Review of the Manuscript:

Adjustments of the Rock-Eval® thermal analysis for soil organic and inorganic carbon quantification

This study presents a methodological protocol to determine soil organic content and soil inorganic content. Using Rock-Eval thermal analysis offers some benefits over Elemental analysis method, where the SOC is estimated by the difference between Total Soil Carbon and SIC. Rock Eval eliminates the chances of calculation error, and experimental error associated with the EA method.

Comments:

1. In general, the formatting needs to be checked. Chemical formula should be written with proper subscript, and units should be given with proper superscript and subscript.
2. Material-The soil type for the 24 samples should be mentioned. This will help to establish the effectiveness of using RE for different soil types.
3. Sec 2.2.1: which acid was used for acid treatment?
4. L105: The authors need to provide a justification on why no replicates were conducted.
5. Figure 1: message from figure not adequate. What is S, S2. Why these temperature ranges were selected. The denotations for S-S3 appears later in the text, but it should be given at the first instance of their appearance.
6. What is the purity of the nitrogen used?
7. L125: The author must specify how these thermograms correlate to SIC and SOC (As mentioned later in Table1).As a reader it’s not directly given and needs to be inferred.
8. L165: I appreciate that the author has taken the recalcitrant carbon into account.
9. Figure 4: why EA was selected for comparison? Since SOC is estimated by the difference between TC and TIC, TOC is an estimate, as mentioned in the introduction. Shouldn’t LOI method come here? Please provide a solid justification for the use of EA. Rather TC from EA and TC from RE could have been co-related.
10. p values should be denoted as p<0.001 for all numbers less than 0.001.
11. Table 2: what is the p-value for these numbers? 12±20 seems statistically insignificant.

The study by Hazera et al. is a methodological manuscript dealing with the quantification of total carbon (TC), soil organic carbon (SOC), and soil inorganic carbon (SIC) in single analysis with the Rock-Eval6 method (RE). The proposed method is compared against the standard method using an elemental analyzer (EA) combined with acid fumigation or heating. The main conclusion by the authors is a necessary adjustment of the standard protocol to ensure complete combustion of SIC by increasing the last isotherm from 4 to 7 minutes.

The scientific significance itself is excellent, as RE offers the advantage to derive additional properties of SOC. Advancing the method, so that TC, SOC and some of its properties, as well as SIC could be measured in a single run would be an excellent methodological step forward.

However, as methodological work requires a very high standard, I have some reservations regarding the scientific quality of the description or application of the methods. As it is, I judge the scientific quality as “Fair”, but I hope the authors can amend their manuscript further. Please find some comments and questions in the paragraphs below.

Overall, I guess that addressing the major comments below would need additional work and analysis. This will take some time. If the authors and editors agree, I suggest the following major change to the manuscript: In my opinion, there are several topics which are addressed more in a “work-in-progress” way. The main conclusion, however, is the necessary prolongation of the last isotherm. By significantly cutting the word count it would be an alternative to focus on the topic of the last isotherm and change the “Research Article” into a “Technical Note”. This would mean to focus on the second panel of samples, for which I can see convincing results.

P.S.: After writing this review, I found the poster of Sebag et al. (2022, DOI: 10.13140/RG.2.2.27606.52800). Using the presented datasets or getting samples and re-analyzing them would render several of my comments unnecessary. Please justify why this large dataset was not used here.

General comments:

Number and range of samples

For a methodological study the number of samples (n = 24) is relatively low. Moreover, the samples are all (1) from agricultural soils, and (2) topsoil samples. Therefore, all results are strictly only valid for this limited sample set. This is discussed in the conclusion (see also “conversion factors” below), but should also be mentioned clearly in the Abstract and Discussion.

There is also no description of soil sample properties. At least, the range of TC (measured by EA) and a corresponding pH value should be given. It is also not clear if all soils contain carbonates. From Results & Discussion I suspect that all samples contained carbonate. It is highly preferable to mix some samples without carbonates into the set to show that these carbonate-free samples do not show any (or only the stated 9.2%) signal in the “MinC” range. It might even be a good idea to treat these carbonate-free samples exactly as the carbonaceous samples (muffle oven, acid pre-treatment) to show caveats of these methods.

There was only one sample of “natural calcite”. The description must be expanded. Was it taken from carbonaceous rock? If yes, which kind of rock? Calcite is also available as certified standard material and for methodological work, this should be preferred over a natural sample with unknown properties (e.g. purity, presence of other carbonates such as dolomite or magnesite, presence of traces of organic matter).

Precision and accuracy

In methodological studies two measures are essential, that is “precision” and “accuracy” (sometimes termed “bias”). EA, for example, is widely accepted as a standard method because it is both accurate (i.e. measures the true value) and precise (i.e. repeated measurements are very close to each other). The manuscript presents some indications for accuracy, but none for precision. A quantification of precision and accuracy often starts by using reproducible standard materials, which should be mixed together, and measured in replication. Bisutti et al. (2007) provide an excellent example for this. Vuong et al. (2013), for example, offer simple methods to compare precision and accuracy with well-known F- and t-tests.

Accuracy in the manuscript is quantified by linear regression. It is a very good start that not only R² is given, and there is more emphasis on the slope. However, I wonder (1) why no intercept is presented (I do hope the regression was not forced through zero, please confirm), and (2) why there is no measure of dispersion (e.g. standard error, confidence interval) for slope and intercept. The dispersion of the regression parameters is important to know. Moreover, the authors present the p-value for the regression, which is correctly interpreted as the probability that the slope differs from 0. In this case this is not of interest. We have to know if the slope is different from 1 to judge if methods correspond with each other. This is addressed in the text, but should also integrated into the figures. Furthermore, a large error of intercept or slope will indicate low precision. Do not only look on significances, but also on variability. With high variability, the difference to 0 or 1 will be insignificant, but the high error may render the method unsuitable for routine analysis.

Use of certified standard materials and device calibration

The use of reproducible standard material is highly encouraged. The readers also must be informed about the calibration of all involved devices; including calibration range, precision, and accuracy. There is some information about the assumed relative error, but the authors need to show this data.

Conversion factors

First of all, I wonder why the authors gave a constant conversion factor, whereas Disnar et al (2003) provided a regression function for correction [this information was taken from Saenger et al. 2013, as I have no access to the manuscript of Disnar et al. 2003, so please correct me if I am wrong].

It is especially important, that the conversion factor (SOTHIS) was stated to be valid for samples “enriched in poorly degraded organic compounds (…)”. In the used sample set the conversion factors seem to work well. However, to really establish the RE method it will be important to ensure that the method works well on a continuum of organic matter states. That is, from the litter, fermented, and humified forest floor layers down the profile of mineral soil. Conditions change through the profile from largely undecomposed to largely decomposed and mineral associated organic matter, often accompanied by a gradient of increasing carbonate content. It seems likely, the “standard” conversion factors will not work for all these conditions. And it is open to question at which depth the “additional 6.2%” shall be applied. Therefore, this will be important future work to address these conversion factors, or possibly change isotherm length or temperature settings.

SOTHIS as presented here seems to be a mixture between Disnar et al. (2003) and Sebag et al. (2022). The latter is a poster, a peer-reviewed source or the actual data would be preferable.

Use of two different devices and settings

LL 117 ff. The authors used devices from different labs, with slightly different settings. They state that “The minor differences between the standard cycles […] do not affect parameter calculations”. This may be so, but it has to be demonstrated with some samples. Overall, it would be essential to develop the method in one lab, using the same device with the same calibration, and after that set up a ring trial with different labs (inter-lab reproducibility).

Additional literature

I suggest to take a look on some additional work in the field of measuring SOC, SIC and TC in a single run (Bisutti et al., 2007; Vuong et al., 2016, 2013). Those references did not use RE, but offer some methodological examples. Bisutti et al. (2007) made excessive use of different material mixtures, which can be reproduced. Vuong et al. (2013) gave an example how to quantify precision and bias on a wider range of soil samples and calcite standard (though it would have been better to add an organic matter standard, and mix it with calcite). Vuong et al. (2016) advance the former work by using dolomite, not only calcite. Although on marine sediments, the work of Carrie et al. (2012) should be recognized, as they already divide RE peaks into OC and IC and discuss some methodological issues, which may be relevant for some special soil conditions (e.g. saline soils).

Additionally, Saenger et al. (2013) conclude that SOC (soil samples of different land use) can be quantified well with help of the correction provided by Disnard et al. (2003). Soucémarianadin et al. (2018 a; 2018 b) provide some insight on the potential for SOC fractionation – which gives reasoning why RE should be advanced in regards of SIC quantification.

Bisutti, I., Hilke, I., Schumacher, J., Raessler, M., 2007. A novel single-run dual temperature combustion (SRDTC) method for the determination of organic, in-organic and total carbon in soil samples. Talanta 71, 521–528. https://doi.org/10.1016/j.talanta.2006.04.022

Carrie, J., Sanei, H., Stern, G., 2012. Standardisation of Rock–Eval pyrolysis for the analysis of recent sediments and soils. Org. Geochem. 46, 38–53. https://doi.org/10.1016/j.orggeochem.2012.01.011

Saenger, A., Cécillon, L., Sebag, D., Brun, J.-J., 2013. Soil organic carbon quantity, chemistry and thermal stability in a mountainous landscape: A Rock–Eval pyrolysis survey. Org. Geochem. 54, 101–114. https://doi.org/10.1016/j.orggeochem.2012.10.008

Soucémarianadin, L., Cécillon, L., Chenu, C., Baudin, F., Nicolas, M., Girardin, C., Barré, P., 2018 a. Is Rock-Eval 6 thermal analysis a good indicator of soil organic carbon lability? – A method-comparison study in forest soils. Soil Biol. Biochem. 117, 108–116. https://doi.org/10.1016/j.soilbio.2017.10.025

Soucémarianadin, L.N., Cécillon, L., Guenet, B., Chenu, C., Baudin, F., Nicolas, M., Girardin, C., Barré, P., 2018 b. Environmental factors controlling soil organic carbon stability in French forest soils. Plant Soil 426, 267–286. https://doi.org/10.1007/s11104-018-3613-x

Vuong, T.X., Heitkamp, F., Jungkunst, H.F., Reimer, A., Gerold, G., 2013. Simultaneous measurement of soil organic and inorganic carbon: evaluation of a thermal gradient analysis. J. Soils Sediments 13, 1133–1140. https://doi.org/10.1007/s11368-013-0715-1

Vuong, T.X., Prietzel, J., Heitkamp, F., 2016. Measurement of organic and inorganic carbon in dolomite-containing samples. Soil Use Manag. 32, 53–59. https://doi.org/10.1111/sum.12233

Specific comments:

This is not exhaustive due to the suggestion to change the manuscript into a Technical Note. As a non-native speaker, I refrain from language editing.

Some of the figures need more precise explanations. As a guideline, figures should be interpretable without referring to the text.

Figures show very different dimensions. This could be streamlined. As a guideline, text should be approximately of the same size in all figures

Ll 96: At which temperature were the samples dried?

Ll 100 ff: Were samples corrected for residual moisture (105°C)? Overall, more methodological details are neede. Which acid was used? Molarity? General conditions, such vacuum etc. Even if Harris et al. (2001) was followed exactly, please provide some details. The same applies to heating.

L 143: The temperature was shifted to be consistent with the decarbonation treatment. However, decarbonation was likely performed in ambient air, pyrolysis without oxygen. Therefore conditions are not comparable. Please justify the shift from 400°C/local minimum to 550°C and estimate the consequences.

L199: As treated samples were used, I would prefer an indication, e.g. EA_decarb

L 247: “…are correlated (R² = 0.9935…” please use the term “related” as “correlated” is used for Spearmen or Pearson correlation coefficient

Figure 5 and text ll 263 ff: This is a good example, why reporting an intercept for the regression is useful. It also demonstrates the issue when correcting values with a fixed factor instead of of a regression (see “Conversion factors” above). Please discuss these consequences.

Table 2, column S3’CO2 MinC: 12+/-20 does not seem to be a reliable estimate. This should be addressed

Ll323 ff, Figure 8: This finding indicates, that including carbonate-free soil samples in the test may be a good idea.

Ll 340 ff: Would it be possible to prolongate the last phase even further? What does this finding mean for more thermostable carbonates?

Ll 351: If the SIC content is known, what is the advantage of the RE method?

Ll 356: Please indicate that further work on a wider range of samples and with different forms of carbonates is needed.