On the role of soil water retention characteristic on aerobic microbial respiration

Ghezzehei, Teamrat A.; Sulman, Benjamin; Arnold, Chelsea L.; Bogie, Nathaniel A.; Berhe, Asmeret Asefaw

doi:https://doi.org/10.5194/bg-16-1187-2019

BG | Articles | Volume 16, issue 6

Biogeosciences, 16, 1187–1209, 2019
https://doi.org/10.5194/bg-16-1187-2019
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

BG | Articles | Volume 16, issue 6

Research article 21 Mar 2019

Research article | 21 Mar 2019

On the role of soil water retention characteristic on aerobic microbial respiration

Teamrat A. Ghezzehei et al.

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Final revised paper (published on 21 Mar 2019)
Preprint (discussion started on 28 Jun 2018)

Interactive discussion

Status: closed

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

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RC1: 'Review', Anonymous Referee #1, 07 Jul 2018
- AC1: 'Responses to Reviewer 1', Teamrat Ghezzehei, 17 Oct 2018
RC2: 'Reviewer evaluation', Anonymous Referee #2, 11 Jul 2018
- AC2: 'Responses to Reviewer 2', Teamrat Ghezzehei, 17 Oct 2018
RC3: 'Physically based modeling of the role of soil matric potential on aerobic microbial respiration', Ali Ebrahimi, 16 Jul 2018
- AC3: 'Responses to Reviewer 3 (Ali Ebrahimi)', Teamrat Ghezzehei, 17 Oct 2018

Peer review completion

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

ED: Reconsider after major revisions (02 Nov 2018) by Michael Weintraub

AR by Teamrat Ghezzehei on behalf of the Authors (03 Nov 2018) Author's response Manuscript

ED: Referee Nomination & Report Request started (19 Nov 2018) by Michael Weintraub

RR by Anonymous Referee #1 (23 Nov 2018)

RR by Anonymous Referee #4 (01 Dec 2018)

Suggestions for revision or reasons for rejection

Major Comments

This paper presents a method for modeling the effects of soil hydrology on soil decomposition rates. The authors do a good job of integrating different water related processes that may limit decomposition into one relatively simple equation. Their approach is therefore more mechanistic than conventional approaches. While the different processes they consider have been explored in previous studies, this study's approach and results are unique in the way the soil water retention characteristics are used to simulate a more complete combined effect of water potential and water content.

I think the manuscript only needs some minor corrections before publication. But I also have some suggestions and comments that the authors may want to consider to improve the final paper.

In Eq. (9), you make oxygen limitation proportional the the relative diffusivity of O2. While this seems like a practical solution, it ignores the fact that a limitation is not only a function of the supply but also of the demand of O2. This means your model may simulate an O2 limitation when none actually exists. Although this may be beyond the scope of your model, it may be good to mention it in the discussion as a potential limitation.

You cite Davidson 2012 who made diffusivity a cubic function of soil water content, but here you make it a function of tortuosity (Eq. 10), i.e. to the power of 1/2. Both have theoretical or empirical evidence to back up the approach, but some justification in the paper of why the tortuosity is preferred over a stronger exponential function often obtained for diffusion in porous media would be helpful.

In the discussion it would be very informative to compare the model results here (as shown in figure 11) with the data-driven results from Moyano et al. 2012.

Minor Comments

Throughout the document, articles and prepositions are missing in several places.

P2L8: the meaning of "average" here is not clear. Averaged over what?

P3L20-21: Matric potential was defined earlier so shouldn't need further explanation. In addition, this sentence is inaccurate (WP is the result of..., not comprised of...).

P4L1: the term "reducing" may be differently interpreted, either as becoming more negative or approaching 0 (if thinking in absolute quantities). I think that using "more negative", here and elsewhere, is clearer.

P5L1-2: The issue of variability in soil properties should also be mentioned as a main factor.

P5L16: SWC does not "exert direct control" on anything. It is simply the relationship between the stated factors.

P5L18-19: There is repetition inside this paragraph and with the intro section.

P6L6: "can be re-written"

P6L10: Pore throat diameter is defined both as 4*sigma/psi and 4*alpha*sigma, although one is defined as the model pore throat diameter. This is confusing and seems inconsistent.

P8L7: a steeper

P9L23: you mean Eq.(7) ?

P11L2: "mechanism through which water"

P11L22: Why do you use tau (which is nowhere defined) and not t at the end of Eq.(11)?

P12L1: for consistency with the new version of Eq(11), update the expression in this line, i.e. make as functions of time.

P16L3: Stay consistent with naming the soils, either using the study names (Arnold, etc) or meadows, etc, reflecting headers in Methods section.

P16L4: In Fig. 6, add in caption what lines represent. Also, caption mentioned Yosemite, which is nowhere else.

P16L22-P17L3: I think the interpretation here is wrong. Soils with higher SOC are likely to result in a higher C_0 (initial C). At the same time a higher C_0 correlates with a lower overall C quality (lower K_0). The incubation duration can affect these numbers. But the duration was equal for all samples of Arnold and therefore does not explain the correlation in panel b.

P17L10: check caption in Fig.10 for consistency.

P17L20: I don't see why this sentence is necessary. Having the SWC parameters, water content could be just as well taken as the independent variable.

P17L24: Replace "However" with "Thus"

P18L9 replace "under" with "when approaching"

P19L5-10: Sensitivities falling to 0 at water contents above 0.2 are not a common observation.

P19L17-18: Describing this study as a first step is inaccurate. This is another step within many studies in this topic.

P20L23: remove the subjective qualifier "particularly valuable"

Update Moyano (2018) citation to final published version:
Moyano, F. E., Vasilyeva, N., & Menichetti, L. (2018). Diffusion limitations and Michaelis–Menten kinetics as drivers of combined temperature and moisture effects on carbon fluxes of mineral soils. Biogeosciences, 15(16), 5031–5045. https://doi.org/10.5194/bg-15-5031-2018

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RR by Anonymous Referee #5 (08 Dec 2018)

Suggestions for revision or reasons for rejection

This manuscript develops a simple but physically-based model of soil water limitation on SOM decomposition. It fits the initial C content and decay rate to the model and tests against four datasets over a wide range of soil textures and water potential. I think this model represents an improvement over other soil water – decomposition relationships that exist since it only has one tunable parameter ka,min. It also implicitly incorporates the effect of aggregation on pore size distribution. Earth system modelers could fairly easily substitute this model for older scalar modifiers of the decomposition rate based on soil water potential.

P3, L15: I’ve always taken kind of a semantic issue ascribing high intracellular osmotic potential to “accumulating electrolytes and …solutes”. Water could also just be exiting the cell due to the existing gradient.

P4, L2: Consistency of terms: At the bottom of the previous page, water content is described parenthetically as matric potential. I like framing the various effects of water by the terms you use to describe the parts of the model: matric potential, dissolved oxygen, and substrate accessibility. This seems a little clearer to me than water potential, soil aeration, and water content (P4, L16), but it depend on the field.

P7, L4: After reading the reviewer comments about the possible bimodality of the other datasets (something I also noticed), I understand why this paragraph is here, but I don’t think the readers will make the connection unless you use this paragraph as a direct justification for assigning two of the datasets to be unimodal.

P8, L7: “They observed that steeper…” should be “They observed a steeper…”?

P8, first paragraph: I think this paragraph is trying to convince me that water potential and diffusion limitation are both important. But I feel like it goes back and forth between water potential is important without diffusion limitation and both are important, maybe you can cut text here a little for a simpler point.

P11, L1: Check the grammar in this first sentence.

P19, L20: What study was this? Peat was not mentioned in the study descriptions.

Table 1: Don study says it lasted 30 days in text, but 1 is listed in the table. Also, why “100?” Were these data already aggregated or something?

In Figure 4, the unimodal model is defined as Van Genuchten, but in Figure 1 the Van Genuchten model is bimodal. Typo?

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ED: Publish subject to minor revisions (review by editor) (18 Dec 2018) by Michael Weintraub

AR by Teamrat Ghezzehei on behalf of the Authors (30 Dec 2018) Author's response Manuscript

ED: Publish as is (21 Jan 2019) by Michael Weintraub

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On the role of soil water retention characteristic on aerobic microbial respiration

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