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Stabilization of organic material from soils and soil-like bodies in the Lena River Delta ( 13 C-NMR spectroscopy analysis)

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The Arctic ecosystem has a huge reservoir of soil organic carbon stored in permafrost-affected soils and biosediments. During the short vegetation season, humification and mineralization processes in the active soil layer result in the formation of specific soil organic substances-humic substances. Humic acids are high molecular, specific, thermodynamically stable macromolecules. The study was conducted in the Lena River Delta, the largest river delta located in the Arctic. Cryosol-type soils on alluvial deposits of the river form an area of about 45 thousand km 2 under permafrost conditions. The vegetation cover is represented by moss-lichen communities with the presence of Salix glauca in the flooded areas, as well as Betula nana in the areas not subject to flooding. The paper presents the elemental and molecular composition of humic acids isolated from soils, integral indicators of humification (stabilization) of organic matter in the soils of the Lena River Delta. The study was conducted using the 13 C (CP/MAS) NMR spectroscopy method. In the work, it was revealed that up to 33% of aromatic and up to 15% COOR fragments are accumulated in humic acids. The AR/AL ratio ranged from 0.69 to 0.89. The studied soils are variants of modern soil formation (not subjected to alluvial processes) and soil-like bodies that melted from the IC of the river delta. A relatively high degree of condensation of humic acid macromolecules in comparison with other polar regions of the Arctic and Antarctic was noted.
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SJSS. SPANISH JOURNAL OF SOIL SCIENCE YEAR 2020 VOLUME 10 ISSUE 2
170
Stabilization of organic material
from soils and soil-like bodies in
the Lena River Delta (13C-NMR
spectroscopy analysis)
Estabilización de la materia orgánica de suelos y cuerpos similares al suelo en el Delta del
Río Lena (Análisis de espectroscopía 13C-NMR)
Estabilização da matéria orgânica do solo e corpos semelhantes ao solo no Delta do Rio
Lena (Análise por espectroscopia 13C-NMR)
Polyakov, V.1,2,@1
slavon6985@gmail.
com
Abakumov E.1
@ Corresponding Author
1Department of Applied
Ecology, Faculty of
Biology, St. Petersburg
State University. 16th
Liniya V.O., 29, St.
Petersburg, 199178,
Russian Federation.
2Arctic and Antarctic
Research Institute.
Beringa 38, St.
Petersburg, 199397,
Russian Federation.
ABSTRACT
e Arctic ecosystem has a huge reservoir of soil organic carbon stored in permafrost-aected soils
and biosediments. During the short vegetation season, humication and mineralization processes
in the active soil layer result in the formation of specic soil organic substances – humic substances.
Humic acids are high molecular, specic, thermodynamically stable macromolecules. e study was
conducted in the Lena River Delta, the largest river delta located in the Arctic. Cryosol-type soils
on alluvial deposits of the river form an area of about 45 thousand km2 under permafrost conditions.
e vegetation cover is represented by moss-lichen communities with the presence of Salix glauca
in the ooded areas, as well as Betula nana in the areas not subject to ooding. e paper presents
the elemental and molecular composition of humic acids isolated from soils, integral indicators of
humication (stabilization) of organic matter in the soils of the Lena River Delta. e study was
conducted using the 13C (CP/MAS) NMR spectroscopy method. In the work, it was revealed that up
to 33% of aromatic and up to 15% COOR fragments are accumulated in humic acids. e AR/AL
ratio ranged from 0.69 to 0.89. e studied soils are variants of modern soil formation (not subjected
to alluvial processes) and soil-like bodies that melted from the IC of the river delta. A relatively high
degree of condensation of humic acid macromolecules in comparison with other polar regions of the
Arctic and Antarctic was noted.
RESUMEN
El ecosistema ártico constituye una enorme reserva de carbono orgánico que se encuentra almacenado en suelos
afectados por permafrost y biosedimentos. Durante la corta estación vegetativa, los procesos de humicación y
mineralización en la capa activa del suelo dan lugar a la formación de sustancias orgánicas especícas en el suelo, las
sustancias húmicas. Los ácidos húmicos son macromoléculas de alto peso molecular, especícas y termodinámicamente
estables. Este estudio se llevó a cabo en el Delta del Río Lena, el mayor delta de río situado en el Ártico. Allí se
encuentran Criosoles formados sobre depósitos aluviales de río que ocupan un área aproximada de 45.000 km2
bajo condiciones de permafrost. La cobertura vegetal está representada por comunidades de líquenes y musgos con la
presencia de Salix glauca en las zonas inundadas, así como Betula nana en las zonas no inundadas. Este trabajo
presenta la composición elemental y molecular de los ácidos húmicos aislados de los suelos, indicadores integrales de
AUTHORS
Received: 07.05.2020 Revised: 17.06.2020 Accepted: 29.30.2020
DOI : 10.3232/SJSS.2020.V10.N2.05
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la humicación (estabilización) de la materia orgánica en los suelos del Delta del río Lena. El estudio fue realizado
mediante el método de espectroscopía 13C (CP/MAS) NMR. Así, se obtuvo que en los ácidos húmicos se acumulaban
hasta un 33% de fragmentos aromáticos y un 15% de fragmentos COOR. La relación AR/AL osciló entre 0,69 y
0,89. Los suelos estudiados son variantes de la formación de suelos modernos (no sometidos a procesos aluviales) y
cuerpos similares al suelo que se derritieron a partir del Complejo de Hielo (IC) del delta del río. Se observó un grado
relativamente alto de condensación de macromoléculas de ácidos húmicos en comparación con otras regiones polares
del Ártico y Antártico.
RESUMO
O ecossistema Ártico é uma enorme reserva de carbono orgânico, que está armazenado em solos afetados por permafrost
e em biosedimentos. Durante a curta estação vegetativa, os processos de humicação e mineralização na camada
ativa do solo dão origem à formação de substâncias orgânicas especícas do solo – as substâncias húmicas. Os ácidos
húmicos são macromoléculas de elevada massa molecular, especícas e estáveis em termos termodinâmicos. Este estudo
foi realizado no Delta do Rio Lena, o maior delta uvial localizado no Ártico. Numa área de aproximadamente
45 000 km2, ocorrem Criossolos formados a partir de depósitos aluviais do rio Lena em condições de permafrost. A
cobertura vegetal é representada por comunidades de líquenes e musgos com a presença de Salix glauca nas zonas
inundadas, bem como Betula nana nas zonas não inundadas. O artigo apresenta a composição elementar e molecular
dos ácidos húmicos isolados dos solos, indicadores integrais da humicação (estabilização) da matéria orgânica nos
solos do Delta do Rio Lena. O estudo foi realizado utilizando o método de espectroscopia 13C (CP/MAS) NMR.
Nos ácidos húmicos acumulam-se até 33% dos fragmentos aromáticos e 15% dos fragmentos de COOR. A relação
AR/AL variou entre 0,69 e 0,89. Os solos estudados são variantes da formação moderna dos solos (não sujeitos a
processos aluviais) e corpos semelhantes a solo que resultaram da fusão do Complexo de Gelo (IC) do delta do rio. Foi
observado um grau relativamente elevado de condensação de macromoléculas de ácidos húmicos em comparação com
outras regiões polares do Ártico e do Antártico.
1. Introduction
Soil organic carbon (SOC) is a product that accumulates in the soil after the partial
decomposition of diverse types of materials, derived from microorganisms and plant
remnants. This constitutes a key element of the global carbon cycle through the atmosphere,
vegetation, soils, rivers and the ocean (Davis 2001; Dutta et al. 2006; Schimel 1995). The
soil organic matter (SOM) supports the key functions of the soil and ecosystem services,
as it is crucial for stabilizing the structure of the soil, retaining, releasing nutrients for plants,
and for ensuring the penetration of water and its storage in the soil. Loss of SOC indicates a
degree of soil dehumification and degradation. Soils represent the largest surface reservoir of
organic carbon in the Earth. Due to the local geogenic features, climatic conditions and land
use, and management (among other environmental factors), soils retain differing amounts of
SOC (Boike et al. 2013; Dai et al. 2002; Kutzbach et al. 2004). It is estimated that the largest
amount of SOC is stored in the northern permafrost region with over 1024 Pg (1 Pg = 1013 kg)
of organic carbon in the soil in a layer of up to 3 m, as well as 34 Pg of nitrogen (Jones et
al. 2010; Zubrzycki et al. 2013; Zubrzycki et al. 2014) mainly in peat soil. The permafrost-
affected zone occupies an area of more than 8.6 million km2, which is about 27% of all
land areas above 50°N. They accumulate in themselves a huge amount of organic carbon,
so they are considered one of the most important elements of the cryosphere. There,
carbon accumulates in soils in huge quantities due to low temperatures, leading to low
KEYS WORDS
Permafrost soils,
SOM, Arctica, CP/
MAS, humic acids.
PALABRAS
CLAVE
Suelos del
permafrost, SOM,
Arctica, CP/MAS,
ácidos húmicos.
PALAVRAS-
CHAVE
Solos de permafrost,
SOM, Ártica,
CP/MAS, ácidos
húmicos.
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[ STABILIZATION OF ORGANIC MATERIAL FROM SOILS AND SOIL-LIKE BODIES IN THE LENA RIVER DELTA
(13C-NMR SPECTROSCOPY ANALYSIS) ]
biological activity and slow decomposition of
SOM (Cauwet and Sidorov 1996; Ejarque and
Abakumov 2016). The corresponding soil type
is called Histosol (IUSS Working Group WRB
2015) and is characterized by SOC content of
12 to 50%. The presence of permafrost and
long-term freezing of soils has a strong influence
on the processes of ion exchange, the water-
physical regime, the solubility of nutrients and
their availability for plants, and on bioproductivity
in general. The loss of SOC negatively affects
not only soil health and food production, but
also exacerbates climate change. When SOM
decomposes, carbon-based greenhouse gases
are released into the atmosphere. If this happens
too fast, soils can contribute to the warming
of our planet. On the other hand, many soils
have the potential to increase SOC reserves,
thus mitigating climate change by reducing
atmospheric CO2 (Knoblauch et al. 2013; Lara
et al. 1998).
Cryoturbation and cryogenic mass exchange
lead to the translocation and further accumulation
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 (Dutta et
al. 2006; Schimel 1995). During 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 (Davidson and Janssens 2006).
The Lena River has a great influence on the
biogeochemistry of the Arctic Ocean. Arctic
rivers are the main suppliers of organic and
inorganic carbon and largely determines the
organic carbon cycle in the Arctic basin (Boike
et al. 2013; Dobrovolsky 2005; Kutzbach et
al. 2004). The soils of the Lena River Delta
are formed in conditions of seasonal freezing/
thawing processes and annual flooding. The
annual supply of nutrients by the river and a mild
climate cause a high level of microbiological
processes in the soil, which contributes to the
relatively high rate of humification of organic
matter in the soil (Bolshiyanov et al. 2013).
The SOM of Arctic soils is very specific in
comparison with soils are not affected by
permafrost, and modern instrumental methods
and approaches are needed to study it. The
intensity of mineralization and humification of
soil organic matter is extremely low due to the
long duration of the frozen state of soils and the
small sum of positive temperatures (Ejarque and
Abakumov 2016; Lodygin et al. 2017; Lodygin
and Beznosikov 2010). The soils of the Lena
Delta are characterized by humus and peat
soils, and there are also buried organic residues
associated with both cryogenic processes and
the specifics of sedimentation in the deltas of
large rivers (Polyakov et al. 2018). Due to the
widespread development of the lake system, silty
particles and organic residues are deposited at
the bottom of the lakes due to permafrost lateral
mass transfer (Boike et al. 2013; Bolshiyanov et
al. 2013).
For specific conditions of soil formation, in
particular humus accumulation, in addition to
traditional methods for the analysis of organic
matter, the recent method of the study of organic
matter based on its molecular composition
has an advantage. The content of molecular
fragments of HAs determined by CP/MAS
13C–NMR spectroscopy contributes to the
understanding of the fundamental processes of
humus formation and the composition of natural
high-molecular weight HAs in soils subjected to
the influence of cryogenesis (Chukov et al. 2015;
Dai et al. 2002; Ejarque and Abakumov 2016;
Lodygin et al. 2014; Lupachev et al. 2017).
The advantage of the nuclear magnetic
resonance spectroscopy method is the ability
to quantify the content of groups of individual
and structural fragments in humic acid (HAs)
molecules. This method is also used to assess
changes in SOM during decomposition and
humification. So far, studies of the quality of
SOM from polar environments have revealed a
generalized, slightly degraded nature of organic
molecules that retain most of the chemical
nature of their precursor material due to the low
progress of humification (Abakumov et al. 2015;
Davidson and Janssens 2006; Dziadowiec et al.
1994).
Arctic soils, according to studies by various
scientists, have a high proportion of aliphatic
compounds in the composition of the HАs
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[ POLYAKOV V. & ABAKUMOV E. ]
(Abakumov et al. 2015; Chukov et al. 2015;
Lodygin et al. 2017; Polyakov et al. 2019b;
Vasilevich et al. 2018; Vasilevich et al. 2019).
The low content of aromatic compounds of
HAs is primarily associated with vegetation
(the precursors of humification), as well as soil-
climatic conditions, a low degree of aeration,
cryogenic processes, and low microbiological
activity that lead to soil mineralization and
store of organic matter in permafrost. Russian
scientists have well studied permafrost peat
soils in north-west Russia, where they note a
low proportion of aromatic compounds. Upon
transition from the Arctic zone to the boreal
zone, an increase in the proportion of aromatic
compounds occurs, which is associated with an
increase in microbiological activity and a change
in plant communities.
The vegetation cover of the Arctic ecosystem is
represented mainly by moss-lichen communities.
The content of aromatic hydrocarbons in them, in
particular lignin, tannin, and flavonoids, is rather
low or close to zero. The content of proteins
and nitrogen-containing fragments is also
extremely low at 2-10%. Thus, the composition
of precursors plays a key role in the composition
of HAs. The predominantly aliphatic character
of the HAs is associated with a high proportion
of carbohydrates among the humification
precursors (Orlov 1990). Antarctic soil-like
bodies are also similar in relation to the aliphatic
and aromatic fragments in the composition
of the HAs by 13C (CP/MAS) NMR and 1H-13C
HETCORE NMR spectroscopy. The spectra
obtained are very homogeneous, due to the low
diversity of the vegetation cover (Abakumov et
al. 2015; Abakumov et al. 2019; Lodygin et al.
2017; Lodygin and Beznosikov 2010; Lodygin et
al. 2014; Lupachev et al. 2017).
Humic acids are heterogeneous systems of
high-molecular condensed compounds formed
from the decay of plant and animal remnants
in terrestrial and aquatic ecosystems. Climatic
parameters, precursors of humification, and
local position in the landscape determine the
diversity of the composition and properties of
HAs (Chukov et al. 2015; Ejarque and Abakumov
2016; Lodygin et al. 2014). The stabilization of
organic material is defined as the transformation
of organic matter into a state inaccessible
to soil microorganisms, and the stabilization
property itself is a characteristic stage of
carbon dynamics (Semenov et al. 2009). Using
13C–NMR spectroscopy we identify the proportion
of aromatic compounds in the composition
of HAs, to assess the stabilization of organic
matter in soils.
This work is the continuation of a long-term
study of the molecular composition of Arctic
soils, using the example of alluvial soils of the
Lena River Delta. Thus, the aim of this work
is to study HAs by 13C–NMR spectroscopy of
permafrost soils of the Lena River Delta, buried
organic matter, and melted soil-like bodies from
the Ice Complex of the delta. To achieve aim of
work, the following objectives were set:
1. to find out the molecular composition of HAs
from study soils
2. to investigate elemental composition of HAs
3. to determine the stabilization rates of HAs
isolated from study soils
2. Materials and Methods
2.1. The study sites
The Lena River Delta, the largest northern delta
in the world, is located in the Arctic zone and has
an area of about 29,630 km2. Due to such a huge
area and location, it has a significant impact on
the hydrological regime of the Arctic Ocean,
since a large amount of fresh water flows from
the delta into the least salty ocean of our planet.
The delta was formed as a result of river activity:
sediment removal, erosion, and abrasion under
the influence of sea level fluctuations and the
movement of the Earth's crust (Bolshiyanov et
al. 2013) (Figure 1).
The Lena River Delta is located in the area
with an Arctic continental climate. The climatic
characteristics of the area are presented in
Table 1.
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Figure 1. The Lena River Delta. Study area. Sample numbers correspond to Table 2.
[ STABILIZATION OF ORGANIC MATERIAL FROM SOILS AND SOIL-LIKE BODIES IN THE LENA RIVER DELTA
(13C-NMR SPECTROSCOPY ANALYSIS) ]
Table 1. Climate parameters of the study region (Data obtained from the station «Samoylov Island»)
Climate parameters Lena River Delta
Mean annual air temperature (°C) -13
Mean air temperature (ºC):
of the warmest month (July) 6.5
of the coldest month (January) -32
Number of days with mean daily Air
temperature:
above 0 °С 73
above 5 °С 35
above 10 °С 11
Freezing depth (cm) 30-50
Snow thickness (cm) 23
Annual precipitation (mm) 323
In summer (mm) 125
The Lena River Delta is covered with various
types of tundra vegetation. The main components
are lichens, mosses, grasses (cereals and
sedges) and some types of shrubs. Here,
Most of the land is characterized by the presence
of a permafrost table at a depth of about 1 meter.
The depth of the active layer varies: on loamy
soils it can reach 30 cm at the end of August,
and on sand soils it can reach 1 meter.
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cereal-sedge-moss coenoses predominate in
relief depressions of the-hypno-sedge polygonal
swamps (Table 2). The vegetation cover has a
mosaic character ("spotted tundra"). The Lena
River Delta is represented by the dominance
of moss-lichen vegetation. Moss groups
predominate on loam and lichen predominates
on rocks. In addition, often near the thermokarst
lakes, moss-lichen vegetation is replaced by
sedge-cannon fodder. On the warm southern
slopes on well-drained sandy soils and in
the river valley there are areas with grassy
vegetation (tundra meadows and floodplain
meadows) (Boike et al. 2013; Kutzbach et al.
2004; Schneider et al. 2009).
2.2. Ice complex (IC)
The issue of the origin of the Ice Complex
(IC) of rocks has not yet been resolved.
There are several hypotheses that explain the
accumulation of sand-silt sediments and their
simultaneous freezing. Some researchers
associate this process with aeolian transport
and the deposition of a huge amount of mineral
material from the atmosphere, and there is also
a theory about the formation of IC as a result
of alluvial accumulation. Another point of view
on the formation of IC is that in front of the ice
sheet on the shelf of the Laptev Sea there was
a stagnant reservoir in which accumulation of
sediments of the IC occurred (Bolshiyanov et al.
2013).
We studied sample 9, a soil-like body extracted
from the IC of the Lena River Delta. The results
obtained during its processing differ from the soils
investigated in this work. The carbon content
in this sample is quite low and more similar in
composition to fulvic acid. Apparently, during the
long-term storage of organic matter in IC, the
carbon content decreases with an increase in
the oxygen fraction in HAs macromolecules.
[ POLYAKOV V. & ABAKUMOV E. ]
Figure 2. Morphological diversity of study soils. Sample numbers correspond to Table 2.
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[ STABILIZATION OF ORGANIC MATERIAL FROM SOILS AND SOIL-LIKE BODIES IN THE LENA RIVER DELTA
(13C-NMR SPECTROSCOPY ANALYSIS) ]
Table 2. Description of the studied soil and soil-like bodies
Site Sample
Description of the studied soil
horizons
Coordinates
Color
index* Vegetation Soil
name**
Kurungnah
isl.
1
Dark material, roots, iron spots,
sandy loam, cryogenic mass
exchange, permafrost table from
27 cm. Top of alas. N72.28920
E126.18025.
10 YR 6/1
Cetraria nivalis,
Sphagnum,
Carex aquatilis.
Turbic
Cryosol
(Siltic)
2
Dark material, roots, gleyic pro-
cesses, sandy loam, water table
from 25 cm, permafrost table 29
cm. Bottom of alas. N72.29042
E126.18191.
10 YR 6/1
Cetraria nivalis,
Sphagnum sp.,
Carex aquatilis.
Turbic
Cryosol
(Loamic)
3
Roots, sandy loam, water table
from 65 cm, permafrost table from
70 cm. Middle of alas. N72.29039
E126.18125.
10 YR 6/1
Sphagnum sp.,
Carex aquatilis,
Salix glauca
Turbic
Cryosol
(Siltic)
4Organic material. Polygon rim.
N72.29063 E126.18423. 10 YR 4/3 Cetraria nivalis,
Sphagnum sp.,
Turbic
Cryosol
(Loamic)
5Organic material. Inside of alas.
N72.29143 E126.18628. 10 YR 4/3
Cetraria nivalis,
Sphagnum,
Carex aquatilis.
Turbic
Cryosol
(Loamic)
6
Buried organic material. Young
alas (~100-200 years). N72.32162
E126.25303.
10 YR 3/2 Trisetum,
Phragmites.
Folic
Cryosol
9
Organo-mineral material from IC.
N72.392402 E125.648261 (~34,299
± 500 years BP)** *.
10 YR 3/2 -Soil-like
body
Interuve
(Kharaulakh
ridge)
7
Organo-mineral material, thixotropy,
loamy sand, rocks. Top of ridge.
N72.39439 E126.76143.
10 YR 3/2
Cetraria nivalis,
Sphagnum,
Leptogium
lichenoides,
Dactylina
arctica
Skeletic
Cryosol
8
Organo-mineral material, thixotropy,
loamy sand, rocks. Top of ridge.
N72.392723 E126.78203.
10 YR 3/2
Cetraria nivalis,
Sphagnum,
Leptogium
lichenoides,
Dactylina
arctica
Soil-like
body
*Munsell Color (Firm) 2010); **Soil name by WRB classication (IUSS Working Group WRB 2015); *** (Schirrmeister et
al. 2003).
2.3. Sampling procedure
Samples of soils were collected in various
elements of the landforms («Old» alas, «young»
alas (buried material), organo-mineral material
from IC and interfluve (Kharaulakh ridge)), soil
description are presented in Table 2. Soil pits are
presented in Figure 2.
Kurungnah Island, located at the central part
of the delta, has a connection with the Olenek
Channel from the west and consists of sediment
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[ POLYAKOV V. & ABAKUMOV E. ]
from the IC and the underlying sand from the
surface. It is built from the surface by deposits
of the IC and the underlying sand. The thickness
is composed of two packs of rocks. The lower
part is composed of fine-grained, sorted quartz
sands. Horizontal layering is inherent in it, and
it is rarely wavy. In the lower part there are
lenticular layers of plant residues with sand.
The entire stratum was formed in the middle
of the Late Neopleistocene (Bolshiyanov et al.
2013). Interfluves (Kharaulakh ridge) consist of
ultra-fine or fine calcarenite and dolarenite (with
minor siliciclastic material) and sandstone. The
lithite-quartz-feldspar sandstone contains minor
dolomite and limey clasts and is intercalated with
calcarenite, mudstone, calcareous mudstone,
and silty mudstone, with intraclasts and breccia
suggesting occasional landslides (Izokh and
Yazikov 2017).
2.4. Soil analysis, 13C NMR spectroscopy and
elemental analysis procedure of HAs
Soil samples were air-dried (24 hours, 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 (EA3028-
HT EuroVector, Pravia PV, Italy) and pH in water
and in salt suspension (soil-dissolvent ratios
1:2.5 in case of mineral horizons and 1:25 in
case of organo-mineral horizons) suspensions
using a pH meter (pH-150M Teplopribor,
Moscow, Russia).
Humic acids were extracted from each sample
according to a published IHSS protocol (Swift
1996). HAs extraction yields were calculated
as the percentage of carbon recovered from
the original soil sample (Vasilevich et al. 2018).
Solid-state CP/MAS 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 frequency was 20 kHz in
all cases and the nutation frequency for cross
polarization was u1/2p 1/4 62.5 kHz. Repetition
delay were 3 seconds. The number of scans
was 6500-32000. Contact time is 0.2 us.
The elemental composition of HAs represents
the percentage of C, H, N, O elements in them.
The high variability of the elemental composition
of HAs among different soils is explained by the
varying degree of accumulation of elements
in the HAs. The highest C content is normally
typical of Chernozems with a well-developed
mollic layer and high degree of organic matter
humification. In the soils of the Arctic zone, the
carbon content is much lower. This peculiarity is
explained by the effect of increased acidity and
humidity. A reduced nitrogen content in soils of
the arctic environment is also observed, which
is associated with its low content in peat and
an increase in hydrogen content. Information
on the elemental composition of organic
substance provides significant information on
the general principles of molecular construction
and some of their properties (Orlov 1990). To
conduct a graphical analysis of the elemental
composition, we used the van Krevelen diagram
(van Krevelen 1950), using the H/C-O/C ratios
to identify the direction of the transformation
processes of various organic compounds in
natural conditions. Thus, we can evaluate the
processes of oxidation/reduction and hydration/
dehydration in HA macromolecules. Elemental
compositions were corrected for gravimetric
water and ash content. Oxygen content was
calculated by difference of whole samples mass
and gravimetric concentration of C, N, H and
ash.
3. Results and Discussion
3.1. Elemental composition of humic acids
isolated from study soils
The elemental composition of HAs is the most
important indicator determining the progress
of humification, oxidation and degree of
condensation of HAs (Abakumov et al. 2015;
Beznosikov and Lodygin 2010). Characteristic
features of HAs formed in cold conditions, and
especially in permafrost-affected soils, are
a relatively high H content and a reduced O
content compared to boreal and sub-boreal soils
(Lupachev et al. 2017).
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From the obtained data, the carbon content
in the study samples confined to modern soil
formation is quite small and in a narrow range
(41-45%), while the carbon content in the sample
The obtained data on HAs elemental composition,
atomic rations and degree of oxidation (W) are
presented in Table 3.
Table 3. Elemental composition of the studied HAs from soils. Gravimetric concentration is given for C,
H, O and N content. C/N, H/C, O/C, H/Cmod and W were calculated from mole fraction of C, H, O and N
content. H/Cmod is the number of substituted hydrogen atoms in HAs; H/Cmod = H/C + 2 (O/C)*0.67; H/C
and W indexes were calculated according to (Orlov 1985). Sample numbers correspond to Table 2. SD ±
0.05 for N, H and C content
Site Sample N, % C, % H, % O, % C/N H/С O/С H/С
mod W
Kurungnah
isl.
1 3.0±0.1 41±2 5.0±0.2 45 13.81 1.50 0.82 2.60 0.15
2 4.0±0.2 44±2 5.0±0.2 42 13.91 1.43 0.71 2.38 –0.01
3 3.0±0.1 44±2 5.0±0.2 42 15.16 1.43 0.73 2.40 0.03
4 3.0±0.1 43±2 5.0±0.2 43 15.18 1.44 0.76 2.45 0.07
5 3.0±0.1 44±2 5.0±0.2 42 16.82 1.38 0.72 2.35 0.06
6 3.0±0.1 45±2 5.0±0.2 42 15.78 1.37 0.70 2.32 0.03
9 3.0±0.1 36±1 5.0±0.2 51 13.93 1.59 1.05 3.00 0.52
Interuve
(Kharau-
lakh ridge)
7 3.0±0.1 44±2 5.0±0.2 42 15.16 1.42 0.71 2.37 0.01
8 4.0±0.2 42±2 5.0±0.2 44 12.65 1.46 0.80 2.53 0.13
bottom also occurs. In buried soils of «young»
alas (№ 6), the lowest H/C ratio was observed,
which indicates that selective degradation of
condensed structures occurs with depth inside
the profile and an increase in the proportion
of herbaceous vegetation, which is the reason
for the condensation of high molecular weight
compounds (Hatcher et al. 1981; Vasilevich et
al. 2019).
From the W index, it follows that most of the
study samples are in an oxidized state, only
sample 2, located in the bottom of the «old»
alas, has reducing conditions associated with
a relatively high level of hydromorphism in the
soil. Oxidative environmental conditions are
associated with the ability of elements to migrate
to the bottom of the profile, in particular, iron and
aluminum ions, which actively migrate in weakly-
acidic and acidic soil solutions. Weak reducing
conditions are due to the production of fresh
organic residues and the process of humification
in the specific bioclimatic conditions of the river
delta (Vasilevich et al. 2018).
One of the methods for graphical representation
of the elemental composition of HAs from soils
from the frozen soil of the IC is noticeably lower
(36%). This may indicate that less organic
residue reached the IC during its formation.
The C/N ratio various from 12 to 16, indicating
a low enrichment of carbon by nitrogen. It is
associated with a nitrogen reservoir in the Arctic
systems where the processes of nitrogen fixation
and ammonification are low due to the low
microbiological activity. Moreover, the oxygen
content in the studied samples is comparable
with the carbon content; there is a high oxygen
content in the sample from the IC (51%). The
high oxygen content is due to the better solubility
of oxygen-enriched hydrophilic HAs molecules
and their migration ability (Lodygin et al. 2014).
The H/C ratio is an indicator of the stability of
HAs in soils. The lower this indicator, the higher
the process of condensation of monomers in
high-molecular substances. From the obtained
data, it can be seen that the sample from the
IC has the highest H/C ratio, which is the result
of a low level of molecular condensation in
the absence of a relationship with the upper
horizons of the soils. In samples from «old»
alas (№ 1-5), depending on the landscape
position, the accumulation of high-molecular
compounds from the top of the alas to its
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Based on the obtained diagram (Figure 3), the
H/Cmod and O/C integral indicator is relatively
low in most of the studied HAs, which indicates
a low content of oxygen-containing fragments
in the HAs and a relatively low migration ability.
As already mentioned above, a high value of
H/Cmod and O/C is noted in the sample from the
IC and indicates that the sample was formed in a
in order to identify patterns of their formation is
based on the method outlined by Kleinhempel
(1970) and van Krevelen (1950). The method is
based on constructing H/Cmod and O/C diagrams
and serves as a technique to demonstrate the
contribution of oxidation and condensation to
changes in the elemental composition of HAs
(Lodygin et al. 2014; Polyakov et al. 2019a).
and the proportion of aliphatic compounds
grows. Freezing/thawing processes lead to
the evolutionary selection of macromolecules
of HAs, which is characteristic of the tundra
and boreal zones (Abakumov et al. 2015;
Beznosikov and Lodygin 2010; Lupachev et al.
2017; Polyakov et al. 2019b).
3.2. Characterization of HAs by 13C–NMR
spectroscopy
Numerous molecular fragments were identified
by CP/MAS 13C–NMR spectroscopy (Table 4):
carboxyl (–СOOR), carbonyl (–C=O); CH3–,
environment with a high level of hydromorphism
with a low level of microbiological activity,
thereby contributing to better conservation of
carbohydrate and amino acid HAs fragments. A
decrease in H/Cmod indicates the accumulation
of aromatic fragments in the composition of soil
soils (Pengerud et al. 2017; Strebel et al. 2010).
The obtained data correspond to the previously
published work by a numerous of scientists.
The Arctic environment is characterized by low
microbiological activity and the composition
of the precursors of humification. The most
characteristic distribution is from west to
east of the country as the H/C ratio increases
[ POLYAKOV V. & ABAKUMOV E. ]
Figure 3. Elemental composition of the studied HAs isolated from study soil. H/Cmod – the number of substituted hydrogen
atoms in the HA. Sample numbers correspond to Table 2.
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CH2–, CH–aliphatic, –C–OR alcohols, esters
and carbohydrates, phenolic (Ar–OH), quinone
(Ar = O) and aromatic (Ar–) groups, which
indicates the great complexity of the structure
of HAs and the polyfunctional properties that
cause their active participation in soil processes
(Lodygin et al. 2014; Yao et al. 2019).
Table 4. Chemical shifts of atoms of the 13C molecular fragments of HAs
Chemical shift. ppm The type of molecular fragments
0-46 C, H-substituted aliphatic fragments
46-60 Methoxy and O, N-substituted aliphatic fragments
60-110 Aliphatic fragments doubly substituted by heteroatoms (including carbohydrate)
and methine carbon of ethers and esters
110-160 C, H-substituted aromatic fragments; O, N-substituted aromatic fragments
160-185 Carboxyl groups, esters, amides and their derivatives
185-200 Quinone groups; Groups of aldehydes and ketones
Six chemical groups in HAs were identified
according to the 13C–NMR spectroscopy
method. Signals from non-polar alkyls
(0-46 ppm), N–alkyl/methoxyl (46-60 ppm),
O-alkyl and anomerics (60-110 ppm), aromatics
(110-160 ppm), carboxyl, esters, amides
(160-185 ppm) and quinone (185-200 ppm).
The obtained spectra are presented in Figure 4.
According to the obtained data, we can identify
three main groups of fragments that accumulate
in the delta soils, these are C,H–alkyl
((CH2)n/CH/C and CH3), aromatic compounds
(С–С/C–H, C–O) and OCH group (OCH/OCq).
The aromatic group is calculated from the sum of
the shifts of 110-185 ppm. Aliphatic fragments are
calculated from the sum of the shifts of 0-110 ppm,
180-200 ppm, AL h,r + AR h,r (total number of
unoxidized carbon atoms) – the signals were
summed over the regions 0-46 and 110-160 ppm,
C,H–AL/O,N–AL. Signals from C, H–alkyls were
summed in the range 0-47 ppm. O, N-alkyl at
regions 46-60 and 60-110 ppm. The presence
of all peaks of the carbon species which
are required for identification of the studied
substances as HAs has been revealed (Yao et
al. 2019). Data of chemical shifts in the studied
soil are presented in Table 5.
Aliphatic fragments of HAs (53-59%) dominate
in the studied soils, which indicates the dominant
mineralization process of organic matter in the
soils of the Lena River Delta. The predominance
of aliphatic fragments indicates the scarcity of
vascular plant remnants or the low maturation of
humic substances in the terrestrial environment.
The predominance of aliphatic structures is
typical of humic substances formed under
reducing conditions, including aquatic humic
substances. The microbial and algal biomass
consists of protein and membrane lipids,
and sometimes carbohydrate. An aliphatic
enhancement in humic substances often occurs
when there is contribution of microbial biomass.
At the same time, relative to other polar sectors
of the Arctic (the Yamal Peninsula, a number of
Russian northern islands in the Barents, Kara
Seas and the Svalbard archipelago), a significant
amount of aromatic fragments accumulate in
soils (41-45%). This ratio is closer to the soils of
the taiga zone (Abakumov et al. 2019; Lodygin
et al. 2014; Polyakov et al. 2019b; Strebel et
al. 2010). We previously studied a number of
samples from the Lena Delta in recent works,
where we noted that a significant amount of
aromatic fragments accumulate in the delta in
the alluvial soils of the first terrace of the river
(annually flooded), as well as samples from the
island of Kurungnah (typical permafrost soils).
From the data obtained, it was seen that alluvial
soils are more enriched in aromatic fragments,
and the ratio of aliphatic to aromatic in some
samples was more than one (Polyakov et al.
2018; Polyakov et al. 2019b).
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[ POLYAKOV V. & ABAKUMOV E. ]
Figure 4. CP/MAS 13C–NMR spectra of HAs from study soils. Number corresponds toTable 2.
Table 5. Percentage of carbon in the main structural fragments of HAs from the studied surface soil
horizons (according to CP/MAS 13C–NMR data). Sample numbers correspond to Table 1; AR – aromatic
fraction; AL – aliphatic fraction; AL h,r + AR h,r % – hydrophobicity degree; C,H–AL/O,N–AL – the degree of
decomposition of organic matter
Sample Chemical shifts, % of total C-13 signal AR AL AR/AL AL h,r +
AR h,r, %
C,H –AL/
O,N – AL
0-46 46-60 60-110 110-160 160-185 185-200
1 28 8 21 27 14 2 41 59 0.69 76 0.97
2 26 8 20 30 13 3 43 57 0.75 76 0.93
3 24 8 21 30 14 3 44 56 0.79 75 0.83
4 25 7 22 30 13 3 43 57 0.75 77 0.86
5 25 7 22 29 13 4 42 58 0.72 76 0.86
6 25 7 20 31 14 3 45 55 0.80 77 0.96
7 24 8 24 28 14 2 42 58 0.72 76 0.75
8 21 9 22 30 15 3 45 55 0.82 73 0.68
9 22 7 21 33 14 3 47 53 0.89 76 0.79
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3.3. Characterization of HAs from «old» alas
(Kurungnach isl.)
In the «old» alas, depending on the position in
the landscape, 5 soil pits were made (Figure 5).
Depending on the height, different amounts
of aromatic and aliphatic HAs fragments
accumulate in soils. First of all, this is due to the
predominance of various types of vegetation;
mosses and lichens, as precursors of
humification, dominate on the windy top of alas
[ STABILIZATION OF ORGANIC MATERIAL FROM SOILS AND SOIL-LIKE BODIES IN THE LENA RIVER DELTA
(13C-NMR SPECTROSCOPY ANALYSIS) ]
Figure 5. Soil catena of “old” alas in Kurungnah isl. AR/AL – aromatic to aliphatic compounds ratio; H –height of alas; D –
distance from point 1 to point 5.
(№ 1); mosses and lichens are characterized by
the presence of aliphatic constituents and lead
to accumulation of aliphatic compounds in the
composition of HAs. Sample 3 is characterized
by a higher content of aromatic fragments due
to more favorable climatic parameters and the
absence of stagnant moisture, the thickness of
the active layer reaches 70 cm. This allows a
greater number of shrubs to develop on the slope
surface, and the southern exposure of the slope
is more warmed up during the summer season,
which allows the soil microbiota to transform the
soil organic matter for a longer season. Sample
№ 2 (the bottom of the slope) is characterized by
a slightly lower content of aromatic fragments.
Permafrost processes are more active here,
and together with a high permafrost table,
stagnic conditions and the mosses and lichens
prevailing in the vegetation cover, this leads to
accumulation of C,H–alkyl fragments in HAs.
According to the composition of HAs, it is most
similar to sample 1 and aliphatic fragments
predominate the humic substances. Samples №
4 and 5, developed under conditions of excessive
moisture and confined to the polygonal tundra,
with increasing stagnification in the soil, and
an increasing proportion of oxygen-containing
fragments according to spectroscopy.
In general, the obtained spectra for this locality
are characterized by a rather close position of
the chemical shifts and the composition of the
HAs, which indicates the homogeneity of the
precursors of humification.
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3.4. Characterization of HAs from «young» alas
«Young» alas landscapes and soils are about
200 years old and zonal cryoturbation processes
here are not as clearly traced as in previous
samples. About 200 years ago, there was a lake
that began to dry out due to soil and coastal
degradation. Today it represents a lowering
with a several sporadic pingos, on the tops
of which vegetation different from the typical
tundra vegetation. The vegetation cover plays
a rather important role in the formation of soil
and its reserves of organic matter; in «young»
alas, it is represented by perennial herbs. At
that time, when there was a lake at the place
of alas, a significant amount of organic residues
accumulated in its bottom sediments, which,
after drainage, began to actively interact with the
atmosphere and already modern soils began to
form in their place.
The SOM buried here turned out the most
humified (45% AR) among the studied samples,
as well as a high content of organic carbon.
Relative to the studied soils, the highest indicator
of the content of aromatic fragments (С–С/C–H,
C–O) is here, this also confirms the thesis that
selective degradation of condensed structures
and condensation of high molecular compounds
occur with depth. With an increase in the
proportion of aromatic compounds, soil organic
matter stabilizes, and so the soils buried here
will not be subjected to active microbiological
effects due to the complex structure of high
molecular compounds. The vegetation cover of
this site is characterized by a predominance of
vascular plants with a developed root system.
This results in an intensive input of lignin-
derived compounds into soils. Herbaceous
plants and their root system show accumulation
rates of approximately 30% lignin compounds.
These substances play an important role in the
formation of HAs. Lignin is crucial component for
formation of HAs macromolecules. Chemically,
it is a three-dimensional polymer with highly
branched molecule composed of phenol units
with strong intramolecular bonding (Mišurcová et
al. 2012). Lignin transformation ways in soils are
presented both by decaying of monomers and
by partial changes in the macromolecule. During
lignin transformation, a decrease in the group
of –OCH3 fragments occurs. Demethylation
is a characteristic elementary process of
humification, during the decomposition of
lignin the accumulation of the COOH group
occurs. Thus, an increase in the proportion of
vascular plants and lignin may indicate the
formation of macromolecules that are more
resistant to microbial decomposition indicates
the maturity of HAs. The vegetation cover of
young alas is a mixture of perennial grasses
with a predominance of cereals; annually, as a
result of the growing season, straw forms here,
which is enriched with nitrogen, carbohydrates,
easily and hardly hydrolysable polysaccharides
and lignin, during the transformation of which
high-molecular compounds are condensed
(Verkhoturova and Evstaf'ev 2016).
3.5. Characterization of HAs from Interuve
(Kharaulakh ridge)
The soils formed here are zonal variant of soil
formation and can be considered the baseline
for this region, this site is not under the active
influence of the river. Soils are formed here
under continuously windy conditions, therefore,
they are represented by low-developed profiles
and soil formation takes place on stony rock
including carbonates. Nevertheless, the organic
matter that is formed here has relatively high
levels of humification and up to 45% of aromatic
fragments accumulate here; the content of
aliphatic fragments is mainly in the C, H–alkyl
and OCH group. The high content of aromatic
fragments may be due to the lack of permafrost
table in the soil, since the soil cover is formed
directly on the rock, while the presence of many
cracks in the stones prevents the accumulation
of excess moisture here and the carbonates
available (from the parent rock). The precursors
component composition of humic substances
depends fundamentally on the type of vegetation
and the microbial biomass.
3.6. Characterization of HAs from soil-like body
of IC
Annually, due to the action of the river coastal
erosion and abrasion, the IC of the Lena River
Delta is destroyed and a huge amount of organo-
mineral substance enters the atmosphere
from the frozen state (Figure 6). During the
interaction of organomineral components with
[ POLYAKOV V. & ABAKUMOV E. ]
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the atmosphere, SOM is mineralized, and carbon
dioxide, water and mineral salts are released.
Due to the activation of microorganisms, an
enhanced cleavage (mineralization) of the
aliphatic structures occurs with an increase in
the degree of benzoidicity (Orlov 1990). We
have conducted an analysis of this organic
matter. According to radiocarbon analysis, the
age of these deposits is about 34,299 ± 500
years BP (Schirrmeister et al. 2003). During the
analysis, we found that this material was the
most humified among the samples we studied.
If we consider theories of the origins of the IC,
our data show that a reservoir could exist in
place of the IC in which the accumulation and
transformation of organic matter took place.
Frozen organic matter from the IC is the most
stable of all the samples we studied. It seems
that as a result of the accumulation of various
organic residues here and their long-term
transformation during the selective degradation
of condensed structures, condensation of high-
molecular compounds occurred, which led to an
increase in the aromaticity of HAs in the soil-like
bodies of the IC.
[ STABILIZATION OF ORGANIC MATERIAL FROM SOILS AND SOIL-LIKE BODIES IN THE LENA RIVER DELTA
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Figure 6. Structure of IC in the Lena River Delta.
3.7. Stabilization rate of organic matter from
study soils
In the study samples up to 47% of aromatic
compounds accumulate, which indicates the
stabilization of organic matter in the soils of
the Lena delta. However, aliphatic fragments
remain dominant and their accumulation is
associated with the predominant mineralization
process in soils. The decrease in the portion of
aromatic fragments is primarily associated with
low microbiological activity and precursors of
humification. This ratio AR/AL in study samples
leads to the accumulation of organic matter in
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the soil. As a result of studying the composition of
structural fragments of the studied soils, we can
conclude the contribution of plant communities
to the composition of HA. Thus, in sample № 6,
which is formed under vascular plants with about
30% lignin, an increase in signals is observed
in the interval of 110-160 ppm. Aromatic and
carboxyl fragments in the structure of HA are
formed around lignin transformations, which
leads to increasing of the stability of HAs to
biodegradation. The highest AR/AL ratio (0.89)
was observed in the sample from the IC (№ 9).
The aromatic fragments content is higher than in
the sample formed under vascular plants (№ 6)
AR/AL (0.8), this trend may be associated with
cryogenic and thawing/freezing processes. The
temperature amplitude in the Arctic environments
can reach 90 ºC. It was suggested that the
humid season allows the formation of soluble
precursors, and the dry season favors molecular
condensation. In areas that are formed under
conditions where the lignin content is minimal,
under a moss-lichen cover, a decrease in
aromatic fragments in the HA is noticeable due
to the low content of aromatic precursors in
the precursors of humification. Moreover, the
condensation of macromolecules is apparently
associated with climatic features in this region.
The Lena delta region is quite different from
the continental part of Siberia; it has a much
milder climate due to the proximity of the sea,
and in the summer, the warm Lena waters with
temperatures up to 18 °C also contribute to the
heating of air, and, accordingly, soils. Thus, the
influence of the river on this region is quite high,
which favors the development of soils and the
formation of HAs with a relatively high proportion
of aromatic and carboxyl groups, which are more
resistant to biodegradation, compared to other
Arctic regions. At the same time, evolutionary
selection of organic compounds takes place and
high-molecular organic compounds condense at
the permafrost table.
The following parameters were used to
standardize the quantitative characteristics
of HAs macromolecules: the ratio of carbon
of aromatic structures to aliphatic, degree of
decomposition of organic matter (C-alkyl/O-alkyl)
and integral indicator of hydrophobicity of HAs
(AL h, r + AR h, r) (Figure 7).
[ POLYAKOV V. & ABAKUMOV E. ]
Figure 7. The diagram of integrated indicators of the molecular composition of HAs. Samples correspond to Table 2; AL h,r + AR
h,r indicates the total number of unoxidized carbon atoms.
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Based on the data obtained, we can conclude
that aliphatic compounds ((CH2)n/CH/C and
CH3) and aromatic compounds (С–С/C–H, C–O)
accumulate in the soils investigated. The ratio
of aromatic to aliphatic fragments ranges from
0.69 for the sample on top of the «old» alas up
to 0.89 for the frozen organics from the IC. The
increased portions of aromatic is associated
with the local position of the soils in the relief,
the exposure of the slope, wind conditions,
altitude, hydromorphism of the territory and plant
composition. It should be noted the cryogenic
activities in soil under the conditions of the
Lena Delta, the parent material of which are
alluvial sands, lead to a lesser development of
cryogenic processes in soils. Under conditions
of good drainage and soil aeration, there is a
rapid heat exchange with the atmosphere, which
affects the level of soil microbiological activity
and thereby increases the rate of humification.
Thus, during relatively active microbiological
processes, a significant proportion of aromatic
fragments accumulate in the delta soils. Figure
7 shows that the most humified and hydrophobic
constituents/structures are those obtained
from the buried organic matter of «young»
alas. This is primarily due to the precursors of
humification, and thermodynamic evolutionary
selection of high-molecular compounds that
accumulate at the permafrost table. In general,
from the obtained diagram we can conclude
that the accumulation oxygen-containing
–OCH and –COOH fragments influences the
redistribution of organic acids along the profile,
which occurs together with the destruction of
the mineral part of the soil. An increase in the
proportion of aromatic fragments of HAs leads
to stabilization of the organic matter in the soils
of the Lena River Delta. The condensation of
macromolecular compounds, which includes
aromatic/unsaturated structures between
110–185 ppm, indicates an increase in the
degree of hydrophobicity of soil organic matter
and its low availability to soil microbiome
(Semenov et al. 2009).
We carried out a statistical analysis of the
principal components in the HAs of the studied
soils (Figure 8).
Figure 8. Representation of the samples of humic acids in the plane dened by the rst two axes obtained by Principal
Component Analysis (88% of the total variance explained).
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Based on statistical analysis, we can conclude
that there is a high correlation between the
studied samples associated with the aliphatic
fragments (С-alkyl PC1 (58.3%) and the
aromatic fragments PC2 (29.3%). Thus, we can
say that the formation processes are primarily
responsible for the accumulation of long
aliphatic chains present in lipids (fatty acids,
paraffins), which are the result of decomposition
of moss-lichen plant residues (Karmanov et al.
2015). The aromatic C component, including
the aromatic group of chemical compounds,
is associated with the transformation of lignin
from vascular plants. In general, n-alkyl and
carboxylic acids predominate among mosses,
which corresponds to the composition of HAs
isolated from the studied soils.
Our data are confirmed by previously
published materials from scientists working in
the Arctic sector and for permafrost-affected
soils (Abakumov et al. 2015; Beznosikov and
Lodygin 2010; Ejarque and Abakumov 2016).
The dominance of aliphatic HAs’ fragments
is associated with the specific composition
of the vegetation cover, soil microbiological
composition, and climatic conditions (Lupachev
et al. 2017; Polyakov and Abakumov 2020;
Szymański 2017). The taiga zones are more
similar in terms of HAs composition; here,
the aromaticity of the studied HAs of podzols
increases to 44% and the European Arctic of
Russia to 50% (Lodygin et al. 2014; Pengerud
et al. 2017). These regions combine the features
of the vegetation cover; the studied tundra and
taiga zone is characterized by the predominance
of moss-lichen vegetation, which is a source of
carbohydrates and various lipids. Thus, the
annual change in climatic parameters, cryogenic
processes, and the precursors of humification
determine the composition of HAs in the
study area. The predominance of moss-lichen
communities contributes to the formation of long
aliphatic chains in the HAs macromolecules.
The change of plant communities to vascular
plants and the alternation of humid and dry
seasons promotes the condensation of aromatic
and carboxylic fragments of HAs, which are
associated with the resistance of organic
material to biodegradation (Abakumov et al.
2015; Beznosikov and Lodygin 2010; Lodygin et
al. 2014).
4. Conclusions
The key role in the formation of HAs in the soil
is related to the processes of condensation and
the polymerization of compounds formed from
precursors of plant and microbial origin. Humic
acids are structurally dynamic macromolecules,
the condensation of which depends on external
and internal environmental factors that determine
their resistance to biodegradation. During the
temporal development of soils, the H/C ratio
decreases, which leads to the accumulation of
aromatic fragments in the composition of HAs.
An increase in the aromaticity of soil HAs lead to
a decrease in the rate of soil mineralization and
as a result, its destabilization (protection) from
microbiological decomposition.
Analysis of the molecular composition of HAs by
13C–NMR spectroscopy also shows that aromatic
fragments (up to 47%) are accumulated in HAs
of the Lena River Delta soils. The ratio AR/AL
varies in soils from 0.69 to 0.89, which indicates
the leading processes of mineralization in the soil.
The predominance of moss-lichen communities
consisting of lignin-lacking plants leads to the
accumulation of aliphatic fragments of HAs,
which are dominant in the Lena River Delta.
In comparison with the HAs that were formed
under the current processes of humification we
can conclude that the organic residues from the
IC are the most humified among the studied
samples, which may be the result of long-term
climatic changes and condensation of HAs
macromolecules in the IC. Plant communities
with a predominance of lignified vascular plants
are characterized by a higher content of aromatic
and carboxylic fragments in the composition
of HAs, which indicates the maturity of organic
matter in the Lena Delta region.
5. Acknowledgements
This work was supported by the Russian
Foundation for Basic Reasearch, project
No 19-05-50107.
[ POLYAKOV V. & ABAKUMOV E. ]
SJSS. SPANISH JOURNAL OF SOIL SCIENCE YEAR 2020 VOLUME 10 ISSUE 2
188
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[ STABILIZATION OF ORGANIC MATERIAL FROM SOILS AND SOIL-LIKE BODIES IN THE LENA RIVER DELTA
(13C-NMR SPECTROSCOPY ANALYSIS) ]
... The stability of various groups of organic compounds from soils, water bodies, and cryoconite has been investigated for a long time (Polyakov and Abakumov 2020a, Chukov et al. 2015, Lodygin et al. 2014, Lodygin and Beznosikov 2010, Dziadowiec et al. 1994, Orlov 1990, Hatcher et al. 1981. Studies have focused on stabilization of organic matter, but addressed also the assessment of the soil regime, the intensity of the soil forming factor, as well as the process of removal of pollutants from ecosystems (Baccolo et al. 2020, Amaro et al. 2015, Celis et al. 2014, Knoblauch et al. 2013. ...
... Organic matter contains a large set of various structural fragments that are formed under conditions of mineralization and humification. In the course of these processes, labile (readily hydrolyzable) and stable substances are formed (Polyakov and Abakumov 2020a, Lupachev et al. 2017, Semenov et al. 2009). These substances can be accumulated and formed in soil and water systems, as well as on glacial surfaces (Polyakov et al. 2019a, Szymański 2017, Szymański et al. 2015, Dziadowiec et al. 1994. ...
... Cryoconite holes, in addition to changing albedo and their contribution to the deglaciation process, have a threat from the accumulation of priority toxicants (Akilan et al. 2019, Amaro et al. 2015, Baccolo et al. 2020, Casey et al. 2017. Trace elements, radionuclides, and polyaromatic hydrocarbons are transported here with particles of BC (Polyakov et al. 2020a). Cryoconite holes are small ecosystems with a closed cycle of nutrients and a specific microbiome community (Bond et al. 2013, Christner et al. 2003, Singh et al. 2020. ...
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... The majority of studies on HAs have focused on the geochemical features of substances extracted from soils (Dziadowiec et al. 1994;Pengerud et al. 2017;Polyakov et al. 2019 a, b;Lodygin and Vasilevich 2020;Polyakov and Abakumov 2020). However, the results obtained for humic substances of lake sediments do not go beyond HAs extracted from lake sediments of permafrostaffected areas (Golebiowska et al. 1996;Belzile et al. 1997;Klavins and Apsite 1997;He et al. 2008). ...
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... (Orlov, 1985). The diagram of Kleinhempel (1970) was used to demonstrate the elemental composition of the studied HAs (Lodygin et al., 2014;Polyakov & Abakumov, 2020). Quantitative treatment was carried out for numerical integration by the areas of 13C-NMR spectra using the program MagicPlot. ...
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... In the samples from the island of Jipyries, negative dynamics were observed, with a decrease in aromatic compounds with depth [58]. This was apparently due to the low content of lignin-containing fragments in the precursors of humification [15,20]. ...
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... The highest nitrogen content was observed in the horizon in the upper CP-1 Oe horizon. The studied soils are characterized by a relatively high C content in the HA composition compared to the Arctic tundra soils, where the C content varies from 36 to 44% [31]. The increased C fraction in peat soils in the middle part of the profile may indicate more homogeneous conditions of humification of plant residues [32]. ...
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... On Samoylov isl., a lot of work was carried out related to the study of the soil cover (Kutzbach et al., 2004;Zubrzycki et al., 2013;Antcibor et al., 2014;Polyakov et al., 2019;Polyakov and Abakumov, 2020). Most of the work is represented by the study of soil organic matter (Zubrzycki et al., 2013), greenhouse gas emissions (Kutzbach et al., 2004), ...
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Permafrost regions account for about 22% of the exposed land area in the Northern Hemisphere (Obu et al., 2019). As one of physical characteristics in the cold environment, permafrost is sensitive to climate change. During the past decades, permafrost in high latitude and high-altitude regions shows obvious degradation, which is indicated by increasing ground temperature, deepening active layer, shrinking of permafrost area, and development of thermokarst features (Biskaborn et al., 2019). Permafrost is distributed beneath the earth’s surface. Permafrost can regulate the regional water cycle and ecology from several mechanisms. First, as a weak impermeable layer, permafrost can prevent water vertical infiltration and increase the surface soil water content. Second, the freeze-thaw cycles of the active layer can store excess water from summer rainfall as ice during winter, and the melting of this ice can supply soil water in the following summer. Third, ground ice melting can provide soil water for plant growth (Sugimoto et al., 2003). Permafrost regions also store a large amount of soil organic carbon, which is almost twice as the carbon currently contained in the atmosphere (Mishra et al., 2021). These carbon pools have been gradually accumulated and preserved during the past thousands of years due to the low-temperature limiting the microbial decomposition of organic matter. The permafrost degradation may remobilize these carbon pools by releasing greenhouse gases into the air. This process contributes one of the great uncertainties in the terrestrial carbon cycle feedback (Schuur et al., 2015). In addition, permafrost regions also store a large number of pollutants and heavy metals (e.g., mercury) which have been sequestrated for a long time. Permafrost degradation poses environmental risksand thawing permafrost may release these biological or chemical substances that can affect human health (Schuster et al., 2018; Miner et al., 2021). To address the issues on how permafrost environment is changing, to what extent the changing permafrost may affect the hydrology, ecology, carbon cycle, and pollutants, eleven multi-discipline studies are collected in this special topic on permafrost environment changes in a warming climate. Permafrost regions have been warming at two to three times the global average (Hu et al., 2021). Using the monthly air temperature reanalysis dataset from the Climate Research Unit (CRU, University of East Anglia), it was found that the air freezing index in the Mongolian Plateau decreased by 4.1 C d yr-1, and the air thawing index increased by 2.3 C d yr−1 during 1901–2019. The northern permafrost regions showed large variabilities in freezing and thawing index than the southern non-permafrost regions (Ma et al. ). Based on the meteorological station records from 1957 to 2019, the annual mean air temperature has increased by 0. 031–0.039°C yr−1 in the hinterland of the Qinghai-Tibet Plateau. The ground temperature within the active layer at 1 m depth increased at an average rate of 0.05°C yr−1 (Zhou et al.). Along with climate warming, frequency of extreme events also changed. On the Qinghai-Tibet Plateau, the warmth indices such as warm days, warm nights, summer days, and tropical nights increased at rates of 1.1, 1.6, 1.4 and 0.3 days per decade from 1960 to 2016. Meanwhile, cold indices including the number of cool days, cool nights, ice days, and frost days decreased significantly (Gong et al.). These results confirmed the rapid warming of the permafrost environment during the past decades and also provide useful data to understand the changing patterns and future projections of permafrost. Three studies (i.e., Yang et al.; Rossi et al.; Polyakov et al. ) examined the detecting permafrost and soil mapping method in permafrost regions. The equivalent anti-flux opposing coils were used to eliminate the blind area for the transient electromagnetic method, and the results showed that this method solved the problem of the shallow detection blind area, eliminated the interference caused by the primary field, and improved the horizontal and vertical resolutions (Yang et al.). In the Russian Arctic, geophysical and geocryological methods including landscape microzonation, borehole drilling, ground temperature measurements, and geoelectric surveys were employed to investigate the active layer thickness. The results showed that the multidisciplinary approach can be also useful for other areas (Rossi et al.). In permafrost regions, soil type is one of the most fundamental properties because it is an important parameter for Earth System Models as well as the carbon stocks estimation. However, due to the harsh natural conditions, field investigation of soil types is usually costly and difficult. Using the unmanned aerial vehicle (UAV) imaging data in the Lena River Delta, classical soil sections, geomorphological observation, and determination of the main chemical parameters of soils are presented. Although accurate mapping of soil types should be based on chemical analysis, this result suggests that the highresolution soil-geomorphological maps based on the Geographic Information System and UAV data are useful for the mapping of soil types under the high variability of the watershed dan cryogenic landscapes (Polyakov et al.). Permafrost significantly affects ecology and hydrology (Woo et al., 2008). A review paper in this topic summarizes that soil water potential is widely used to describe the energy state of liquid water. The movement of liquid water in the soil is mainly determined by soil matric potential. The process of ice lenses development in permafrost has been explained by mathematical models, however, existing models might be too simplified (Fu et al.). Therefore, new model development for ice formation for micro landscapes is still largely needed. To investigate the effects of hydrology on peat permafrost and carbon process, a process-based model, i.e., HPM-Arctic, was used the simulate the past and future changes in a peatland ecosystem in the Canadian Arctic. The results showed that the regional hydrology and basin characteristics strongly determined peat accumulation history and its future changes in organic carbon stocks under different climate scenarios (Treat et al.). For the carbon cycle in the Arctic permafrost, a pilot study showed that extensively grazing by large animals can cool the ground temperature by modifying ground cover properties. In addition, the soil organic carbon content is also higher in the extensively grazing sites than that of non-grazing sites, which is likely attributed to the higher carbon input (Windirsch et al.). Heavy metals are anthropogenic contaminants that can be transported for long distances. Due to the atmospheric circulation and deposition, large heavy metals have been transported to the Arctic, Antarctic, as well as the Qinghai- Tibet Plateau. A review paper in this topic issue pointed out that heavy metals on the Qinghai-Tibet plateau are mainly from surrounding heavily-polluted regions. The shrinkage of the cryosphere may increase the release of these heavy metals in the future. This work highlights the importance of heavy metals in permafrost environments. This special topic has collected the studies of permafrost regions located in the Arctic, Mongolia, and the Qinghai- Tibet Plateau. The results deepen our understanding of changing trends of climate and permafrost, interactions among permafrost, hydrology, ecology, carbon cycle, and risks of heavy metals. We hope this special topic could provide valuable references to the researchers with relevant interest and play an active role in promoting the research of permafrost changes and their environmental impacts.
... On Samoylov isl., a lot of work was carried out related to the study of the soil cover (Kutzbach et al., 2004;Zubrzycki et al., 2013;Antcibor et al., 2014;Polyakov et al., 2019;Polyakov and Abakumov, 2020). Most of the work is represented by the study of soil organic matter (Zubrzycki et al., 2013), greenhouse gas emissions (Kutzbach et al., 2004), ...
... Stagnic processes develop in soils, which lead to the migration of iron and aluminum oxides in the soil profile and their accumulation on the permafrost table. This process takes place with the participation of organic acids (fulvic and humic), its formation occurs in the upper humic horizons under the influence of soil microbiota [37][38][39][40]. Cryosols formed in first terraces and third terraces are different from each other. ...
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