The Molecular Composition of Humic Acids in Permafrost Peats in the European Arctic as Paleorecord of the Environmental Conditions of the Holocene
Round 1
Reviewer 1 Report
This manuscript investigated the transformation regularities of molecular composition of HAs in the peatlands under different climatic conditions in the Holocene. The methods were reasonable. The results are interesting. Generally, the manuscript was well written. I suggest some explanations and discussions of results are improved. There were three issues I concerned.
1. The 14C of peat represent the age of the growing plant, which means it does not represent the time of peat buried.
2. The change of molecular compositions in peat was determined by both botanical compositions of peat and the decomposition process. How did authors suggest one of them contribute more than the other? For example, line 320-321. I found the botanical compositions of peat are different between these two sites. But authors suggest climatic condition of its formation is more related to the difference in HAs between two sites, which means decomposition process contribute more. The similar concern was also for the HAs change in the peat profile.
3. From the title and aim of this study, we are informed that author would demonstrate the change of molecular composition of HAs in peatlands could be as a marker of climatic changes. However, I cannot see how changes of HAs indicate climatic change. The authors state climatic change first, then showed the change of HAs in each time scale. It cannot be reversed.
Author Response
The authors are very grateful to the reviewer for reviewing our manuscript. The responses to the comments are given below:
1) 14C radiocarbon dating is the standard method for determining the age of peat deposits (Zaccone et al., 2011, doi: 10.1016/j.chemgeo.2011.03.018; Klavins and Purmalis, 2013, doi: 10.1016/j.molstruc.2013.07.021; Routh et al., 2014, doi: 10.1016/j.chemgeo.2013.12.022; et al.)
2) Yes, we agree with the reviewer and corrected some phrases.
3) Humic acids, along with palynological data (van der Knaap et al., 2000) and peat botanical composition (Barber et al., 1994; Hughes et al., 2000), are a reliable paleoclimatic markers. It was previously believed that such structural and functional parameters of HAs as optical properties and atomic ratios of elements are reliable parameters of the degree of humification (Zaccone et al., 2011). The most reliable molecular parameter of the degree of peat HA humification is the proportion of aromatic fragments or the degree of aromaticity. Obviously, according to the content of aromatic fragments in HAs from specific peat layers, it is possible to correlate with climatic features in a given time period. This approach has already been successfully implemented in the study of peat HAs from other climatic zones (Vasilevich et al., 2017, 2018, 2019, 2021).
All changes in the text are highlighted in yellow.
Sincerely, authors
Reviewer 2 Report
This study investigated the molecular composition of humic acids of four profiles in Permafrost peatlands. The results reveal that the top soils (modern peat) contain Has with long-chain carbohydrate and paraffin structures. For the subsoils, a large amount of lignin components can be found. The authors also compared the molecular composition of humic acids of peat with and without vegetation cover. This is a very interesting and important study. Indeed, the studies on molecular composition of humic acids are limited for peat soils, especially for permafrost peat. However, I believe that the quality of the manuscript should be further improved before publication:
1. It is unclear how far away between the plots. I do see the spatial variance between the two plots within one site (Figure 4). Therefore, I would like to see some discussion on the spatial variance of the peat. It is quite important as it determines the core samples are sufficient to generate a solid conclusion or representing a whole peatland.
2. The authors gave error bar for figure 6 but not figures 3 and 4?
3. The figure caption of figure 1 should be revised because we cannot see 1-1,1-2 from the figure.
4. Lines 88-91, winter temperature is also quite important.
5. Lines 135-147, could you give any reference for the peat decomposition determination? In general, people use von Post, bulk density or 13C to indicate the peat decomposition stage.
6. It is well known that the subsoils contains a large amount of lignins. Because the easily decomposed organic carbon has been decomposed. I think the authors should give a discussion on this. See Liu et al. 2019.
Reference:
Liu, H., Zak, D., Rezanezhad, F., Lennartz, B. 2019. Soil degradation determines release of nitrous oxide and dissolved organic carbon from peatlands. Environmental Research Letters. 14, 094009.
Author Response
The authors are very grateful to the referee for reviewing our manuscript. The responses to the comments are given below:
1) Probably the reviewer meant Fig. 1. Plots 1-1 and 1-2, as well as Plots 2-1 and 2-2 are located a few meters apart and the scale of Fig. 1 does not allow to distinguish their location. Therefore, we have added explanatory phrases in section 2.1. Geographic setting.
2) In Figs. 3 and 4 show NMR spectroscopy data. Each 13C-NMR spectrum of the HA preparation is taken once, but the spectrum is an accumulation of a large number of individual scans, in our case 8000 scans. Therefore, when integrating and calculating the proportion of certain functional groups in the composition of HA, the relative error of this determination is quite small and does not exceed 1% for a particular sample. Therefore, as a rule, when demonstrating the results of NMR spectroscopy, error bars are not used.
3) We believe that the figure caption in Fig. 1 is correct.
4) We did not indicate the average temperatures in summer and winter, but only indicated the average sum of positive temperature values. Since this parameter is the most important when considering the process of humification of soil organic matter. At negative temperatures, the chemical reactions of HA transformation proceed at very low rates.
5) We carried out the determination of peat decomposition in accordance with the interstate standard - GOST 10650-2013. Peat. Methods for the determination of the disintegration degree. Reference added.
6) Yes, we agree with the reviewer. The accumulation of aromatic fragments in the structure of HAs from the lower older layers of peat can also be due to the fact that the easily decomposed fragments (e.g., carbohydrate and paraffin) are more quickly consumed by microorganisms. The relevant phrase and reference have been added to the Discussion.
All changes in the text are highlighted in yellow.
Sincerely, authors
Round 2
Reviewer 1 Report
Authors have answered all comments.
