Vertical partitioning and controlling factors of gradient-based soil carbon dioxide fluxes in two contrasted soil profiles along a loamy hillslope

Wiaux, F.; Vanclooster, M.; Van Oost, K.

doi:https://doi.org/10.5194/bg-12-4637-2015

Articles | Volume 12, issue 15

https://doi.org/10.5194/bg-12-4637-2015

© Author(s) 2015. This work is distributed under
the Creative Commons Attribution 3.0 License.

https://doi.org/10.5194/bg-12-4637-2015

© Author(s) 2015. This work is distributed under
the Creative Commons Attribution 3.0 License.

Articles | Volume 12, issue 15

Research article

|

06 Aug 2015

Research article |

| 06 Aug 2015

Vertical partitioning and controlling factors of gradient-based soil carbon dioxide fluxes in two contrasted soil profiles along a loamy hillslope

F. Wiaux, M. Vanclooster, and K. Van Oost

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Final revised paper (published on 06 Aug 2015)
Preprint (discussion started on 22 Sep 2014)

Interactive discussion

Status: closed

AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment

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- Supplement

RC C6685: 'Review: Wiaux et al,. 2014', Anonymous Referee #1, 13 Nov 2014
AC C6885: 'Response to reviewer #1: this study does not overlap our previous publications', François Wiaux, 20 Nov 2014
RC C7612: 'Review: Wiaux et al., 2014', Anonymous Referee #2, 24 Dec 2014
EC C7616: 'Editor Comment and Additional Reviewer Comment', Daniel Obrist, 24 Dec 2014
EC C7676: 'Editor Comment', Daniel Obrist, 29 Dec 2014
RC C7802: 'Review', Anonymous Referee #3, 05 Jan 2015
AC C8565: 'Response to the Editor: description of how we intend to improve the manuscript (by K. Van Oost and F. Wiaux)', François Wiaux, 08 Feb 2015
- EC C8695: 'Decision: Major Revisions', Daniel Obrist, 11 Feb 2015

Peer-review completion

AR: Author's response | RR: Referee report | ED: Editor decision

AR by François Wiaux on behalf of the Authors (18 Mar 2015) Author's response Manuscript

ED: Referee Nomination & Report Request started (27 Mar 2015) by Daniel Obrist

RR by Anonymous Referee #3 (11 Apr 2015)

RR by Anonymous Referee #1 (17 Apr 2015)

Suggestions for revision or reasons for rejection

This is the second time I have reviewed this manuscript. I think the shortening and focusing of the manuscript has greatly improved this paper, which is now focused much more on soil CO2 profiles, vertical contributions to fluxes, and controlling variables. I still think this is a strong dataset with very sophisticated processing of results, I am particularly impressed how the authors could differentiate between contributions of various depths layers to CO2 fluxes, that is really valuable and highly interesting.
Having said that, the manuscript lacks a clear hypothesis and reasoning for the need of this study. The manuscript still has a lot of editorial issues (lines of argumentation, implications, typos, etc), and I suggest that the senior authors of this manuscript really help with editorial issues. The introduction and results sections is quite thin and very short, and should be expanded in certain areas. The results section should present quantitative relationships between variables (e.g., correlations, or other methods to clarify how much variability is explained by variables, this can be added in the text, no need to add additional figures). This would allow for better quantitative description of the controlling factors and therefore would give the discussion section more credibility (i.e,. more than qualitative description of how physical processes control fluxes/emissions). Finally, I am doubtful how the authors can clearly differentiate between contributions of physical factors versus microbial factors, as they are both strongly interlinked.
I am recommending “major review” to fix these remaining issues, although I think the required changes can be implemented easily as they mainly are related to editorial issues and improving discussion/introduction, and don’t reflect a structural deficit of the data or analysis performed.
Abstract:
Line 22. Start with the purpose and goal of this study, it needs to be clear why this study was conducted, rather than just stating “We assessed…” A clear hypothesis might help as well.
Line 30-31: I don’t think that the fact that 90-95% of the soil flux originated from the surface 10 cm alone supports that soil OC at deeper depth is stabilized – there just could be very little carbon at depth, so you need to state there is plenty of OC carbon (even labile pools) present at higher depths (e.g., Figure 1) but apparently these pools aren’t mineralized and don’t contribute to surface flux.
Line 32-35. You should clarify the new results of this study, e.g., that CO2 fluxes at the footslope are controlled by water content and therefore diffusivity. What are the major implications and meanings of this?
A key conclusion/discussion point of this study might be how surface flux measurements compare to gradient-based diffusion approaches, and what vertical measurement resolution and soil parameters (porosity/diffusivity/soil water content) need to be characterized to obtain reliable fluxes based on soil CO2 characterization?
Introduction:
Line 50: can you expand on this study, how much did subsoil fluxes contribute.
Line 54-57: Can you pls. clarify why this needs to be done in agro-ecosystems, and why you expect patterns to be different than in forests? Please expand on the results from forests, what the implications of this are, and why this needs to be repeated/studies in ago-ecosystems. E.g. how important are agro-ecosystems for CO2 fluxes worldwide? Etc. While the revised paper now is shorter and more focused, the introduction should be expanded to clarify why this study is needed, what the anticipated differences are to forest sites, and what the anticipated differences may be .
Line 63: some clear hypotheses of your expected patterns would help here.
Line 6: can you clarify here what their major difference is?
Materials and Methods:
Entire section: we measured….we inserted….we adjusted… I normally don’t mind active language, but starting each sentence with “we” is not good style. This section (and the full manuscript) should be edited for language and flow. It seems that the writing was left to a student without appropriate internal review by all involved authors – please fix and edit.
Lines 79 to 86: could you clarify why these parameters were measured. It is clear, but a short statement introducing the need for these measurements would help the flow of this section.
Lines 120: We measured VWC at a depth of 10, 25, 35, 50, 70 and 95 cm depth – should be changed to “we measured VWC at depths of….”
Lines 126-128: Is it really the only goal to calibrate the soil gas diffusion model. The authors might want to expand on this stating that a goal is to compare gas diffusion-based fluxes with surface fluxes. I think the authors have a unique ability to clarify how detailed diffusion measurements and soil properties are needed in order to appropriately predict/model surface fluxes.
Lines 147-150: I don’t understand these statements.
Lines 190-199: ok, I guess I now understand, that the discrete surface flux measurements are used to calibrate the diffusion model, and that the measured soil CO2 gradients are used for larger temporal coverage. Still, I think this study has a unique opportunity to compare these two methods and add a good discussion about the challenges/needs when using soil CO2 gradients to predict surface fluxes (i.e., characterizing diffusivities and their temporal/spatial variability).
Results
Entire section 3.1. What I am missing is how these figures link. All (most) of these variables are directly linked, e.g., soil water content affects diffusivity which in turns affects CO2 concentration profiles and fluxes. The authors need to link these figures, e.g., they could explain how much of the variability of certain factors are directly driven by others (e.g., how much of the variability in CO2 concentrations or diffusivity are directly linked to soil water content)? Maybe I am missing something here, but just showing/presenting all these variables separately without showing the connections does not make a lot of sense to me.
Lines 215: What statistical tests were done to evaluate differences? Please clarify.
Line 218: in air? Surface soil? Please clarify.
Lines 228 to 230: soil gas diffusivities are directly linked to water content, so the word “in contrast” doesn’t seem correct. The authors should refer to the correlations (and dependence) of diffusivity to soil water content in describing the patterns (i.e., link Figures 6a and 6c).
Line 243-244: Clearly, these differences are highly significant, but a test should be done and significance levels reported.
Line 246: what model? Clarify what is done here.
Lines 253-255. The authors should expand on the spatial/temporal patterns of CO2 fluxes, what are temporal and spatial patterns, correlations to soil parameters (temperature, SWC, diffusivity, CO2 profiles), etc.
Discussion:
Lines 272 to 276: can you give the predictive power (e.g., r2, or percent variability explained) that is explained by temperature at both locations, rather than just describe this qualitatilvely. The correlations/%variability explained could be added to the results section as suggested above, and then could be discussed here.
Lines 278 – 380: you should quantify this, rather than suggest. You could add the correlation coefficients or %variability explained by each in the results section when reporting patterns in figure 6.
Line 283 – 288: This section needs referencing, there is a large amount of literature on this, the authors need to discuss their results with those of the literature.
Lines 293-297. The authors should put this discussion in respect to the fact that 90-95% of fluxes occurred from the top 10 cm at this location. So does the diffusion barrier prevent contribution from deeper soil levels, and would contributions be likely higher without the diffusion barrier? This discussion should be expanded.
Line 295 to 297: This discussion is a bit thin (are key) – please discuss in detail how continuity and diffusion barriers regulate soil gas emissions, this seems a key point of this study but it is not reference or discussed well.
Lines 298-310: this section needs improvement, and I doubt the authors clearly separate between physical processes and microbial processes since both are highly are interconnected. For example, earlier in the discussion, the authors state that a the footslope, high VWC limits the transfer of CO2 along the sol profile and/or reduces production of CO2 due to a lack of oxygen - so microbial respiration is controlling CO2 production also here, although the underlying reason is not substrate or temperature limitation but likely oxygen limitations – this seems to directly contradict the statements here (lines 307-310). This discussion should be clarified.
Line 312: can you clarify how? I assume you need both respiration AND carbon amount to infer about persistence?
Line 320-3232: I would disagree with this statement, why should forest soil and agro-ecosystems not be comparable. The differences should be clearly discussed, what could cause/contribute to the differences? I assume if other sites don’t have such high water saturation, or not such strong diffusion barriers, then CO2 fluxes probably are related to substrate concentrations? Please clarify.
Line 323-324. The authors should quantify relative controls of different variables in the methods section, then they could be quantitative rather than qualitative.
Line 355: add: and likely the transfer of O2 to deeper soil depths.

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ED: Publish subject to minor revisions (Editor review) (17 Apr 2015) by Daniel Obrist

AR by François Wiaux on behalf of the Authors (04 Jun 2015) Author's response Manuscript

ED: Publish as is (09 Jun 2015) by Daniel Obrist

AR by François Wiaux on behalf of the Authors (17 Jun 2015)

Short summary

In this study, we highlight the role of soil physical conditions and gas transfer mechanisms and dynamics in the decomposition and storage of soil organic carbon in subsoil layers. To illustrate it, we measured the time series of soil temperature, moisture and CO2 concentration and calculated CO2 fluxes along 1 m depth soil profiles during 6 months throughout two contrasted soil profiles along a hillslope in the central loess belt of Belgium.