DRIFTS band areas as measured pool size proxy to reduce parameter uncertainty in soil organic matter models

Laub, Moritz; Demyan, Michael Scott; Nkwain, Yvonne Funkuin; Blagodatsky, Sergey; Kätterer, Thomas; Piepho, Hans-Peter; Cadisch, Georg

doi:https://doi.org/10.5194/bg-17-1393-2020

Articles | Volume 17, issue 6

https://doi.org/10.5194/bg-17-1393-2020

© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/bg-17-1393-2020

© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 17, issue 6

Research article

|

20 Mar 2020

Research article |

| 20 Mar 2020

DRIFTS band areas as measured pool size proxy to reduce parameter uncertainty in soil organic matter models

Moritz Laub, Michael Scott Demyan, Yvonne Funkuin Nkwain, Sergey Blagodatsky, Thomas Kätterer, Hans-Peter Piepho, and Georg Cadisch

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Final revised paper (published on 20 Mar 2020)
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Preprint (discussion started on 07 Aug 2019)
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Interactive discussion

Status: closed

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

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RC1: 'Excellent contribution to our understanding of SOM model initialization', Sander Bruun, 29 Aug 2019
- AC1: 'Response to Sander Bruun', Moritz Laub, 09 Oct 2019
RC2: 'Chemical recalcitrance of major importance if you combine DRIFTS and first-order pool model?', Anonymous Referee #2, 30 Aug 2019
- AC2: 'Response to Referee 2', Moritz Laub, 09 Oct 2019
RC3: 'Reservation on the rationale of the DRIFTS stability index of soil organic matter (SOM) in mineral soil, and its use for partitioning the C kinetic pools of SOM dynamics models', Lauric Cécillon, 31 Aug 2019
- AC3: 'Response to Lauric Cécillon', Moritz Laub, 09 Oct 2019
  - AC4: 'Updated Response to Lauric Cécillon', Moritz Laub, 11 Nov 2019

Peer-review completion

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

ED: Reconsider after major revisions (23 Oct 2019) by Michael Weintraub

AR by Moritz Laub on behalf of the Authors (11 Nov 2019) Manuscript

ED: Referee Nomination & Report Request started (29 Nov 2019) by Michael Weintraub

RR by Lauric Cécillon (14 Dec 2019)

RR by Anonymous Referee #4 (13 Jan 2020)

Suggestions for revision or reasons for rejection

In general I like the idea of your study, however, (i) several mistaken terms need to be corrected (“vibrational peaks of absorbance”, peaks instead of bands, “ratio of.. stretching vibrations”), (ii) the innovative aspect, and importance need to be clarified, and (iii) lots of details need to be clarified within material and methods (:most important is the DSI suggested to be a measure for SMB-C or SOM pools?). The paper need a carful revision (e.g. correction of mistakes terms, clarification of innovative aspect and importance, and addition of missing/ incomplete information in material and methods) before a complete review will be possible.
Some details:
There are some sematic mistakes:
a) the term “ratio of aliphatic C-H (2930 cm-1) to aromatic C=C (1620 cm-1) stretching vibrations” is not correct. According to textbooks on spectroscopy it should be called as a ratio between the area under aromatic and aliphatic absorption bands (which is also in accordance with the respective description in the materials and methods section in the manuscript under review.
b) “vibrational peaks of absorbance (at L74 for example)”. Should be called absorption bands: Please note that if infrared light of a certain wavelength introduces a vibrations at molecular scale that light will loss intensity which is recorded in the (FTIR/DRIFT) spectra by respective signals. However, since that light is absorbed by the sample such signals are called absorption bands (not peaks since this term is in general used to describe much sharper signals (textbooks on spectroscopy). Mistaken terms like the ones mentioned above need to be corrected. Standard terms need to be used. An introduction of new (incorrect / mistaken) ones need to be avoided.
c) In the abstract the “ratio of aliphatic C-H (2930 cm-1) to aromatic C=C (1620 cm-1)” which means A1620 to A2930 ratio is mentioned as a measure for DSI, but according to equation (2) it is the ratio between (A1620) and (A1620 +A2930), THIS is different! Please revise the text accordingly throughout the whole manuscript.

Abstract
- should consist of short statements on introduction, problem of interest, objective, material and methods, results, discussion and the conclusion. However, the problem of interest is missing.
L25-26. Please clarify the meaning of: “DRIFTS stability index ….., was used to divide SOM into fast and slow cycling pools in the soil organic module of the DAISY model.” How was this done? How could a ratio be used to divide different pools? It may be usable to reflect the RATIO between SLOW and FAST pool or the proportion.

Introduction
Clarify the innovative aspect and importance of your study.
L73 Clarify the meaning of the sentence: “The interaction of mid-infrared energy with molecular bonds in soil produces typical vibrational peaks of absorbance at distinct wavelengths.“ It seems to be incomplete, refer to textbooks on FTIR/DRIFTs?
L74 note the term “vibrational peaks of absorbance” is wrong. Absorption bands results from vibrations at the molecular level, but they are called in any textbook as absorption bands. All statement like the one above need to be changed throughout the whole manuscript..
L74 Clarify the meaning of the term “These”
L75 This is incorrect, the wavelength of the CH absorption band is related to vibrations of C-H bonds in the SOM but NOT to elements. Please correct the wording and CLARIFY THE MEANING:
L77 Stretching peak need to be corrected into absorption band thoughout the whole manuscript…..
L75 to 80 please clarify and consider that this problem is already described by textbooks on spectroscopy but also by authors (e.g. Capriel 1997, Celi et al. 1997 among others) who interpret the CH absorption bond with respect to soil wettability. These authors already mentioned the overlap of CH bands by those of OH bands. BUT they also stated that the OH bands are not only present in water the OH is also part of SOM, and soil minerals (e.g. clay minerals, oxides, hydroxides, etc.). These OH band will remain in air dried soil samples as well in the ones dried at 37 to 105°C. But drying at higher temperature may affect the OH content of SOM and soil minerals thereby affecting the absorption bands in DRIFT/FTIR spectra (see Yeasmin, Reeves among others). Please correct this statement AND clarify how you ensure that the drying procedure did not affect the spectral properties of your samples. Add respective spectra into the supplement.
L86 explain the background of the idea of SOM pool initialization.

Material and Methods:
PLEASE describe in more detail the soil sampling. Which soils were taken where, at which time? Which is the year of establishment for each of the experiments, at which year did the bare-fallow started, and which are the sampling year(s)?
L145 ff seem mostly belong to the discussion.
How did you ensure that drying at 105°C did not result in artifacts? Add references different to your own work.
Add the limits of the band areas, how did you chose them?
L171: meaning of the term “DAISY pools” ? The ones mentioned in table 2 but table 2 show turnover rates, “maint” and “CUE”?

Describe the DSI evaluation, - initialization….
L194: Where is the description of SMB-C measurement? Please add a short s of SMB-C determination. Please explain your reasons for dividing SMB-C into a fast and a slow cycling microbial pool. Why are they important? According to the statements at L194 to 203 the SMB-C seemed to be assumed here as a measure for SOM. Is this correct, please explain! Please clarify the differences between the SMB-C pools (L194) and the SOM pools (L199)! AND CLARIFY which pool ratios will be described when using the A1620/(A1620+A2930) ratio?
L405 please add background information for this statement (table, Fig.?, ref).

Did you consider somewhere in the discussion that the Bad Lauchstädt soil is a Chernozem? Please ad missing information, old carbon present in Chernozem may cause difference in SOM turn-over (papers on black carbon etc…).

I stopped reading here since lots of information need to be clarified and added before a careful review will be possible.

Table 1 please add the studied sites, AND note SOC stand for SOIL organic carbon content please correct the capture accordingly refer to standard captures…
Explain the terms - UTM (abbreviation for ?), -“Depth of measurement” possibly the depth of sampling?, - the meaning of initial (SOC in soil in the year the bare fallow is established, of the SOC of soil in the year the experiment was established?
Please add year of establishment and sampling; number of replicates
Table 2 which of the pools are assumed to represent the fast/slow cycling SMB-C pools mentioned at L194?
Table 3 show starting and ending year of the field experiment as well as … SOC etc … of the soil sampled at….. site. Please correct accordingly.
Correct the capture of table 4 and 5 accordingly…
Fig. 1 explain the meaning of the term “A_XXXX”
Fig. 2 I assume the capture should be: baseline corrected and vector normalized DRIFT spectra of 105°C dried soil samples from soils under bare fallow at … site. Please indicate which figure shows what kind of spectrum. How did you ensure that drying at 105°C did not result in artifacts? If these spectra are baseline correct WHY did they start at different Y-axis values?

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RR by Anonymous Referee #2 (13 Jan 2020)

Suggestions for revision or reasons for rejection

I criticized in my earlier review that Laub and colleagues use the chemical recalcitrance hypothesis to explain SOM cycling. Now, they cite an earlier study that their index, which is based on the aromatic and aliphatic peaks, is correlated with aggregation.

Mechanistically, the interpretation should be chemical recalcitrance. I don’t get what we learn from DRIFTS spectra when we deviate from the mechanistic interpretation. Demyan et al. (2012) suggest a relation to mineral-association (correlation to the sum of the heavy fraction > 1.8 g cm-3 and clay fraction compared to light fraction < 1.8 g cm-3) and not to aggregation (the occluded light fraction is not measured in that study).

Arguably microaggegrate occluded soil organic matter can be a part of the abovementioned heavy fraction > 1.8 g cm-3 and clay fraction but is possibly only a smaller portion (Castellano et al., 2015) of physicochemically stabilized SOC.

The sentences that Laub and colleagues added should reflect that the actual correlation is with physicochemically stabilized SOC (mainly mineral-association) and not aggregated SOC.
Conceptually it is unclear for me why aliphatic SOC should not be preferentially found in the mineral-associated fraction since this should be mainly microbial residues.

I would appreciate if the authors could discuss this a little bit more also in light of the results from 13C-NMR which would suggest that the light fraction is enriched in aromatics (lignin) and the heavy fraction fractions in aliphatics (microbial cell walls). I think it might be worthwhile to discuss this further when I am missing it also other readers might.

Apart from a more open discussion in this direction, I am happy with the changes made to the manuscript.

Castellano, M.J., Mueller, K.E., Olk, D.C., Sawyer, J.E., Six, J., 2015. Integrating Plant Litter Quality, Soil Organic Matter Stabilization and the Carbon Saturation Concept. Global Change Biology.
Demyan, M.S., Rasche, F., Schulz, E., Breulmann, M., Müller, T., Cadisch, G., 2012. Use of specific peaks obtained by diffuse reflectance Fourier transform mid-infrared spectroscopy to study the composition of organic matter in a Haplic Chernozem. 63, 189-199.

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ED: Reconsider after major revisions (13 Jan 2020) by Michael Weintraub

AR by Moritz Laub on behalf of the Authors (11 Feb 2020) Author's response Manuscript

ED: Publish as is (13 Feb 2020) by Michael Weintraub

AR by Moritz Laub on behalf of the Authors (14 Feb 2020) Manuscript

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Short summary

Loss of soil carbon to the atmosphere represents a global challenge. We tested an innovative way to reduce the high uncertainty related to turnover of carbon stored in soils. With the use of infrared spectra of soils from model bare fallow systems, we were able to better assess the current state of soil carbon and predict its behavior in overdecadal time spans. In agreement with recent studies, carbon turnover seems faster than earlier assumed, with potential for high loss under mismanagement.