Comparing models of microbial–substrate interactions and their response to  warming

Sihi, Debjani; Gerber, Stefan; Inglett, Patrick W.; Inglett, Kanika Sharma

doi:https://doi.org/10.5194/bg-13-1733-2016

Articles | Volume 13, issue 6

https://doi.org/10.5194/bg-13-1733-2016

© Author(s) 2016. This work is distributed under
the Creative Commons Attribution 3.0 License.

https://doi.org/10.5194/bg-13-1733-2016

© Author(s) 2016. This work is distributed under
the Creative Commons Attribution 3.0 License.

Articles | Volume 13, issue 6

Research article

|

21 Mar 2016

Research article |

| 21 Mar 2016

Comparing models of microbial–substrate interactions and their response to warming

Debjani Sihi, Stefan Gerber, Patrick W. Inglett, and Kanika Sharma Inglett

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Final revised paper (published on 21 Mar 2016)
Preprint (discussion started on 10 Jul 2015)

Interactive discussion

Status: closed

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

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- Supplement

RC C4421: 'Review of Sihi et al. 2015', Katerina Georgiou, 18 Aug 2015
- AC C6726: 'Response to Reviewer 1 comments', Debjani Sihi, 20 Oct 2015
RC C5443: 'Sihi et al. 2015 Review', Will Wieder, 16 Sep 2015
- AC C6747: 'Response to Reviewer 2 comments', Debjani Sihi, 20 Oct 2015

Peer-review completion

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

ED: Reconsider after major revisions (05 Nov 2015) by Sönke Zaehle

AR by Debjani Sihi on behalf of the Authors (18 Dec 2015) Author's response Manuscript

ED: Referee Nomination & Report Request started (12 Jan 2016) by Sönke Zaehle

RR by William Wieder (04 Feb 2016)

Suggestions for revision or reasons for rejection

General comments
Sihi and co-authors have done a nice job revising their manuscript. The reorganization better leads the reader through the experiments & findings.

Specific comments
I’m not sure why the authors made the quasi-steady state assumption for microbial biomass in their microbial explicit model (Section 2.1.2 & 3.3)? I don’t recall this analysis in the first submission, or see where previous reviews asked for the analysis? More, removing M as a state variable in the model hardly seems to simplify things, when M has to be approximated by the short time-scale equation in Table 2. The first half of the text in 3.3 seems more appropriate for the methods, and the second half of the text discusses results that are never show (as far as I can tell). This makes evaluating the claims being made challenging, but I think they are shown in Fig 3 (although this isn’t referenced in the text)? Finally, these results are sparingly mentioned in the discussion, and I wonder if they contributed to, or distract from the story being presented here? If it’s the former, minor changes are needed to better integrate these results throughout the paper.

The ideas in the discussion are well developed and organized, but I find it helpful to refer to figures in the discussion w/ relevant text (same is true in the results).

I’m not really clear how these conclusions were reached. It seems like the authors are suggesting if we take a more complicated model (reverse M-M) and add even more complexity (optimized for yield) we can produce a model w/ identical form and function to a simple first order model. Why not just use a first order model? I wonder if those simplifying assumptions listed in the conclusion seem reasonable? Is this how we think soils work, or may the assumptions that led to this conclusion be unrealistic? How do they compare to assumptions behind a first order model? There seems to be some rich ideas here if you’re interested in exploring them (although it may not be necessary in the text).

There are distracting grammatical errors in the text, which should be carefully proofread before publication.

Technical corrections
P4 L5- The authors contend that “As microbial models are considered critical towards improvement of Earth System model”. I’m not sure this statement is widely agreed upon, it’s also somewhat misleading for the scope of the study presented here. Instead, it may be safer to state “As microbial models are considered for broader application in models…”?

P6 L10-21 The use of KE and KM (for forward and reverse models, respectively) in eq. 3 & 4, their subsequent description in the text, and in appendixes is somewhat confusing because it does not follow conventions used the papers on which this study is largely based. Specifically for the forward model, German and others (2012) state “Km is the substrate concentration at half-maximal velocity”, while the reverse model of Schimel and Weintraub (2003) use “Kes half saturation constant for enzymes on substrate”.
Eq. 13. This isn’t the first time someone has looked at temperature sensitive CUE. I see a reference in Table 3, but it’s likely worth citing Allison et al. 2010 (or others) in the text here.

Section 3.4. also seems to refers to results, but never references a figure. I’m assuming it should refer to Fig. 3, but shouldn’t have to.

Discussion (and Fig. 2). I wonder what are we assuming by assigning really low Km value for the OPT model- that the soil environment is basically saturated w/ respect to enzymes, such at addition of more microbes (or enzymes) yields no fitness advantage growth? I’m assuming you could parameterize a similar model with different Vmax and Km values that have non-zero Km values, in which case the optimization would produce qualitatively different results? This is suggested in the discussion, and while I’m not sure it needs greater attention in the text, seems like an interesting result.

P21, L21. Check accuracy of text in revering to KM, KE, and Kp values values.

Table 2 What is the value for Kp *c in the OPT model? Also, check units for parameters that are given for accuracy.

Fig. 1 Is kind of busy and difficult to understand. Several suggestions to bring greater clarity follow:
• Is that ‘decreasing marginal return’, green line in Fig 1, is also the reverse M-M model? If so, please use consistent language throughout (on the figure and in the caption)?
• Why are the E, E-S, and DOC pools shown at all, my reading is that these pools not actually being simulated? If so, it seems misleading to show these pools at all- or is the point to demonstrate that these pools are implicitly represented in the model, but because the ‘fast’ parts of the model each are assumed to be in steady state, and thus omitted from explicit representation. I think it’s the later, but maybe this can be clarified in the text & caption?
• would mapping parameters from Table 1 onto Fig. 1 would be more useful for readers, or make the figure too busy?
• I’m also not sure Figs 1b and 1c are needed. Information about adding growth and maintenance respiration fluxes could be handled w/ small dashed lines or colored lines to communicate this relatively minor modification to the basic model structure.

P47 L1- should be ‘growth’

Fig. 3. As text introduces results from quasi steady-state results and then partitioning between maintenance & growth respiration should the caption for Fig. 3 be similarly organized?

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RR by Anonymous Referee #1 (08 Feb 2016)

ED: Publish subject to minor revisions (Editor review) (08 Feb 2016) by Sönke Zaehle

AR by Debjani Sihi on behalf of the Authors (22 Feb 2016) Author's response Manuscript

ED: Publish as is (02 Mar 2016) by Sönke Zaehle

AR by Debjani Sihi on behalf of the Authors (06 Mar 2016)

Short summary

Simple microbial decomposition models show distinct responses to warming under different assumptions of how complex organic matter is broken down. If there are limitations other than microbial enzyme availability, the short-term respiration response is dampened and the decomposition dynamics resemble traditional first-order decay used in most biogeochemistry models. Further, microbial adjustment to respiratory cost for enzyme production reduces overall sensitivity to temperature.