This needs to be acknowledged along the text, downplaying some general comments.

Finally, the manuscript would benefit of proofreading to amend minor mistakes.

Other comments (by line; not exhaustive)

L 60-66. What soils were considered? Please indicate soil types used and include a suitable reference (e.g. WRB, Soil Taxonomy) to help an international audience to identify the soils included in this study.

L 70-71. What does “mixing” refer to? Mixing topsoil with subsoil and vice versa or just physical disruption of the soil (e.g. as in sieving). This needs to be clarified. Anyway, any disturbance of the physical integrity of aggregates should influence the protection of soil organic matter. Did the authors consider this possibility? More details on the choice of this treatment will improve the understanding of the work. Are authors mimicking some situations were soil organic matter can be incorporated in depth (e.g. soil redistribution by erosion or tillage) and buried? How realistic is this in a forest?

L 99-100. Do the soils contain inorganic carbon? If so, this must be indicated. Dichromate oxidation may underestimate soil organic carbon.

L 107-108. Unclear what authors consider “four time measurements”. Four replicates? If so, why, as the number of replicated columns was 6 (L66)? This point needs to be clarified. Was time considered a factor assessing changes in soil properties?

L 111-113. Keep the number of significant digits used for each variable consistent.

L 123-124. Other authors express respiration in units as micromole/kg or micromole/m²; this may help to represent data in a more consistent range of values.

L 132-134. This is an example of statistical information that is necessary to describe earlier in the manuscript, as a coherent and detailed sub-section.

L 147-148. Surprising not find any differences between soils (e.g. BF shows at least 30% more C in topsoil than CF); what soils were used? Soil type may be an important factor to consider in the study.

L 164-166. What date the data on Table 1 or Table 2 refer to? Before the start of the experiment, or after measurements? A reader may find difficult to follow that the data presented is the average values of four sampling dates (i.e. the “four times” mentioned in L 107-108). Again, it must be stressed that changes with time may be key factor to consider for some variables (e.g. Table 2). Was time considered a factor assessing changes in soil properties? If yes, this must be unequivocally indicated.

L 183-185. This would be contrastingly different on the “mixing” treatment, as soil was disturbed. Any comment on this possibility?

L 221-226. The authors should link these results with a more mechanistic understanding and not single properties. Soil bulk density is not necessary a good proxy for soil texture. A reader would be interested in the clay content (and type) in the soils. Were changes in soil water assessed in detail (e.g. changes with time; or was water content kept constant)? These obscure points can be easily clarified to gain a deeper understanding of the outcome of the experiment.

Assessment

As consequence of all the comments detailed above, the general assessment is that the current version of the manuscript needs review.

Author Response

Response to Reviewer 2’ Comments

Point 1: First, the experimental section shows some potential caveats: (1) a thorough description (e.g. differences in physical properties or clay type) and characterisation of the (presumably) contrasted soils involved in the experiment is of importance; (2) a more in-depth assessment of the “mixing” treatment would be helpful: what information is obtained by attending at this treatment? What is the hypothesis a priory justifying the inclusion of the “mixing” as treatment? (3) a clear-cut description of the number of replicates used (6 replications vs 4 measurements; see details below) is paramount; (4) following (2), it is important that the statistical analyses applied are explained in greater detail and compiled e.g. as a new sub-section (”statistical analyses”).

Response 1: (1) The soil is an Orthic Acrisol (FAO, 2006) and the surface soil pH is about 4.0. The clay content is about 38.5% in BF (data from Heshan station).The soils SOC were 13.08 and 19.26 g.kg^-1 in CF and BF, respectively. The soils TN were 0.99 and 1.11 g.kg^-1, respectively (See L82-84);(2) Mixing disturbance like tillage is usually used in the process of restoration in damaged ecosystems;(3) The number of replicates was 6. We sampled the soil at May and November respectively both in 2008 and 2009. This was the“four time measurements” (See L117); (4) We are very sorry for the lack of description to the statistical analyses and we added the sub-section of “Data analysis” as suggested (See L130-135).

FAO. World Reference Base for Soil Resources 2006. World Soil Resources Report 2006, 103, FAO, Rome.

Point 2: Second, as commented above, the description of the statistical methods used in this work is key to understand the full data analysis and interpret results completely. Statistical results must then be included in figures and tables where possible to help readers to understand the differences found (if any) as depending on the treatments considered in the experiment.

Response 2: we agree with the reviewer and added the sub-section of “Data analysis” as suggested (See L130-135). We also added statistical methods in figures’ and tables’ caption.

Point 3: Third, as consequence of previous paragraphs and the details following, the conclusions in the study are not fully supported by the information available in the current version of the study. This fact may constrain the actual relevance of this work. This needs to be acknowledged along the text, downplaying some general comments.

Response 3: Thank you for your comments. We improved conclusion section as suggested and hope the correction will meet with approval (See L263-271).

Point 4: Finally, the manuscript would benefit of proofreading to amend minor mistakes.

Response 4: As suggested, We checked  the manuscript again and made some corrections as possible.  

Point 5: L 60-66. What soils were considered? Please indicate soil types used and include a suitable reference (e.g. WRB, Soil Taxonomy) to help an international audience to identify the soils included in this study.

Response 5: We are very sorry for the lack of description of the soil. The experiments were performed at Heshan Station, Chinese Academy of Sciences, Guangdong Province, China.  This station is located at an altitude of 80 m above sea level, and the soil is an Orthic Acrisol (FAO, 2006) and the surface soil pH is about 4.0 (See L82-84). The clay content is 38.5% in BF. These information were added into the revised version of the manuscript.

Point 6: L 70-71. What does “mixing” refer to? Mixing topsoil with subsoil and vice versa or just physical disruption of the soil (e.g. as in sieving). This needs to be clarified. Anyway, any disturbance of the physical integrity of aggregates should influence the protection of soil organic matter. Did the authors consider this possibility? More details on the choice of this treatment will improve the understanding of the work. Are authors mimicking some situations were soil organic matter can be incorporated in depth (e.g. soil redistribution by erosion or tillage) and buried? How realistic is this in a forest?

Response 6: The soil in “mixing” treatment was mixed the topsoil with subsoil thoroughly (no sieving) mimicking soil ploughing under the forest (e.g. understory farming). It is true as reviewer suggested that soil mixing should influence the protection of soil organic matter. In real world, soil mixing (e.g.tillage and tree planting) occurs when an ecosystem undergoes restoration from degraded status. For example, tea or coffee plantations could be established and interplanted with forest. The field site of our study is exactly a forest restoration ecosystem.

Point 7: L 99-100. Do the soils contain inorganic carbon? If so, this must be indicated. Dichromate oxidation may underestimate soil organic carbon.

Response 7: The soil of the field site is an Orthic Acrisol and the soil pH is about 4.0. Inorganic carbon content in this type of soil could be neglected.

Point 8: L 107-108. Unclear what authors consider “four time measurements”. Four replicates? If so, why, as the number of replicated columns was 6 (L66)? This point needs to be clarified. Was time considered a factor assessing changes in soil properties?

Response 8: The number of replicated columns was 6. We sampled the soil at May and November respectively both in 2008 and 2009 (See L117). This was the“four time measurements”.

Point 9: L 111-113. Keep the number of significant digits used for each variable consistent.

Response 9: Thank you for the suggestion. We have made correction according to your kind comment (See L139).

Point 10: L 123-124. Other authors express respiration in units as micromole/kg or micromole/m2; this may help to represent data in a more consistent range of values.

Response 10: Thank you for this concern.  I think the unit (μL·L^-1) of that line express CO₂ concentration, not for soil respiration. Did I misunderstand your question?

Point 11: L 132-134. This is an example of statistical information that is necessary to describe earlier in the manuscript, as a coherent and detailed sub-section.

Response 11: Thank you for the suggestion. We have added the sub-section of “Statistical analysis” as suggested (See L130-135).

Point 12: L 147-148. Surprising not find any differences between soils (e.g. BF shows at least 30% more C in topsoil than CF); what soils were used? Soil type may be an important factor to consider in the study.

Response 12: Thank you for this comment. It reminds us that this may be attributed to the difference in temperature, as “mean soil temperature at 5 cm depth was 24.73℃ in CF in the study period, which was 1.29℃ higher than in BF”. The soil is an Orthic Acrisol (FAO, 2006).

Point 13: L 164-166. What date the data on Table 1 or Table 2 refer to? Before the start of the experiment, or after measurements? A reader may find difficult to follow that the data presented is the average values of four sampling dates (i.e. the “four times” mentioned in L 107-108). Again, it must be stressed that changes with time may be key factor to consider for some variables (e.g. Table 2). Was time considered a factor assessing changes in soil properties? If yes, this must be unequivocally indicated.

Response 13: Soil along the profiles was sampled in May and November respectively both in 2008 and 2009 (See L117). Time was not considered as a factor when assessing changes in soil properties

Point 14: L 183-185. This would be contrastingly different on the “mixing” treatment, as soil was disturbed. Any comment on this possibility?

Response 14: The soil in “mixing” treatment was mixed the topsoil with subsoil thoroughly (no sieving) mimicking soil ploughing under the forest (e.g. understory farming and tree planting).

Point 15: L 221-226. The authors should link these results with a more mechanistic understanding and not single properties. Soil bulk density is not necessary a good proxy for soil texture. A reader would be interested in the clay content (and type) in the soils. Were changes in soil water assessed in detail (e.g. changes with time; or was water content kept constant)? These obscure points can be easily clarified to gain a deeper understanding of the outcome of the experiment.

Response 15: Special thanks to you for your good comments. The soil is an Orthic Acrisol. The clay content is about 38.5% in BF (data from Heshan station). Average water contents during the experimental period were 26.2% and 20.4% in CF and BF (at 500px-depth), respectively. We have examined the relationship between soil respiration and soil water content changes with time and found the R²<0.01 (Data not shown). We suggested that the studied parameters affected CO₂ profile presentation and made a comprehensive effect.

Reviewer 3 Report

The manuscript Forest soil profile inversion intensifies the vertical stratification of soil CO2 concentration without altering soil surface CO2 flux explores CO2 fluxes in soil columns in case of inversion of the soil profile or mixing of the soil.  The manuscript is written in clear and sound English and presents a solid experimental design and dataset. The idea of exploring the effects of forest soil disturbance in CO2 fluxes might be interesting but the introduction is very short and doesn't explain the importance of the chosen manipulation. Selected parameters to correlate with CO2 flux should be better presented integrating literature on the importance of the parameters like SOM or microbial biomass in soil carbon cycle should be added to allow the reader to appreciate the significance of the work done. Also, the inversion and soil mixing should be related to realistic situations like landslide and soil ploughing. Data are well presented both oi terms of tables and graphs. 

The discussion needs to be improved better explaining how the investigated parameters influenced the CO2 flux recorded. Results could be better discussed comparing with results from similar disturbance/undisturbed soil situations like:

Bailey, N.J., Motavalli, P.P., Udawatta, R.P., and Nelson, K.A. (2009).  Agroforestry Systems 77(2), 143-158. doi: 10.1007/s10457-009-9218-x.

or 

Cardinali, A., Carletti, P., Nardi, S., and Zanin, G. (2014). Applied Soil Ecology 80, 67-76. doi:10.1016/j.apsoil.2014.04.003.

Finally, the conclusion paragraph states that all the studied parameters (temperature, precipitation, SOC, TN; soil density and microbial community) affected CO2 profile presentation without exploring most relevant parameters and the consequences of these foundings. Also reports that soil properties and environmental factors regulate CO2 production, basically simply repeating the premises of the study presented in the introduction.

The paper could be accepted after a major revision of introduction, discussion and conclusion sections.

Author Response

Response to Reviewer 3’ Comments

Point 1: The manuscript Forest soil profile inversion intensifies the vertical stratification of soil CO2 concentration without altering soil surface CO2 flux explores CO2 fluxes in soil columns in case of inversion of the soil profile or mixing of the soil.  The manuscript is written in clear and sound English and presents a solid experimental design and dataset. The idea of exploring the effects of forest soil disturbance in CO2 fluxes might be interesting but the introduction is very short and doesn't explain the importance of the chosen manipulation. Selected parameters to correlate with CO2 flux should be better presented integrating literature on the importance of the parameters like SOM or microbial biomass in soil carbon cycle should be added to allow the reader to appreciate the significance of the work done. Also, the inversion and soil mixing should be related to realistic situations like landslide and soil ploughing. Data are well presented both oi terms of tables and graphs.

Response 1: Special thanks to you for the comments. We have re-written this part according to you and other Reviewers’ suggestions (See L54-75). The soil in “mixing” treatment was mixed the topsoil with subsoil thoroughly mimicking soil ploughing under the forest (e.g. understory farming). In real world, soil mixing (e.g.tillage and tree planting) occurs when an ecosystem undergoes restoration from degraded status. For example, tea or coffee plantations could be established and interplanted with forest. The field site of our study is exactly a forest restoration ecosystem.

Point 2: The discussion needs to be improved better explaining how the investigated parameters influenced the CO2 flux recorded. Results could be better discussed comparing with results from similar disturbance/undisturbed soil situations like:

Bailey, N.J., Motavalli, P.P., Udawatta, R.P., and Nelson, K.A. (2009).  Agroforestry Systems 77(2), 143-158. doi: 10.1007/s10457-009-9218-x.

or

Cardinali, A., Carletti, P., Nardi, S., and Zanin, G. (2014). Applied Soil Ecology 80, 67-76. doi:10.1016/j.apsoil.2014.04.003.

Response 2: Thank you for the suggestions. We improved discussion section and compared our results with results from tillage studies (See L241-258):

Zhang, Z.S.; Cao, C.G.; Guo, L.J.; Li, C.F. Emissions of CH4 and CO2 from paddy fields as affected by tillage practices and crop residues in central china. Paddy Water Environ. 2016, 14(1), 85-92.

Li, C.F.; Kou, Z.K.; Yang, J.H.; Cai, M.L.; Wang, J.P.; Cao, C.G. Soil CO2 fluxes from direct seeding rice fields under two tillage practices in central china. Atmos. Environ. 2010, 44(23), 2696-2704.

Point 3: Finally, the conclusion paragraph states that all the studied parameters (temperature, precipitation, SOC, TN; soil density and microbial community) affected CO2 profile presentation without exploring most relevant parameters and the consequences of these foundings. Also reports that soil properties and environmental factors regulate CO2 production, basically simply repeating the premises of the study presented in the introduction.

Response 3: We have made a correlation analysis among all the studied parameters and CO₂ profile presentation and found that the relationships were confusion or inconsistent among treatments. So we suggested that the studied parameters affected CO₂ profile presentation and made a comprehensive effect when the soils were disturbed in this study. Thank you very much for your comments and suggestions.

Round  2

Reviewer 3 Report

The manuscript has been improved.

Author Response

Point 1: The manuscript has been improved. English language and style are fine/minor spell check required.

Response 1: Thank you for your kind comments and suggestions. We have had our manuscript checked by a professional English editing service and made a revision (MDPI- english-8064).