1. Introduction
Polar soils play a key role in the global carbon balance as they contain maximum stocks of soil organic matter (SOM) within the whole pedosphere [
1]. The accumulation of humus in the profile of arctic soils is associated with permafrost retinization processes, cryogenic mass exchange processes, in situ organic matter formation from root remnants, as well as with an inheritance from parent rocks [
2]. The area occupied by permafrost-affected soils amounts to more than 8.6 million km
2, which is about 27% of all land areas north of 50° N [
3]. The storage of SOM in high latitudes was estimated at 1672 × 10
12 kg, which comprises about 70% of all SOM in the world [
3,
4,
5].
Low average temperatures and a short vegetation season in the arctic zone cause the accumulation of organic matter throughout the Quaternary period [
6]. The biomass formed during the short vegetation season initially accumulates in the upper active soil layer. Thus, there is an annual accumulation of organic matter to which the alluvial sedimentation of organic residues contributes [
4]. Cryoturbation and cryogenic mass exchange also lead to the incorporation of organic matter into deeper soil horizons. Another process is the movement of organic matter in a dissolved state and its accumulation on the border with the permafrost table [
5,
7].
According to cryoturbation processes, small fragments of organic matter separate from the lower parts of the surface horizons under the influence of ice penetration, move inside the profile, and mix with the mineral part of the underlying horizons. Such movement of organic masses along the profile leads to its compaction, homogenization, and destruction of plant remnants [
8,
9]. As a result of slope processes, organogenic horizons often turn out to be sealed under the material that arrived as a result of solifluction [
9,
10]. Unlike cryoturbated material, buried organogenic horizons are characterized by high porosity; plant remnants, including roots, are relatively much less destroyed [
11].
Only 0.35% (45,000 km
2) of Antarctica is ice-free [
12]. The extreme variation in Antarctica’s climate has important effects on soil properties and distribution. In continental Antarctica, plant life is restricted to mosses, lichens, and algae, with vascular plants limited to the Antarctic island north of 67° S, particularly in the South Orkney and South Shetland Islands [
13]. Birds play an important role in modifying the soils of coastal Antarctica. Seabirds and nesting birds constitute the dominant factor influencing the soil organic carbon (SOC) and nutrient levels in Antarctic soils. Specific soil-forming processes in the South Orkney Islands (SOI) and South Shetland Islands (SSI) include cryoturbation, phosphatization, brunification, podsolization, sulfurization, and andosolization [
14,
15]. At higher landscape positions, eutrophic, alkaline soils prevail that are normally devoid of vegetation and with low SOC [
16]. Birds are the main source of soil organic carbon in the soils of Antarctica; specific ornithogenic soils are formed in the rookery sites [
12]. The organic carbon content in the upper organo-mineral horizons (up to 20 cm) can reach 38%. Significant accumulation of organic matter was found earlier in ornithogenic soil types, and the minimum organic carbon content was identified in Leptosol on basalts and andesites under communities of mosses and lichens [
17]. The organic matter of Antarctic soils is poorly humified; fulvic acids and detrital forms of undecomposed humus prevail [
18]. The organic matter of Antarctic Leptosol contains a significant proportion of water-soluble fragments. In general, these features are associated with the low microbiological activity typical of Antarctic soils [
19].
The circumpolar environments are characterized by a low degree of humification of organic matter, which is associated with a short vegetation season, as well as low levels of plant remnant in the soil. In the soils of the coastal zones of Antarctica, the formation of huge carbon stores has been noted, comparable to those in soils of similar regions of the Arctic [
20,
21].
Various methods have been used to investigate the humic acids (HAs) of soil organic matter. There are also many methods for determining HAs composition. Fourier-transform infrared spectroscopy (FTIR), ultraviolet-visible spectroscopy (UV-ViS), cross-polarization, magic-angle spinning, molecular fluorescence spectroscopy (MF), and electron spin resonance spectroscopy (ESR) are all useful and often applied in SOM studies [
22,
23]. Here, we use two modern instrumental methods:
1H-
13C HETCOR (heteronuclear-correlation) and
13C CP/MAS (cross peak magic-angle spinning) NMR spectroscopy. One-dimensional (1D) solid-state
13C NMR techniques have provided some structural information on HAs, including quantitative determination of various types of chemical groups [
19,
24,
25,
26,
27,
28]. However, this does not provide the opportunity to characterize detailed differences between the atoms included in such groups and in many cases does not allow for the signals to be clearly distinguished as a result of their considerable overlap due to broad lines and the diversity of structures of functional groups [
29,
30,
31,
32]. By the application of two-dimensional (2D) NMR techniques, such problems of peak overlap can be reduced, and the chemical structure corresponding to a given peak can be identified more specifically. As dipolar interactions act through space, the correlation of unprotonated carbons with unbound protons is possible, which provides additional structural information [
30]. Two-dimensional correlation of
13C chemical shifts with
1H chemical shifts, termed 2D heteronuclear-correlation
1H-
13C HETCOR NMR, can provide more structural information than
13C or
1H NMR spectroscopy. The HETCOR method has been used to determine organic compounds in solutions of organic substances and has also been adapted for solids using 1H homonuclear dipolar decoupling and can now be used to determine humic acids in soils [
29,
30,
31,
32,
33].
The HETCOR experiment correlates chemical shifts of
1H with chemical shifts of X-nuclei (for example,
13C,
15N). The experiment provides excellent resolution in an indirect
1H measurement. Homonuclear decoupling during the
1H evolution is achieved using the FSLG (Frequency Switched Lee Goldburg) sequence, which works even at relatively high MAS frequencies. Decoupling from protons during evolution is not necessary, since the high MAS speed already provides this [
34,
35].
Mixing is carried out during the contact magnetization transfer pulse. Since the transfer of magnetization from protons to X (for example,
13C) occurs quickly, the contact time should be short in order to avoid transfer over long distances, which leads to a nonspecific cross-peak pattern. There is a modification of the base sequence that uses magnetization transfer at the LG bias frequency for protons. In this case, the magnetization to the X nucleus passes only from closely located protons [
34,
35].
The bioclimatic conditions determine the soil formation in the polar regions, and their specific composition of Has; however, their high diversity, low degree of knowledge of the regions, and the use of classical methods for studying organic matter do not allow us to state the molecular composition of HAs in polar soils with a high degree of confidence. The formation and transformation of HAs is a complex process in which a group of factors is involved, such as climate, composition and activity of the microbiological community, quality of plant residues, pH, and hydrophobicity of environment [
26,
27,
28,
36]. At present, there are a number of works devoted to the study of taiga and tundra soils using
13C (CP/MAS) NMR spectroscopy; however, there are few studies on Antarctic and Arctic soils. Studies on the organic compounds of HAs for the soils of the polar area by the
1H-
13C (HETCOR) NMR spectroscopy have not been carried out to current time. The advantage of this method is that, when analyzing the spectra of HAs, we can observe cross-peaks of H-C bonds, while for the
13C (CP/MAS) NMR spectroscopy we can only observe chemically bound carbon. The HETCOR method allows the study of single HAs fragments. Thus, the combination of the two methods
1H-
13C (HETCOR) and
13C (CP/MAS) NMR spectroscopy can reliably determine the molecular structure of HAs [
30].
For further study of the fundamental processes of humus formation and the accumulation of specific organic compounds in the polar regions, modern instrumental methods are required. The methods of analysis of molecular composition that we have proposed will help to understand the fundamental processes of soil formation and create new ideas about the complex composition and structure of natural high molecular compounds of HAs in permafrost-affected soils [
26,
27,
28].
1H-13C (HETCOR) and 13C (CP/MAS) NMR spectroscopy are powerful tools for studying molecular-level structure and dynamics in HAs. Thus, this study aimed to determine the molecular composition of organic matter in selected soils of the Russian Arctic and Antarctic using 13C-1H(HETCOR) and 13C NMR spectroscopy.