Decreased Soil Microbial Biomass and Changed Microbial Community Composition following a Defoliation Event by the Forest Tent Caterpillar

Round 1

Reviewer 1 Report

This manuscript entitled “Decreased soil microbial biomass and changed microbial community composition following a defoliation event by the Forest tent caterpillar” is a research paper aimed at detecting the effects of defoliation induced by tent caterpillar on the soil microbial community in Canada. Overall, I think the aims of the study were clear and straightforward, and the methods used to answer the research questions were suitable. In addition, I completely agree that gaining a better understanding of the aboveground herbivores affect the belowground community, especially the legacy effects of episodic insect outbreak events is a worthwhile and important study. The introduction and discussion are written in a nice logical way.

 

However, there are still a few issues that I would like the authors to address.

 

General comments:

 

The hypothesis of this study is that N input by frass would stimulate microbial activity, and the soil N of 2017 was tested which showed higher amounts of N in soil under defoliated trees. Did the authors collect frass samples and test their C and N content? Is there some experiment or method to estimate the amount of frass input?

 

The references in the manuscript were mixed in “numbers style” or “author-date style”, of course, this is allowed in the draft and the editor will deal with them in the end. The titles of references 9, 12, and 35 are shown in upper case letters while others are lower case letters. I think a unified format would make your manuscript friendly to read.

 

Specific suggestions:

L23, In the abstract, authors state that soils of 2018 were sampled in July 2018, but in the main text, it is said samples were collected in August 2018. Please check and verify.

 

L133, Were any pesticides applied during the insect outbreak events which perhaps would affect soil microbes?

 

L141, How many trees were sampled in each stand?

 

L142, What was the diameter of the soil core?

 

L182, “Only 2018 samples were analyzed” is ambiguous, is it ‘the number of samples was two thousand and eighteen’ or ‘the samples of the year 2018’. 

 

L190, A mixed model analysis was performed to test the effects of defoliation and sampling time on soil microbial biomass and so on. Please give more information of the model, is it a “Linear mixed effects model” or “Generalized linear mixed effects model” or some other kind of mixed model?

 

L198, Which time was “spring” and “late summer” refer to? Please use the sampling time, just like shown in the x-axis in the figures. This description is unfriendly to people living in the Southern Hemisphere.

  

Figures:

In the Results section, the p-values were shown to imply significant effects, however, I can not find any information of P in Figure 1 and Figure 2. Please add this information, and use different letters or asterisks to show significant differences.

 

L260, the title of Figure 2 (Mean microbial respiration) and the y-axis of Figure 2 (Microbial biomass) are conflicted.

 

L265, what’s the meaning of asterisks in figure 3, please add explain information in the figure title.

 

Figure S1, The chemical analysis of soil N, P, K, Ca, and Mg is shown, were they available or total element concentration? Please state in the figure title.

 

Figure S1, The first panel is the concentration of soil N which unit is shown in %, and the N concentration of control is around 0.7%, it is 7 g kg-1 which is already a super high soil N level, however, the FTC defoliated soil N is around 1.5%, i.e. 15 g kg-1, it is an amazing N level. Perhaps it is induced by the frass input, anyway, please check the data or the unit. The units of other panels are shown in g kg-1, you should make them unified.

 

 

L380, What’s the soil-to-water ratio when testing the soil pH?

Author Response

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Author Response File: Author Response.docx

Reviewer 2 Report

This manuscript aims to shed light on the impacts of the forest tent caterpillar on soil microbial communities, expecting an increase in soil microbial biomass following defoliation. For this, the authors compared microbial activities from soil under 16 trees in total, which were located in one of two forest stands: one defoliated by the caterpillar and one undefoliated as control, one and two years after a 3-year caterpillar defoliation episode. Other than expected they found a decrease in microbial biomass and a shift from malic acid consumption to glucose & fructose consumption.

Generally, this is a nice study trying to answer an interesting research question. The largest weakness it the small number of trees sampled and that they are only from only two forest stands in combination with the lack of pre-defoliation measurements. There remains uncertainty whether the observed patterns are due to forest stand specific differences or whether defoliation was the true driver of the observed patterns. This should at least be mentioned in the discussion. The second weakness is the freezing of the soil before measurements. The living cells will lyse and whatever microbial activity that is measured afterwards is from microbes that re-colonized the soil from some resting stage and are likely not identical to the active microbial community pre-freezing. The references cited for the method use fresh soil for their measurements.

The methods descriptions need to be substantially re-worked. It was very difficult to understand what the authors did exactly. For example, figuring out the actual sampling design was difficult and only with the figure legends and the sample sizes explained there I could make sense of it. As someone who is not familiar with the substrate-induced soil respiration method, I had difficulties following the descriptions of the respiration measurements. E.g. what example do you read from the colorimetric microplate in the spectrophotometer? How does this work? Similarly, I did not understand how exactly the relative respiration was calculated. Further, the chosen measures for soil community composition and activity need to be better justified. For example, why did you chose the four substrates that you chose? Is there an ecological explanation why the consumption of e.g. glucose would vary between defoliated and undefoliated forest soils? Are different microbial groups specialized on different resources? Which ones eat what? Based on the information the authors give, the described statistical model makes sense. However, only for the measurements, that have been repeated over time and not for the measurements that were only taken once. Please explain better.

The results are presented clearly. However, I would like to see the model output table at least in the supplementary. In the figures, instead of means and error bars from the raw data I would prefer to see plots of the model predictions with the raw data points plotted on top of it.

 

Detail comments:

Line 131 strange transition. Where are the defoliated and undefoliated stands coming from here? How many defoliated and undefoliated stands were used? How big are they?

Line 140 somehow make clear that one tree is one sampling site. Not multiple sampling sites under one tree or multiple trees within one sampling site. This became only clear to me after the statistical model description (Line 191). Maybe, it is enough to call them trees and drop the term site overall. Also based on this description I didn’t understand if you sampled defoliated AND un-defoliated trees at each forest stand or in other words whether there were only defoliated trees in a defoliated stand or whether there were also undefoliated trees in a stand that was classified as defoliated (and vice versa).

Line 153 How does the microplate spectrophotometer measure respiration? Or what exactly does it measure and how does it link to respiration?

Line 154 You measure respiration. Why this awkward calculation of microbial biomass carbon, based on a most-likely outdated reference (Anderson and Domsch 1978)? You compare fungal and bacterial respiration. Are you sure that the relationship between biomass carbon and respiration is identical for fungi and bacteria? Based on the correlation in the other cited reference (Sassi et al. 2012, Fig. 2), the relationship between microbial carbon and respiration of the entire community is not identical than for fungal communities only. Based on this it would maybe be better to just frame the entire study in light of fungal and bacterial respiration/activity, rather than their abundance.

Line 155 I assume you measured glucose induced respiration in two subsamples of the original samples: once with only glucose added and once with bronopol added as well? If so, please specify. Also somewhere in the introduction explain why it matters whether the soils are dominated by fungi vs. bacteria. The introduction makes the point that fungal to bacterial ratio could potentially change (Lines 92-96), but there is no clear hypothesis in which direction and based on which ecological mechanism. Only the mention of two studies that found contrasting effects of herbivory on fungal abundance. à justify better why you measure this.

Line 162-163 Ok here we get the total sample size, but I didn’t understand it without seeing the numbers in the description of Figure 1. Clearly specify somewhere: 2 forest stands, 8 trees each, 3 sampling time points = 48 samples in total.

Line 172 Could you please explain a more what the different carbon sources mean in an ecological sense? Do they come from different sources? Are different microbial groups specialized to degrade different sources? Which microbes eat which C source? Are some of the C sources harder to use than others? And which ones would be expected to be consumed more/less after defoliation?

Line 182 Why did you measure this only on the 2018 samples?

Line 183-187 I don’t understand what you calculated here. What is “total respiration”? Here you call it “relative utilization”, later “relative respiration”. Please be consistent!

Line 185 Defoliation is not a treatment. You did not experimentally manipulate defoliation.

Line 191 What were the model specifications for the relative utilization? In line 182 you mention that this was only measured in 2018, so having time as fixed effect and tree as random effect doesn’t make sense.

Line 201 Are control stands undefoliated stands? Please be consistent with terms!

Line 303 You measured soil activities one and two years after defoliation, so how is it more similar to the conditions in the study by Streminska than the other studies. Based on this information, I would have expected that you would first see an increase in microbial biomass and later a decrease compared to undefoliated stands.

Line 354 Were the micronutrients measured under the same trees than where the soil samples were collected? In this case I would highly recommend to test if the observed respiration measurements can be explained as response to the nutrient availability, to strengthen this line of argumentation.

Author Response

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Author Response File: Author Response.docx