SESolid EarthSESolid Earth1869-9529Copernicus PublicationsGöttingen, Germany10.5194/se-9-1329-2018Stability of soil organic matter in Cryosols of the maritime Antarctic: insights
from 13C NMR and electron spin resonance spectroscopyStability of SOM in soils of the maritime AntarcticAbakumovEvgenye_abakumov@mail.rue.abakumov@spbu.ruhttps://orcid.org/0000-0002-5248-9018AlekseevIvanalekseevivan95@gmail.comst014661@student.spbu.ruhttps://orcid.org/0000-0002-0512-3849Department of Applied Ecology, Saint-Petersburg State University, 199178, 16-line 2, Vasilyevskiy Island, RussiaOtto Schmidt Laboratory for Polar and Marine Research, Arctic and Antarctic Research Institute, 199397, Beringa str. 38, RussiaEvgeny Abakumov (e_abakumov@mail.ru, e.abakumov@spbu.ru) and Ivan Alekseev (alekseevivan95@gmail.com, st014661@student.spbu.ru)19November2018961329133912May201830May201822October201823October2018This work is licensed under the Creative Commons Attribution 4.0 International License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/4.0/This article is available from https://se.copernicus.org/articles/9/1329/2018/se-9-1329-2018.htmlThe full text article is available as a PDF file from https://se.copernicus.org/articles/9/1329/2018/se-9-1329-2018.pdf
Previously, the structure and molecular composition of
the Antarctic soil organic matter (SOM) has been investigated using
13C-NMR methods, which showed that in typical organo-mineral soils the
aliphatic carbon prevails over the aromatic one, owing to the non-ligniferous
nature of its precursor material. In this study, the SOM was analysed from
different sample areas (surface level and partially isolated supra-permafrost
layer) of the tundra-barren landscape of the Fildes Peninsula, King George
Island, Western Antarctica. We found that the humic acids (HAs) of the
cryoturbated, buried areas had lower amounts of alkyl aromatic and protonized
aromatic compounds. In contrast, the HAs from the surface layers contain less
alkyl carbon components. The free-radical content was higher in the surface
layers than in the buried layers due to the presence of fresh organic
remnants in superficial soil samples. New data on SOM quality from these two
representative Cryosols will enable a more precise
assessment of SOM stabilization rate in sub-Antarctic tundras. Comparison of
the 13C-NMR spectra of the HAs and the bulk SOM revealed that
humification occurs in the Antarctic and results in accumulation of aromatic
and carboxylic compounds and reductions in alkylic ones. This indicates that
humification is one of the ways of soil organic matter
stabilization.Highlights.
Stabilization of soil organic matter was studied; humic acids of superficial
horizons contain more aromatic carbon; humification is one of the ways of
soil organic carbon stabilization.
Introduction
Polar soils play a key role in global carbon circulation and
stabilization as they contain maximum stocks of soil organic matter (SOM)
within the whole pedosphere (Schuur et al., 2015). Cold climate and active
layer dynamics result in the stabilization of essential amounts of organic
matter in soils, biosediments, and grounds of the polar biome (Zubrzycki et
al., 2014). Global climate changes and permafrost degradation have led to the
exposure of huge pools of organic matter to microbial degradation (Schuur et
al., 2015) and other environmental risks (IPCC, 2007). Polar SOM represents a vulnerable
carbon source, susceptible to remobilization under increasing temperatures
(Schuur et al., 2015; Ejarque and Abakumov, 2016). In order to better
understand the implications of permafrost SOM for greenhouse gas emissions,
accurate knowledge of its spatial distribution, both in terms of quantity and
quality (e.g. biodegradability, chemical composition, and humification stage)
is needed in addition to effective evaluation of SOM temporal dynamics (Fritz
et al., 2015; Vasilevitch et al., 2018).
Current estimations of soil organic carbon (SOC) stocks are around 1307 Pg
throughout the northern circumpolar region (Hugelius et al., 2014). These
amounts surpass previous estimates (Tarnocai et al., 2009) and grossly exceed
the total carbon contained in the world's vegetation biomass (460–650 Pg)
or in the atmosphere (589 Pg) (Tarnocai et al., 2009). However, the
aforementioned SOM or SOC stock estimations are still poorly constrained
(Hugelius et al., 2014). This uncertainty is largely caused by the estimates
having been calculated from observations that are highly spatially clustered
(Hugelius et al., 2014) while extensive land areas remain uncharacterized due
to the logistic difficulties of reaching these sites. Additionally, the
calculation of these stocks are based on estimated data on soil bulk density
and carbon values derived from dichromate oxidation methods (Abakumov and
Popov, 2005; Polyakov et al., 2017).
The stocks of SOM in the Antarctic are underestimated compared to the Arctic
because of the lack of data for many parts of this continent, due to the high
content of stones in the soils and the high variability in the carbon content
of the fine earth. Stocks of organic carbon in the Antarctic soil have been
reported as 0.5 kg m-2 in its polar deserts, about 1.0 kg m-2
in its barrens, up to 3–5 kg m-2 in the sub-Antarctic tundra, and up
to 30 kg m-2 in the penguin rockeries of the maritime islands
(Abakumov, 2010; Abakumov and Mukhametova, 2014; Abakumov et al., 2016). To
date, investigation on structural composition of the SOM from both
superficial and partially isolated areas has only been performed on Cryosols
of the Kolyma Lowland (Lupachev et al., 2017), where the organic matter of
modern and buried soils vary greatly in terms of their molecular composition
and quality.
Stability and biodegradability are the key features of SOM that should be
taken into account when estimating current and future carbon stocks and
organic matter quality and dynamics. Stability is related to humification
degree, as more advanced stages in the humification process involve depletion
of the labile molecules, as well as an increase in the bulk aromaticity,
which confers higher stability to the SOM. A number of proxies have been used
to trace humification rate and SOM stability, including aromaticity level
(Vasilevitch et al., 2018; Kniker, 2006). Also, the ratio of
C-alkyl : C-aryl and C-alkyl : O-N-alkyl have been successfully used to
assess humification degree (Kinker, 2007). The C / H ratio from humic
acids (HAs) has been used as an index of molecular complexity, as more
degrees of conjugation imply less hydrogenation of the carbon chains (Zaccone
et al., 2007) and C / N has been used as a measure of histic material
degradation (Lodygin et al., 2014). 13C-NMR spectrometry provides
information on the diversity in carbon functional structures (carbon species)
and has been used to evaluate changes in SOM during decomposition and
humification. More specifically, high phenolic (150 ppm), carboxyl-C
(175 ppm), and alkyl-C (30 ppm) groups – combined with low O-alkyl carbons
– have been associated with advanced humification stages (Zech et al.,
1997). So far, studies of SOM quality from polar environments have revealed
generally partially decomposed organic molecules
(Dziadowiec et al., 1994; Lupachev et al., 2017), which preserve much of the
chemical character of their precursor material due to slow progress of
humification (Davidson and Jansens, 2006). This is very important because
polar soils are characterized by the specific composition of the humification
precursors.
The structure and molecular composition of the Antarctic SOM has been
investigated using 13C-NMR methods (Beyer et al., 1997; Abakumov, 2017b)
and it was shown that in typical organo-mineral soils the aliphatic carbon
prevails over the aromatic one, owing to the non-ligniferous nature of its
precursor material (Calace et al., 1995). Also, analyses of cryptogam
extracts were conducted towards identification of individual organic
precursors (Chapman et al., 1994). This feature was then shown to be typical
for soils from different regions of the Antarctic (Abakumov, 2010), including
soil formed on the penguin rockeries (Abakumov and Fattakhova, 2014). The northernmost soil of the Arctic polar biome shows the
same trend in organic molecule organization: higher prevalence of aliphatic
structures over aromatic ones. The diversity of the individual components in
aromatic and aliphatic areas is usually higher in Arctic soil because of the
increased diversity of humification precursors (Ejarque and Abakumov, 2016;
Abakumov, 2010). A selective preservation of the alkyl moieties in the deeper
soil layers has been suggested, and little transformation processes of the
SOM are detectable because soil temperatures are not high enough to stimulate
further microbial breakdown, even in the summer (Beyer et al., 1997). It has been shown that
ornithochory plays an essential role in redistribution of plant remnants
in the Antarctic (Parnikoza et al., 2016) as birds transport considerable
amounts of variably composed organic material within its inland landscapes.
However, published data on SOM composition for the Antarctic are rare, and
further studies that detail its structural compounds and their distribution
are needed. Recently, 13C NMR was successfully used to detail the soils
found in endolithic
communities in Eastern Antarctica and revealed that endolithic organic matter
is characterized by a low prevalence of alkyl aromatic compounds (Mergelov et
al., 2018).
This study aimed to compare the structural composition of the SOM from both
superficial and partially isolated (i.e. buried spots on the border with
permafrost) areas and to evaluate the stabilization rate of Antarctic
Cryosols. The objectives of the study were (1) to evaluate the alterations in
the elemental compositions of the HAs under partial isolation, (2) to assess
the ratios of aromatic and aliphatic carbon species in the topsoil and
isolated areas, and (3) to characterize the biochemical activity of the HAs
(e.g. free-radical concentration).
Location of the Fildes Peninsula.
Materials and methodsStudy sites
King George Island is the largest in the South Shetland Islands archipelago
and only around 5 % of its 1400 km2 area is free of ice (Fig. 1;
Rakusa-Suszczewski, 2002). The Fildes Peninsula and Ardley Island, together
around 33 km2, comprise the largest ice-free area on King George Island
and the second largest of the South Shetland Islands. It has a gentle
landscape consisting of old coastal landforms with numerous rocky ridges
(Michel et al., 2014). According to Smellie et al. (1984),
this area mainly consists of lava with small exposures of tuffs, volcanic
sandstones, and agglomerates. The climate is cold and humid with a mean
annual air temperature of -2.2 ∘C and mean summer air temperatures
above 0 ∘C for only up to 4 months (Wen et al., 1994). The mean
annual precipitation is 350–500 mm year-1. The Fildes Peninsula and
Ardley Island are among the first areas in maritime Antarctica to become ice
free after the Last Glacial Maximum (Birkenmajer, 1989). The onset of
deglaciation in the Fildes Peninsula started, as in the
South Shetland Islands, by 8000–9000 ka and spread during the mid-Holocene (Oliva et
al., 2016). The patterned ground in this region dates from 720 to 2640 BP.
In the South Shetland Islands, permafrost is sporadic or non-existent at
altitudes below 20 m a.m.s.l. and occurs discontinuously in altitudes from
30 to 150 m a.m.s.l. (Bockheim et al., 2013). Mosses, lichens, and algae
are common to this area along with two vascular plants (Deschampsia antarctica and Colobanthus quitensis). Penguins, seals, and
seabirds inhabit the coastal areas and greatly impact the soil development.
Major cryogenic surface-forming processes in this region include frost creep,
cryoturbation, frost heaving and sorting, gravity, and gelifluction (Michel
et al., 2014). Eight separate sites on the Fildes Peninsula have been
collectively designated as an Antarctic specially protected area (ASPA 125)
largely due to their paleontological properties (Management Plan for
Antarctic Specially Protected Area No. 125, 2009). The average thickness of
the soil is about 15–25 cm. Soils from King George Island have been divided
into six groups (WRB, 2014): Leptosols, Cryosols, Fluvisols, Regosols,
Histosols, and Technosols; this corresponds well with previously published
data (Navas et al., 2008; Abakumov, 2017a).
Three soils were selected for humic substance isolation and further
investigation in this study. All soils have top humus layers with a high
carbon content and distinguishable layers of supra-permafrost accumulation of
organic matter. All three soils are classified as Turbic Cryosols (Histic,
Stagnic; WRB, 2014). Soil profiles 1, 2, and 3 (labelled SP1, SP2, and SP3)
were collected from locations described by the following coordinates:
62′14′′391∘ S,
58′58′′549∘ W; 62′13′′140∘ S, 58′46′′067∘ W; and 62′10′′578∘ S, 58′51′′446∘ W,
respectively. The sampling depth was 0–10 cm for the superficial layers and
50–55, 15–20, and 20–25 for SP1, SP2, and SP3, respectively. Images of the
soil profiles are presented in Fig. 2. SP1 is from under the mixed
lichen–bryophyta cover, SP2 and SP3 are formed under species of
Bryophyta and Deshampsia antarctica, respectively.
Soil morphology.
Sampling and laboratory analysis
Soil samples were air-dried (24 h, 20 ∘C), ground, and passed
through 2 mm sieve. Routine chemical analyses were performed using classical
methods: C and N content were determined using an element analyzer (Euro
EA3028-HT analyser) and pH in water and in salt suspensions using a pH-meter
(pH-150 M).
HAs were extracted from each sample according to a published protocol
(Schnitzer, 1982;
http://humic-substances.org/isolation-of-ihss-samples/, last access: 11 May 2018). Briefly, the soil samples were treated with
0.1 M NaOH (soil : solution mass ratio of 1:10) under nitrogen gas.
After 24 h of shaking, the alkaline supernatant was separated from the soil
residue by centrifugation at 1516×g for 20 min and then acidified
to pH 1 with 6 M HCl to precipitate the HAs. The supernatant, which
contained fulvic acids, was separated from the precipitate by centrifugation
at 1516×g for 15 min. The HAs were then dissolved in 0.1 M NaOH
and shaken for 4 h under nitrogen gas before the suspended solids were
removed by centrifugation. The resulting supernatant was acidified again with
6 M HCl to pH 1 and the HAs were again isolated by centrifugation and
demineralized by shaking overnight in 0.1 M HCl / 0.3 M HF
(soil : solution ratio of 1:1). Next, the samples were repeatedly washed
with deionized water until pH 3 was reached and then finally freeze-dried. HA
extraction yields were calculated as the percentage of carbon recovered from
the original soil sample (Vasilevitch et al., 2018; Abakumov et al., 2018).
Isolated HAs were characterized for their elemental composition (C, N, H, and
S) using the Euro EA3028-HT analyzer. Data were corrected for water and ash
content. Oxygen content was calculated by difference from the whole
sample. The elemental ratios reported in this paper are based on
weight. Solid-state 13C-NMR spectra of HAs were measured with a Bruker
Avance 500 NMR spectrometer in a 3.2 mm ZrO2 rotor.
The magic angle spinning speed was 20 kHz in all cases and the nutation
frequency for cross polarization was u1 / 2p 1/4 62.5 kHz. Repetition
delay between the number of scans was 3 s.
Groups of structural compounds were identified by their chemical
shift values: alkyl C (-10 to 45 ppm), O / N-alkyl C (45 to 110 ppm),
aryl / olefine C (110 to 160 ppm), and
carbonyl / carboxyl / amide C (160 to 220 ppm) (Kniker, 2007). The
13C-NMR study was also conducted in bulk soil samples towards
characterizing changes in the initial soil material during humification.
The ESR spectra (only for HAs due to low ash content) were recorded on a JES
FA 300 spectrometer (JEOL, Japan) in X-diapason with a free-radical
modulation amplitude of 0.06 mT and a microwave power in the cavity of
1 mW. Magnesium powder with fixed radical concentration was used as an
external standard. The concentration of the paramagnetic centres in powdered
samples was determined by comparison to relative signal intensities of the
external standard using the program JES-FA swESR v. 3.0.0.1 (JEOL, Japan;
Chukov et al., 2017).
Basic characteristics of soils. [CRH] represents buried humic cryogenic horizon.
SampleTOC, %N, %C / NpHH2OpHCaCl2Colour1 O27.63±0.235.18±0.425.336.355.3010 YR 4/72 [CRH]19.05±0.152.20±0.058.665.674.892.5 YR 4/43 O20.04±0.171.16±0.0917.134.804.8010 YR 4/45 [CRH]12.33±0.240.78±0.0915.804.704.502.5 YR 4/34 O10.16±0.090.84±0.0711.984.904.2110 YR 5/36 [CRH]6.66±0.070.81±0.098.204.704.352.5 YR 5/3Statistical analysis
Statistical data analysis was performed using the STATISTICA 10.0 software
(TX, USA). One-way analysis of variance (ANOVA) was applied to test the
statistical significance of the differences between the data, based on
estimation of the significance of the average differences between three or
more independent groups of data combined by one feature (factor). Fisher's
least significance test (LST) was used for post hoc analysis to provide a
detailed evaluation of the average differences between groups. A feature of
this post hoc test is inclusion of intra-group mean squares when assessing
any pair of averages. Differences were considered significant at the 95 %
confidence level. Concentrations of organic and inorganic contaminants were
determined with at least three replicates. The calculated average
concentrations are provided as mean ± standard deviation.
Elemental composition (%) and atomic ratios in HAs. Data
presented in atomic values.
Sample no.CNHOC / NH / CO / C149.53±0.565.55±0.076.90±0.1138.02±0.648.920.130.76247.14±0.454.30±0.066.79±0.0941.77±0.2110.960.140.88345.55±0.325.14±0.095.80±0.0943.51±0.358.860.120.95443.77±0.244.72±0.116.90±0.0844.61±0.219.270.151.01549.99±0.414.78±0.086.56±0.0838.67±0.3410.450.130.77644.45±0.0343.99±0.076.77±0.1044.79±0.2511.140.151.01P, one-way ANOVA, superficial/buried0.140.050.290.05n.d.n.d.n.d.Results and discussion
It was previously suggested that temperature and humidity are the most
important factors determining most soil-forming processes in cold climate and
humid environments (Campbell and Claridge, 1982; Matsuoka et al., 1990). However, maritime Antarctica is different from the other
regions of Earth by the high influence of seabirds and seals on soil-forming
processes as they provide additional sources of biogenic elements and
significantly change the chemistry of soils. Seabird and seal colonies
significantly change biotic activity in marine terraces of maritime
Antarctica (González-Guzmán et al., 2017). Periglacial features are
dominant on Fildes Peninsula (King George Island; Lopez-Martinez et al., 2012). Total organic carbon
(TOC) content was high in both the superficial and buried soil layers. This
is indicative of the low degree of decomposition and transformation of the
precursor material and is comparable to the data on soils from the Yamal
tundra (Ejarque and Abakumov, 2016) and the Argentinian islands (Parnikoza et
al., 2016). High TOC content is typical for the Antarctic Peninsula compared
to soils of the Eastern Antarctic (Beyer et al., 1997; Mergelov et al., 2017).
While both were elevated, the TOC was higher in the
superficial levels relative to the lower ones. Previous studies describe high
variability in the TOC content from the soils of King George and Galindez
islands, mainly depending on the diversity of the ecotopes and the sources of
organic matter (Abakumov, 2010; González-Guzmán et al., 2017; Moura
et al., 2012; Parnikoza et al., 2016). TOC was previously found higher in
ornithogenic soils of rocky platforms compared to non-ornithogenic soils
(Moura et al., 2012). Isolated (buried) soil spots are not connected with
fresh sources of organic matter, explaining why the TOC content in these
layers is lower.
The 13C-NMR spectra of the HAs, isolated from soils
(labelled 1–6 according to Table 1). [CRH] represents the buried humic cryogenic horizon.
13C NMR spectra of bulk organic matter of soils (labelled 1–6
according to Table 1). [CRH] represents the buried humic cryogenic horizon.
Typical ESR spectrum of humic substances investigated.
The carbon to nitrogen ratio was narrowest in SP1, which was affected by the
skua activity (evidenced by remnants of nests). This is in line with
previous studies that documented the well-pronounced ornithogenic effects on
soil nitrogen content (Otero et al., 2013; Parnikoza et al., 2016; Simas et
al., 2007). Organic matter is one of the main soil components that
contributes to the development of many of the physical, chemical, and
biological properties and is of particular importance in Antarctic soils
(Beyer et al., 1997). The fine earth of the investigated
soils were characterized by acid reaction, which is expected for soils of
this region. Values of pHH2O varied from 4.70 to
6.35. These values coincide well with those previously obtained for maritime
Antarctica (Moura et al., 2012; Navas et al., 2017).
In terms of elemental composition, soil HAs are comparable with those
previously reported for the Arctic and Antarctic soil (Table 2). Current exposed
organic layers contain HAs with higher carbon and nitrogen and lower oxygen
content. Conversely, the HAs of isolated soil patches show increased levels
of oxidation. In comparison to soils of the tundra in the Komi Republic
(Vasilevitch et al., 2018), HAs found in this study were more oxidized,
comparable to those of the Kolyma Lowland (Lupachev et al., 2017) and
previously published data from the Fildes Peninsula (Abakumov, 2017b).
Carbon species integration in molecules of the HAs, %.
Data on the distribution of carbon species in HAs (Fig. 3, Table 3) and in bulk soil
(Fig. 4, Table 4) samples indicated that aromatic compound content is generally lower
than the alkyl components. This is
a well-known peculiarity of the soils of the polar biome (McKnight et al.,
1994; Beyer et al., 1997). At the same time, the degree of aromaticity of the
isolated HAs is 3 fold higher than in the bulk organic matter. This suggests
the presence of the humification process in the soils of Antarctica since
humification involves increasing the aromatic compound content in
macromolecules. This supports the classical humification hypothesis instead
of new arguments, which are critical for this approach (Lehman and Kleber,
2015). Our data show that SOM is on a continuum and HAs are the main acting
constituent of this continuum, thereby confirming that this model is
applicable even in Antarctica. The degree of aromaticity was higher in both
isolated HAs and bulk soil samples from superficial levels compared to
samples from isolated patches. Carbonyl / carboxyl / amide area
(160–220 ppm) was more prevalent in the HAs of topsoils and less abundant
in the organic matter of bulk samples (this region was mainly presented by
carboxylic and amide carbon in the interval between
160–185 ppm) (Kniker, 2007). HAs extracted form SP1, located under the
Deshampsia antarctica, exhibited wide peaks around 110–140 ppm
(H-aryl, C-aryl, olefinic-C) and at 140–160 ppm (O-aryl and N-aryl-C),
while aromatic components of SP2 and SP3 were mainly represented by peaks
between 110 and 140 ppm. This difference can be explained by the organic
remnants of Deshampsia antarctica serving as the precursor for
humification. All HA samples showed intensive areas of alkylic carbon
(0–45 ppm), aliphatic C and N and methoxyl C (45–110 ppm), O-alkyl of
carbohydrates and alcohols (60–95 ppm), and acetal and ketal carbon of
carbohydrates (95–110 ppm). Carbon composition of the bulk samples was
different from isolated HAs as evidenced mainly by the presence of alkyl
carbon (0–45 ppm) and O- and N-alkyl carbon (45–110 ppm). Characteristic
features of the bulk organic matter include carboxylic carbon and that the
aryl compound content was low relative to isolated HAs. Only soils with prior
ornithogenic interactions showed increases in carboxylic peaks, which
corresponds well to data on relic ornithogenic soil (Beyer et al., 1997).
Mass concentration of free radicals in humic acids.
The C-alkyl : O-N-alkyl ratio used to indicate the degree of organic matter
transformation was quite variable in all samples investigated. This can be
caused by diversity in the origin and composition of the humification
precursors. In the case of comparisons with humic and fulvic acids of tundra
soils (Vasilevitch et al., 2018), HAs of soils are intermediated between HAs
and fulvic acids of tundra Histosols with partially decomposed organic
matter. These data are in line with a previous report (Hopkins et al., 2006)
that showed soils of the McMurdo Dry Valleys have a low
alkyl-C : O-alkyl-C ratio using solid-state 13C-NMR spectroscopy), and
therefore can serve as a labile high-quality resource for micro-organisms.
Beyer et al. (1997) showed that both the cross-polarization magic angle
spinning 13C NMR and the
Py-FIMS (pyrolysis-field ionization mass spectrometry) spectra of
the Terri-Gelic Histosol were dominated by signals from carbohydrates and
alkylic compounds, which is corroborated by our findings. They also suggest
that the 13C-NMR data reflected decomposition of carbohydrates and
enrichment of alkyl C in deeper soil layers. In regards to the bulk SOM, this
was true for SP2 and SP3 but not for SP1 that formed under the vascular plant
Deshampsia antarctica.
A representative electron spin resonance ESR spectrum of HAs is presented in
Fig. 5 and the ESR parameters are similar to HAs and fulvic acids (FAs) of temperate
soils (Senesi, 1990; Senesi et al., 2003). The spectra show a single, wide
line with a g-factor ranging from 1.98890 to 1.99999, attributable to the presence of stable semiquinone free
radicals in the HA-containing macromolecules (Table 5). The free-radical
content was higher in the superficial levels than in the isolated ones. This
corresponds well with previous reports (Chukov et al., 2017; Abakumov et al.,
2015) that connect the isolation of buried organic matter in the
supra-permafrost with declining free-radical content. This reveals the
increased biochemical activity of HAs in topsoil. Compared to data from
Lupachev (2017), the differences between exposed and isolated areas are less
pronounced but, in general, the HAs of the Antarctic soils contain more
unstable free radicals on average than the tundra soils of the Kolyma Lowland
(Lupachev et al., 2017) and are comparable to the soils from the Yamal tundra
(Chukov et al., 2017). Taken together, the free-radical content found in our
study was lower than in anthropogenically affected boreal and forest steppe
soils of the East European Plain (Abakumov et al., 2018).
Conclusions
High TOC content was fixed for the three studies representatives of Turbic
Cryosols on King George Island, northwest of the Antarctic Peninsula, Western
Antarctic. High amounts of TOC are characteristic for both superficial and
partially isolated soil materials. HAs contained 3 times more aromatic carbon
than bulk SOM, which indicates that humification appears and is active in
soils of the Antarctic. Moreover, the amounts of aromatic carbon and carboxyl
groups were higher in the HAs of the superficial layer, which is likely
caused by the greater diversity of their organic precursors and more active
humification than in sub-aerial conditions. The HAs of the superficial sample
layers contained lower concentrations of free radicals, an indicator of
active transformation in the topsoil. In general, the organic matter from
partially isolated areas is less stable in terms of carbon species and free
radical content. This likely results from the relative lack of fresh organic
precursors and the different aeration and hydration conditions of
stagnification bordering the permafrost table.
Our underlying research data belong to the
Research Centre of Saint Petersburg State University, since we performed the analysis there.
IA contributed humic substances isolation;
EA contributed field soil survey, NMR spectra collection, and interpretation.
The authors declare that they have no conflict of interest.
Acknowledgements
This work was supported by the Russian Foundation for Basic Research, project
nos. 16-34-60010 and 18-04-00900 and the Saint Petersburg State University
Internal Grant for the Modernization of Scientific Equipment
no. 1.40.541.2017. Analyses were carried out at the Magnetic Resonance
Research Centre and at the Chemical Analysis and the Materials Research
Centre of the Research Park of St. Petersburg State University,
Russia.
The authors would like to thank Alexey
Lupachev for assistance with field research and for providing the images in
Fig. 2. Edited by: Marc
Oliva Reviewed by: three anonymous referees
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