Organo-Mineral Interactions Are More Important for Organic Matter Retention in Subsoil Than Topsoil

Round 1

Reviewer 1 Report

Reviewer comments for Organo-mineral interactions are more important for organic matter retention in subsoil than topsoil by Poirier at el.

The topic of the paper is very interesting. While is the paper is generally well written, I found some major issues within each section: 1) the introduction lacks some important discussion points about factors controlling organo-mineral formation, 2) materials and methods section lack very important details or clarifications regarding the experimental design and finally the discussion and conclusion sections appeared to be very weak and mostly repeating the results sec While the authors started their introduction talking about the important of this work for climate mitigation, there was no mention in the conclusion section about implications of such results ( even though such results were expected) , as the authors mentioned. In other words, I don’t think this paper bring new significant insights and I feel it would highly benefit from a much better  discussion section.

Abstract: well written, describes the main objectives, results and conclusions. I would only like to see one extra statement about what does "these conclusions mean or impact climate change since the authors started the abstract suggesting that “Accumulation of carbon (C) and nitrogen (N) from crop residues in mineral-associated 17 soil organic matter (SOM) can increase soil fertility and mitigate climate change”

 

Introduction: Well written introduction, will benefit from removing some of that statements that the authors repeat continuously. My major comment for the introduction is that the authors don’t mention SOM quality as another important factor impacting organo-mineral complexation. The authors focused on SOM quantity and how it plays a major role in determining whether organo-mineral association will occur or not, but how about SOM quality? In typical soil profiles, the top soil is dominated by labile compounds from litter decomposition, such as glucose and fructose that are known not to bind to mineral surfaces, however non-polar compounds are known to be left behind and might drive binding especially in the subsoil. It will be great to discuss SOM quality versus quantity here. Also since in most soil profiles the water table and redox conditions are highly different between subsoil and topsoil, do the authors think that binding is also impacted by redox chemistry?

Line 44-45 repetition

Line 62 extra space

Line 65 extra space

Materials and methods:

In the materials and methods section, more information about the experimental design is required. How many samples were incubated? How many biological replicates? Was there any biological replication?

Why did the authors decide to use a labeled substrates? How did they benefit from it? Did I miss that?  

If the soils were under barley crop, why did the authors decide to use corn resisude?

Was the incubation done aerobically or anaerobically?

After reading all of the materials and methods section, I am still confused on the number of samples and/or if any replication has been done which is especially important for density fraction results reproducibility . I just realized though that In the statistics section, the authors mention three replicates. It would be great to add such information earlier on in the text with other important experimental details (o2, no O2, why 51 days, in the dark? At room temp , etc)

Line 191, where did this number come from? Is 2 soil types * 2 treatments* 3 replicates? If yes, please add clarifications.

Thanks for adding figure 1, very helpful!

Line 294 , is 88~ based on all 12 soil samples? What is the stdeviation?

Lines 306-308 : what is the % recovery for each soil sample? Was it comparable? Did you observe more loses for subsoil compared to top soil? Please clarify

Section 4.2 in the discussion, reads more like a results section.

Aren’t these two statements contradictory? Or does the first part refers to data before incubation and the second after incubation? “an subsoil, but both soils had  similar amounts of residue-N in OLF (Figures 4a and 4b). The OLF contained a larger proportion of residue-C and –N in the subsoil than in the topsoil (Figure 3), but its mass was eight times lower in subsoil than in topsoil (data not shown).”

Also why did the incubation go for 51 days? Was this done under room temp? did you continuously adjust the soil moisture?

Also if subsoil is more prone to preserving SOM within aggregates (occlusion), why initially (line 310) , you say that OLF was greater in topsoil than subsoil, but both soils had  similar amounts of residue-N in OLF ? wouldn’t you expect from the beginning that subsoil has more occluded carbon?? Also how you explain differences between c and N data. Do you think its dependent on compound type and hence compound diversity?

Line 364 , add “of”

Discussion is very short And vague. It kind of repeats the results section again with no in-depth discussion about processes of stabilization. I was left unsatisfied with the discussion section.

 

Hope the authors find these comments helpful.

Author Response

Please see the attachment

Author Response File: Author Response.pdf

Reviewer 2 Report

The paper is very interesting and adds to the present knowledge in the subject. It consists of an experiment in which corn residues containing C and N isotopes are incubated during 51 days in samples of Top- and Subsoil of the same soil. It tests the hypothesis that soil with less autochtonous SOM will have greater formation of SOM-mineral complexes than oclusion of the recent added residue into soil aggregates.

The clever use of topsoil / subsoil combination is a bit entangled by the observation that 2:1 phyllosilicates in the fraction with density less than 2.1 in topsoil and less than 2.3 in subsoil had not weathered similarly (see how the peaks are different in the low angle -less than 15 2theta-  range). 
From 2.5 to smaller density, the 2:1 phyllosilicates peaks in the less than 15 2theta makes a band that may be caused by interestratification or overlapping of peaks of minerals that are increasingly more expansive, or both, in the topsoil, but are more defined around montmorilonite spacing in the subsoil.

Questions:

1. Based on the area under the peaks in this range (less than 15 2theta), it seems that the amount of 2:1 phyllossilicates in the subsoil is greater than in the topsoil. Could the author discuss this in more detail ?

2. L358: Considering the theoretical formula of muscavite, it does not contain Fe (as its trioctahedral mica syster, biotite, does). Please justify or correct this information.

Suggestions

L351: ... and NH4 could be sequestered into siloxane cavities in the partially weathered illite or vermiculite (if it has great tetrahedral substitution), as also happens with K+.

Because Gleysols are subjected to waterlogging periods, it would be worth to informe the paper audience that the autochtonous SOM evolved under such conditions.

In addition, the XRDs seems not contain pedogenetic metal oxides. Is this so because of previous DCB treatment or because the reductive environment does not allowed their genesis (considering that in some Gleysols there is accumulations of Fe oxides in certain areas - mottles, nodules, etc -), that is, the samples naturally lack pedogenic oxides in clay fraction?

 

Although I am not an native English speaker, please check some typos and potential grammar issues below:

L218: "amontmorillonite" should be "a montmorillonite"

L240 "Topsoil Fc values was" should be "Topsoil fc values were" ?

L265: "latter" should be "letter" ?

L327: "mostly retain in " should be "mostly retained in " ?

L351 ".." should be "."

 

Author Response

Please see the attachment

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

I would like the thank the reviewers for their careful response to each and every question/comment provided by the reviewers. I found most of the answers to be satisfying. No further comments are needed.