Autotrophic component of soil respiration is repressed by drought more than
the heterotrophic one in dry grasslands

Balogh, János; Papp, Marianna; Pintér, Krisztina; Fóti, Szilvia; Posta, Katalin; Eugster, Werner; Nagy, Zoltán

doi:https://doi.org/10.5194/bg-13-5171-2016

Articles | Volume 13, issue 18

https://doi.org/10.5194/bg-13-5171-2016

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

https://doi.org/10.5194/bg-13-5171-2016

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

Articles | Volume 13, issue 18

Research article

|

19 Sep 2016

Research article |

| 19 Sep 2016

Autotrophic component of soil respiration is repressed by drought more than the heterotrophic one in dry grasslands

János Balogh, Marianna Papp, Krisztina Pintér, Szilvia Fóti, Katalin Posta, Werner Eugster, and Zoltán Nagy

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Preprint (discussion started on 20 Oct 2015)

Interactive discussion

Status: closed

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

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RC C8065: 'Reviewer comment', Anonymous Referee #1, 27 Nov 2015
- AC C8197: 'reply to referee#1', János Balogh, 02 Dec 2015
RC C8211: 'Referee #1, reply to authors' reply', Anonymous Referee #1, 03 Dec 2015
RC C9755: '12, 16885–16911, 2015', Anonymous Referee #2, 16 Feb 2016

Peer-review completion

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

ED: Reconsider after major revisions (18 Feb 2016) by Ulrike Seibt

Dear Authors,
In its current form, your manuscript is unfortunately not at the standard of quality expected and required for publication in Biogeosciences. Both reviewers gave low to very low ratings for Scientific Quality. Please revise your manuscript thoroughly following the recommendations of the reviewers. This will require substantial rewriting to better structure the text for clarity, and better describe the justification, background, context and implications of the study, the methods used, error propagation and analysis, and the limitations to isotopic results. Please note that following the exchange of comments and replies you had with reviewer 1 during the discussion phase, it is still unclear to me whether the isotopic results are robust. For example, you did not clarify whether you had more than one chamber for isotopic measurements of the components, one of the major concerns of reviewer 1. Also, in your reply you mention R2 values between 0.01 and 0.09 for the linear regressions used to obtain the component isotopic ratios that are then used in the mixing model. These are very low R2 values, i.e. indicating that the isotopic signal is mostly not explained by the independent variable. It is difficult to judge without seeing the plots, but if the linear regressions have large uncertainties in the intercepts, this would lead to very large uncertainties for the results from the mixing model (much larger than currently presented). Please use Gaussian error propagation as requested in an earlier round of reviews, and please consider showing all of your data processing steps by adding supplementary material. If you decide to submit a substantially revised and rewritten manuscript, please pay particular attention to the comments and suggestions from the two reviewers.
Kind regards,
U. Seibt

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AR by János Balogh on behalf of the Authors (14 Apr 2016) Author's response Manuscript

ED: Referee Nomination & Report Request started (13 Jun 2016) by Ulrike Seibt

RR by Anonymous Referee #3 (10 Jul 2016)

Suggestions for revision or reasons for rejection

This paper summarizes findings of a study on total soil respiration and its components in a grassland over various moist and dry periods during one growing season. The authors used near-continuous automatic measurements of CO2 fluxes and their isotopic composition to distinguish between sources of CO2 in the soil. Those sources included respiration by heterotrophic microorganisms, and respiration by roots + rhizosphere organisms and by mycorrhizae that together compose autotrophic respiration. The results showed a decrease in the autotrophic contribution to Rs during drought, mainly because of a greatly reduced flux from roots + rhizosphere. The main issues raised in earlier reviews were addressed by the authors as effectively as possible (see points 1 and 2 below), yet some additional revisions still need to be performed.

1. Spatial replication and capture of spatial variation: Authors made an effort to rotate measurement chambers among different locations. Thus, they obtained a minimal set of 2-3 replications during each of the 5 selected periods. While this approach seems to make optimal use of their measurement capacities, there still remains a spatial mismatch between the larger undisturbed area dedicated to measurements of total Rs and the strip used for exclusion treatments. In addition, the aim of the larger area no. 9 is unclear. These issues need to be clarified. In general, the above-mentioned mismatch may not be fatal, considering the short distances between the control and the treated areas. Maybe more importantly, root and root + mycorrhizae exclusion involved major disturbance of the soil, which might be of greater consequences than the spatial issues (this points should be discussed, see below).

2. Calculated isotopic signatures and error propagation: The isotopic values and the error terms seem reasonable now, considering the large variation inherent in such measurements. However, the estimation of the isotopic signatures of autotrophic respiration and its components is not uniform. As to Fig. S4, δ13C of Rmycrhiz and Rrhizo are -28.9 and -28.6‰, respectively. In Fig. 2 and p12 l19, the opposite was presented, with δ13C of Rmycrhiz being higher than that of Rrhizo. The second version makes more sense, if δ13C of Rmyc is -27.2‰, because only in that way can the isotopic signature of Rmycrhiz be combined by those of Rrhizo and Rmyc.

3. δ13C of Reco is lower by about 1‰ than δ13C of Rs, which indicates that δ13C of plant shoot respiration is lower than δ13C of Reco (as obvious also from autotrophic belowground respiration, calculated as Rmycrhiz). The isotopic signature of shoot respiration might be constrained by results from isotopic analysis of shoot biomass. Moreover, it seems that shoot respiration could be estimated in the same way as the autotrophic belowground components, which would add an important aboveground component to the paper. Drought induced decrease in GPP (or photosynthesis) was mentioned in the text, but no data were presented and no relevant paper was cited.

4. Mean respiration rates could be summarized in a table, for each period and for the entire season. This table could not only contain the measured, but also calculated absolute rates and their error term (using the relative data).

5. The Discussion unnecessarily repeats many results, and a large part of this section was dedicated to numerical comparisons with results from other publications. Only the most important results should be repeated at the beginning of the Discussion, and comparisons with findings of other studies may be noted in the margin. The bulk of this section should be dedicated to explaining the obtained results, suggesting mechanisms underlying the findings, outlining potential consequences, etc. While it shortly mentions such consequences, the main part of the conclusion again repeats the results.

6. Potential drawbacks of the methods and their consequences should also be mentioned. E.g., the great disturbance of the soil to exclude roots and mycorrhizae might be expected to affect decomposition rates of SOM and, thus, Rh. In addition, microbial activity was greatly reduced by root + mycorrhizae exclusion as compared with the undisturbed treatment. How might this fact affect Rh?

7. Automatic soil respiration system: More information is needed on the function of the little chambers, as request by other reviewers. Are the chambers automatically closed during measurements? What was the measurement frequency? How could it be guaranteed that soil enclosed by the chambers received an appropriate amount of rain through the holes? What were the temperature differences between the chambers and the environment? The flow rate of the air through the chamber was relatively high. The potential generation of a vacuum should be discussed, which might have forced air out of the ground. More importantly, because the chambers were very shallowly introduced into the ground (according to Nagy et al. 2011), such a vacuum could possibly suck air from the environment.

8. The English language needs improvement. Most urgently, the title should be edited. The term “ratio” was used inappropriately at several locations in the text. It should be replaced by “fraction” or “percentage”, e.g. in the title of subchapter 3.3, at the beginning of the Discussion and in the y-axis title of Fig. 3.

9. Additional comments
P3, l9: This sentence is expected to appear towards the end of the Introduction, together with the study’s objectives. Additionally, reference to GPP as part of an objective raises expectations of data presentation, but no data on GPP were shown here.
P3, l24: The statement on the percentage of plant species associated with AMF should be referenced.
P5, l24: Rather Fig. S1.
P6, l8: Acronyms must to be explained at first appearance, here and elsewhere.
P7, l17: Please turn these short and repeating sentences into on coherent sentence.
P7, l23: It is unclear where reference air was sampled.
p14, l21: Neither GPP nor leaf photosynthesis rates were presented in this work. Therefore, a study relevant for this site needs to be cited.
Fig. 1: The meaning of the horizontal lines needs to be explained.
Fig. 2: All parts of the box plots should be explained.
Fig. 4: Please indicate the statistical analysis used for this figure.

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ED: Publish subject to minor revisions (Editor review) (30 Jul 2016) by Ulrike Seibt

AR by János Balogh on behalf of the Authors (12 Aug 2016) Author's response Manuscript

ED: Publish as is (02 Sep 2016) by Ulrike Seibt

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Short summary

In the dry grassland investigated in this study the components of the soil CO₂ efflux decreased at different rates under drought conditions. During drought the contribution made by the heterotrophic components was the highest and the rhizospheric component was the most sensitive to soil drying. According to our results, the heterotrophic component of soil respiration is the major contributor to the respiration activities during drought events.