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Review

Estimation of Carbon Stocks and Carbon Sequestration Rates in Abandoned Agricultural Soils of Northwest Russia

by
Vyacheslav Polyakov
and
Evgeny Abakumov
*
Department of Applied Ecology, Faculty of Biology, St. Petersburg State University, 16th Liniya V.O., 29, St. Petersburg 199178, Russia
*
Author to whom correspondence should be addressed.
Atmosphere 2023, 14(9), 1370; https://doi.org/10.3390/atmos14091370
Submission received: 24 July 2023 / Revised: 21 August 2023 / Accepted: 25 August 2023 / Published: 30 August 2023
(This article belongs to the Special Issue Greenhouse Gas Emission: Sources, Monitoring and Control)
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Abstract

:
The fallow agricultural soils of Northwest Russia represent an evolutionary model of the development of ecosystem components in time and space with multidirectional dynamics of agrogenic impact during the long history of agricultural land development. There has been both large-scale land development and uncontrolled conversion of arable lands to a fallow state along with their removal in recent times. All this has led to the formation of a chrono-series of different-age soils with varying degrees of exposure of agrogenic factors. This paper presents a current review of the humus state of fallow soils in Northwest Russia, and examines the main factors (self-restoration, humus transformation, acidification) influencing the transformation of the soil cover under the process of post-agrogenesis. Effective farming techniques aimed at fixing carbon in soils as part of increasing the sequestration potential to mitigate the impact of climate change are considered. The ongoing process of the transition of lands into a fallow state could lead to organic carbon losses and changes in the main physical and chemical parameters, which negatively affects the self-restoration of fallow lands. We offer some recommendations for the effective rewetting of fallow lands in Northwest Russia with the purpose of carbon sequestration in the soil cover.

1. Introduction

Worldwide, agricultural land conservation is the leading problem in the global fight against hunger [1,2]. Decisive steps related to conservation, involvement of natural lands in crop rotation, and reclamation of existing crop rotations, as well as observation of the water balance of these territories are underway. According to the FAO, the area of agricultural land is about 4.8 billion hectares (37% of world land area); within agricultural land, the area of arable land is 1.6 billion hectares (12% of world land area), and 3.2 billion hectares (25% of world land area) are grasslands and pastures [3]. Over the past 30 years, agricultural land has declined by 1%, just as agricultural land per capita has declined [3]. Today, there are 0.6 hectares of farmland per capita, 30% less than in 1990. The leaders in the rate of agricultural land involvement are Indonesia (+60% over the past 30 years), Nigeria (+26%), Argentina (+22%), and Brazil (+13%), with the United States, which led the way in arable land, seeing a 15% decline over the past 30 years. The rapid growth of urban areas is displacing all agricultural land uses in developed countries [1].
According to a United Nations report, the global population will increase to 9.7 billion by 2050, which will create significant pressure on the agro-industrial complex [4]. Modern agriculture is a complexity of interconnections between natural ecosystems and human society. Hence, the environmental and economic problems that arise from the current state of soil and vegetation cover include logistical operations and economic factors. The increasing population may require more agricultural land, which could have a negative impact on climate change for the planet [5]. Expected global warming according to estimates of experts from various countries could lead to an increase of 2.7 °C by the end of the century, affecting 80% of the land and 85% of the world’s population [5,6]. Every year, anthropogenic emissions from agriculture release the equivalent of up to 17 billion tons of CO2 into the atmosphere, which is about 31% of all anthropogenic greenhouse gas emissions into the atmosphere; combined with problems caused by chemical treatment, agriculture has a significant impact on the pollution of soil cover and water resources, as well as the planet’s climate [5].
Currently, according to official sources (State Report “On the State and Use of Land in the Russian Federation in 2021” [7]), about 30 to 40 million hectares of arable land in Russia is excluded from circulation and not used, which is transferred into fallow land and transformed under the influence of natural and man-made processes of soil formation, forest overgrowth, sodding, grassing, waterlogging, etc. [8,9]. According to Rosreestr (State National Report “On the State and Use of Land in the Russian Federation in 2021”) [7], as of 1 January 2022, the total area of agricultural land in Russia was 379.7 million ha—22.2% of the country’s total land fund (381.7 million ha as in 2020). An important issue is the inventory of the current state of fallow lands, virgin lands, and lands already used in agriculture. For example, in the Northwestern Federal District of Russia, which is one of the leaders in the proportion of unused agricultural land, up to 6367.4 thousand hectares—20.8%—were converted to fallow land [10].
This is due to both environmental and economic problems in agriculture, in terms of natural problems, land erosion, dehumification, spontaneous revegetation, and overstuffing and afforestation, which leads to a decrease in soil fertility [11]. The relevance of this study is the need for comprehensive monitoring of the soil cover of fallow lands, especially in the Northwest of Russia, which are currently in the process of degradation and a rapid decline of soil fertility. The territory of Northwest Russia (Leningrad, Pskov, and Novgorod region) is a region with direct and indirect dependence on the history of surface development during glacial and post-glacial times. Due to the relatively young geological age of the Russian plain, a variety of landforms and glacial deposits, poorly altered by weathering, have been preserved here, which determines the diversity of the soil cover, as well as the development of agriculture in the northern regions of Russia. An analysis of recent published data shows that there is a necessity to monitor agricultural land in the fallow state to analyze its fertility and pollution, as well as its possible contribution to climate change on the northern hemisphere and entire planet level. There is also a need for a comprehensive approach to analyzing the current state of fallow land in key areas of the country in the interest of localizing agriculture and ensuring food security. In general, the study of fallow lands and soils, their condition, and fertility is in high demand for the effective re-involvement of fallow lands in agricultural practices. Based on a review of the key recent publications, this article assesses the state of modern and fallow lands in Northwest Russia, their current humus state, and the rate of post-agrogenic evolution of agricultural lands for the effective re-involvement of fallow lands in modern agriculture.

2. Materials and Methods

The methodology consisted of work in the literature on the characterization of fallow lands in Northwest Russia, as well as our own field data. Own field data were collected between 2021 and 2023 during various expeditions.
The organic carbon was analyzed on a CHN analyzer (EA3028-HT EuroVector, Pravia PV, Italy). Humus stocks (volumetric concentration) within each horizon were determined by the formula:
H u m u s   s t o c k s   ( k g / m 2 ) = ρ × C o c × H × 0.1 × 1.72
where (kg/m2); ρ-horizon density; Coc—organic carbon content (%); 0.1—conversion factor; H—depth, m.
The humus calculation was carried out to compare the existing humus reserves with the data obtained from the literature.

2.1. Features of Soil Formation in the Territory of Northwest Russia

The territory of Northwest Russia is located within 55°17′ N and 27°30′ and 36°27′ E and two geomorphological provinces—the Baltic Shield and the Russian plain (Eastern European plain) (Figure 1). The modern surface of the Northwest is a stepped plain, which repeats the relief of the pre-Quaternary surface, but with a general flatness and low absolute altitudes, the territory is characterized by considerable diversity [12,13].

2.2. Soils on Massive Crystalline Rocks

The thin soils formed on massive crystalline debris are distributed in the north of the Leningrad region, and are located on the border of the Middle and Southern taiga, formed under the influence of coastal climate on granitic rocks of the Archean and Proterozoic [14,15]. Parent materials here are represented by granites, acidic moraine sediments, and banded clays. The soil cover is represented by typical and Entic Podzols, which is due to the acidic reaction, with pH values ranging from 3.6 to 5.7. The humus state of the background soils is presented in Table 1.
The soils are characterized by a relatively high content of coarse humus with a predominance of fractions of aggressive fulvic acids, which contributes to the migration of iron from the upper soil horizons to the lower ones with the formation of ferrous films on the surface of mineral grains [18]. The carbon content in hyperskeletic Entic Podzol decreases with depth, the high carbon content in the Oe horizon is due to the formation of forest litter.

2.3. Soils on Moraines

Soils on moraines (glacial tills) are most widespread in the northwestern part of the Russian Plain [19]. Due to the lithological diversity of moraines, the soils associated with them are very diverse, especially soils of local moraines, the properties of which are determined by the underlying sedimentary rocks [20]. The zonal soil type here is Podzol, but due to the presence of underlying carbonate rocks, carbonate soils are widespread, which are associated with the Izhorskaya Upland (an elevated section of the ancient plateau consisting of Ordovician limestones and dolomites in the south of St. Petersburg) [16,21]. Another area of distribution of carbonate soils is the southwestern coast of Ilmen lake, which is composed of Devonian limestones. The soils formed here are called Rendzinas or humus–carbonate soils [22]. Sod carbonate soils develop normally on the relatively low carbonate content of the parent material. The pH values of such soils varies from neutral to alkaline, and the value increases with depth, which indicates leaching and development of eluvial processes. Carbonate soils are characterized by a relatively high content of humus (up to 15%); it is explained by the relative stability of humus substances to biodegradation at high carbonate soils and the formation of organo-mineral complexes. With the pronounced processes of leaching and eluviation, the humus content decreases and ranges from 4.5 to 7% [23]. The carbonate content in the fine earth and skeletal fraction of soils of the Northwest Russian plain creates special conditions for the development of the soil formation process. In these soils, most actively develop processes of humus accumulation and carbonate leaching. With high natural fertility, soils lose significant humus content during plowing, so depending on the content of carbonate in the soil, the carbon content can decrease to 2.8–3.2%, indicating the need for the input of organic fertilizers.
In the zone of occurrence of bedrock, represented by blue Cambrian clay, the formation of clayey textured Gleysols and Retisols occurs; they are characterized by the development of gley processes, as well as a clear differentiation of water and physical properties, as the underlying clays have low filtration capacity, which leads to stagnation of water in the soil [24,25].
On the red-colored moraine, which includes Devonian sandy–silty sediments, Retisols with a red-colored middle part of the solum are widely spread. These soils are characterized by a relatively high level of fertility inherited from the iron-enriched red-colored sandstone [25].
One of the types of local moraines in the Northwest Russian Plain is a clay carbonate moraine with relativity low content of carbonates, underlain by limestone dolomites and Upper Devonian dolomitic marls. These soils are less carbonate than those formed in the south of St. Petersburg, the humus content in natural undisturbed soils is 5.58–6.86%, while in arable soils, the content decreases to 3% [14].
The humus state of soils on local moraines is presented in Table 2.
The soils formed on moraine rocks are quite different from soils formed on massive crystalline rocks; this is due to the presence of carbonates in the composition of parent rocks, which determines the pH reaction close to the neutral, relative stability of humus to biodegradation at a high buffer capacity.

2.4. Soils on Lake-Glacial Clays

These soils are widespread in the territory of the Russian Plain, are Retisols, and are characterized by the richness of nutrients, relatively homogeneous relief, and the absence of boulders; however, these soils require significant expenditures for their involvement in the agricultural complex, which is associated with a high density of compaction, as well as low water infiltration. The humus state of the soils is presented in Table 3.
The soils are characterized by an acidic reaction with a gradual increase in depth to reach a neutral state; the humus content is relatively high, up to 7% [14,16]. However, soils have a relatively low humus stock; this is due to the development of stagnic processes as well as high density.

2.5. Soils on Water-Glacial Sands

Soils formed on sands and sandy loams occupy about 35% of the area of Northwest Russia and are represented mainly by Entic Podzols. These soils are characterized by low-intensity soil formation, migration of haloxides, and accumulation of poorly decomposed organic residues. The reaction is acidic, where the pH value can fall below 4. Humus content is high (up to 89.7%) due to the formation of histic horizons, as well as coarse humus; but with depth, this indicator drops sharply to an average of 1%. The humus profile is thin, with the bulk of the organic residues concentrated in the upper 20 cm [27]. The humus state of the background soils is presented in Table 4.
Soils are characterized by a significant humus reserve related to the accumulation of organic residues on the soil surface and their slow transformation.

2.6. Histosols

These soils occupy up to 26% of the territory of Northwest Russia; this is due to high precipitation, low evaporation, the presence of soil groundwater table close to the soil surface, and high relative air humidity [14,16]. The soils are characterized by oligotrophic and eutrophic Histosols. The reaction is very acidic; the organic matter is represented by undecomposed organic residues. Most of the peat soils have high potential fertility due to high concentration of organic residues and are of great value as a source of organic fertilizers. The content and stocks of carbon in Histosols is presented in Table 5.
These soils have a significant humus stock, which is associated with the long-term formation of peat moss in hydromorphic conditions.
Thus, the lithological factor of soil formation plays a significant role in the formation of soil cover in Northwest Russia and determines the high soil diversity in Northern Russia. The relatively high level of fertility of these soils has been favorable for the formation of agriculture with a long history here, but at present, the area of fallow lands in this region is increasing every year. This is due to the development of erosion processes, dehumification, and overgrowing with shrubs and afforestation, which leads to a decrease in soil fertility. An important stage of involving both fallow and natural soils in agricultural turnover is an inventory of soil resources, their chemical and physical parameters, as well as peculiarities of local lithology.

3. Results and Discussion

3.1. Evolution of Post-Agrogenic Soils

The process of post-agrogenic evolution of fallow agricultural lands follows classical successional schemes in the direction of forming zonal types of ecosystems [30]. About 50% of all fallow lands in Russia are located in the taiga, in the zone of distribution of podzols, and about 91% of the lands are subject to self-restoration. The most effective method for studying the self-restoration of post-agrogenic soils is the chrono-sequential approach [9]. The process of self-restoration depends on the climatic parameters of the territory, the genesis of soils, and the history of land use, as well as the presence of wild seeds in the vicinity. Parallel to the self-restoration of zonal vegetation, a natural change in the morphogenetic characteristics of soils and their physical, chemical, and biological properties also occurs during post-agrogenic evolution [31] (Figure 2). It has been shown that removing arable soils from agricultural use leads not only to an increase in their microbiological (respiratory) activity [32,33], but also to changes in the structure of the microbial community [34].
In most cases, the evolution of post-agrogenic soils occurs according to the zonal type of soil formation, e.g., the process of podzolization is often activated on fallow sandy soils of the southern taiga; however, soils formed on local moraines underlain by Devonian and Cambrian sediments are able to preserve their agrochemical qualities for a longer period of time. This is due to the formation of organic mineral carbonate complexes, as well as conditions with relatively high acid–base buffer soil capacity, which contribute to the accumulation of organic carbon in the soil. Changes in the dynamics of nutrition, as a result of the cessation of fertilization, as well as acidification and podzol formation, significantly affect the soil biota [35]. Until now, the question on the rate of transformation of fallow lands remains debatable, since it depends not only on the time of transition of soils into fallow lands, but also on the local lithological conditions [36]. In addition to the transformation of the soil cover, succession of the vegetation takes place. In the first 3–5 years, there is a change from cereal crops to ruderal vegetation, which leads to the accumulation of plant residues in the soil, which contributes to the accumulation of humus. Within 20 years, the formation of meadow vegetation with solitary pines and shrubs takes place. The plant residues are actively humified, which contributes to the formation of a stable pool of carbon [21,37]. In 55 years, the formation of young pine forests occurs; at this time, the process of zonal podzol formation can develop in the soil on sandy sediments. On loamy and clayey sediments, the processes of leaching and eluviation are activated due to the large amount of precipitation [16]. After 100 years in a young pine forest, spruce appears in the vegetative undergrowth, soil acidification occurs, and the soil may lose morphological features of arable horizon. After 170 years, the scotch pine forest is replaced by spruce forest with dwarf shrubs and moss, processes of podzol formation, and the formation of ferrous nodules in the middle horizon are noted [28]. Such a picture is characteristic of the soils formed on water-glacial deposits. In soils forming on Devonian sandy and sandy loam sediments after 120 years practically not traced processes of podzol formation, the accumulation of humus under subsoil horizon is noted. The most active transformation processes occur mainly in the upper humus-accumulative horizons, due to the fact that in this horizon the bulk of the roots and the processes of humification and mineralization of organic residues are located [38]. During the first 20 years, there are significant physical and chemical changes, which are accompanied by a decrease in pH, and saturation of the soil with bases; only after 100 years, the soil’s physico-chemical characteristics become similar to natural podzols [9]. Studies conducted in Germany, the United States, Scotland, and Poland indicate a decrease in pH, and the content of exchangeable cations in the soil cover of post-agrogenic soils in the zone of distribution of podzols on sandy rocks [39].
As a result of self-restoration, there is a sharp decrease in the main biogenic forms of phosphorus and potassium in the soil in the first 50 years and corresponds to the formation of a young pine forest [21]. Together with the leaching of nutrients, the source of plant nutrition shifted from mineral horizons to humus-accumulative horizons and this process was accompanied by a change from meadow vegetation to conifers. Post-agrogenic soils are able to retain phosphorus for a long time, as is noted in the studies of Kalinina et al. [21]. Related to nitrogen, the same trend is observed in the first 50 years; there is a sharp decrease in soil nitrogen content, below the natural level, but after 170 years, the level of nitrogen becomes identical for background podzols. In carbonate soils, the process of degradation is more prolonged, it is associated with a high natural fertility of soils, relativity high acid–base buffer capacity, and the formation of stable organo-mineral complexes. However, as a result of water erosion, the upper humus horizon of these soils can be completely washed away. Therefore, in areas of the distribution of carbonate rocks on heterogeneous forms of relief, carbonate, leached, and podzolated soils are found. As a result of the prolonged erosion process, silt particles can be involved in the arable horizons that affects the water and air regime of soils; overcompaction occurs and gley processes can take place. Figure 3 shows soil profiles of natural, fallow, and arable soils in Leningrad and Novgorod regions.
Self-restoration is the main problem of degradation of post-agrogenic soils. The change in physico-chemical parameters, vegetation cover, erosion, and dehumification, and decrease in the content of biogenic elements leads to radical changes in post-agrogenic soils in the direction of zonal variants of soil formation.

3.2. Transformation of Soil Organic Carbon in Fallow Ecosystems

The problem of agricultural land decline and degradation of soil organic matter is a worldwide problem. During the 20th century, the decrease in agricultural land reached a huge scale, the area of land decreased by 2.2 million km2, and the largest amount of land (up to 706 thousand km2) was reduced in Russia [30]. The forest zone of the European territory of Russia is characterized by the conversion of arable land to pasture, as well as self-restoration to woodland; with such transformations of the landscape, there is an active deposition of soil organic matter [40,41,42]. The organic matter transformation rate (mineralization and humification) depends on multiple factors—quantity and quality of organic matter, soil type, particle size distribution, temperature, and water regime [32]. The conditions of organic matter transformation and accumulation in natural soils are relatively stable, whereas the conversion of an agroecosystem to a fallow state results in a significant transformation of vegetation cover and organic matter accumulation conditions until a new balance is reached in the ecosystem [43]. As a result of the change in vegetation cover, there is an increase in the amount of organic matter entering the soil, enrichment of deeper soil layers due to an increase in underground phytomass and increased activity of soil biota, and the formation of organo-mineral complexes that protect organic matter from microbial decomposition [44]. Thus, the rate of accumulation of organic carbon in the soil increases, depending on the bioclimatic zone, varying from 3.1 to 113.5 gC/m2 with an average rate of 33.2 gC/m2 [44]. However, as a result of fires, meadow vegetation, shrubs, and small trees can completely burn out, as well as litter and the upper part of the humus horizon, which leads to a decrease in organic matter content due to a deficit of plant residue input.
According to different estimates, from 64 to 870 Tg (1 Tg = 1012 g) of carbon accumulated due to self-restoration [45,46]. Carbon accumulation during posta-grogenesis is heterogeneous; arable lands accumulate mainly passive carbon pool, which is associated with the formation of stable organo-mineral complexes, while at the same time, during the conversion of soil to a fallow state and the change of plant communities, active accumulation of the active carbon pool occurs [47]. During the formation of the active carbon pool, poorly decomposed organic residues are accumulated, and aggregates with poorly decomposed organic residues enclosed in them are formed; such dynamics are characteristic of soils that have passed into a fallow state in the first few years [30]. In such a soil, fresh plant remnants are actively accumulated, which undergo an active process of mineralization, and only a small part of the carbon goes to a stable state. Then, with the change from meadow vegetation to woody vegetation, change of soil biota, water, and air regimes of soils, the process of humification is more active, and organo-mineral complexes are formed in which carbon is deposited. In the later stages of the transformation of fallow soils, most of the organic matter is stored in the passive carbon reservoir [30,48]. The passive reservoir includes decomposed organic residues as part of organo-mineral complexes, soil aggregates, and as part of pyrogenic carbon. The formation of active and passive pools is determined by the climatic parameters of the territory, the quality of organic residues, and the composition and activity of soil microorganisms [47,49]. Thus, during the self-restoration of coniferous forests and Podzols, we will observe the formation of the active carbon pool in the early stages of the transformation of fallow lands, while on Retisol with relatively high levels of natural fertility, the formation of the passive carbon pool will proceed at a more rapid rate [30]. The active pool is characterized by relatively rapid rates of organic matter transformation, while the passive pool is more resistant to biodegradation and can persist longer in the ecosystem [50,51]. During self-restoration, some portion of the active pool can transition to the passive pool; depending on environmental conditions, this takes from 1 to 20 years, depending on the composition of the rocks on which the soil develops [30,52]. Cultivation of the soil leads to the stabilization of carbon, through the formation of stable organo-mineral complexes.
In the initial stages of ecosystem self-restoration, the total amount of carbon in the soil increases due to the change of plant communities and the entry of large amounts of plant residues into the soil. According to Kalinina et al. [30], it has been noted that SOC stocks do not fully recover in 120 years. Soil texture, haying, and the initial carbon content of arable soils make the greatest contribution to carbon sequestration and the formation of different pools of SOCs (Figure 4).
As a result of anthropogenic transformation of natural soils in arable lands, there is a significant decrease in the carbon stock, it is associated with a decrease in the input of organic residues into the soil, as well as a relatively high level of transformation of plant residues. Thus, in the transition of arable land to a fallow state, in the conditions of self-restoration there is an increase in carbon stock, degradation of arable horizons, and transition to background soil. Perhaps, when carbon stocks comparable to forest background values are reached, there is no need to return such soils to an arable state. The first priority should be to return lands that have recently been converted to a fallow state and require less expenditure on their restoration [53].
Moving from the north to the south of Russia, it is notable that the taiga zone is characterized by a relatively rapid accumulation of the active carbon pool, which is the result of the formation of forest cover, as well as increased activity of earthworms, which participate in the formation of granular soil structure [30,54,55]. Moving southward, the proportion of the passive carbon pool increases, which is associated with the formation of deciduous forest vegetation. The same is typical for Chernozems, here, under the condition of the active formation of soil aggregates, a slight increase in the passive pool of carbon associated with silt particles is noted. According to Semenov [55], the formation of the active pool of carbon is maximum in the tundra zone (7–557 mgC/100 g), taiga, Chernozems are characterized by accumulation from 75 to 150 mgS/100 g, relatively low (35–75 mgC/100 g) characterized alluvial soils, and the lowest carbon accumulation rate (less than 35 mgC/100 g) corresponds to saline soils. Arable soils contain less active pool carbon compared to natural soils, depending on the bioclimatic zone from 1.2 to 2.4 times, which is associated with the absence of natural vegetation cover, as well as low organic matter input to the soil [51].

3.3. Carbon Sequestration by Cropland and Fallow Land

Today, the burning of fossil fuels, deforestation, and agricultural intensification are the main sources of carbon dioxide in the atmosphere. Since the Industrial Revolution, the concentration of carbon dioxide in the atmosphere has gradually increased, and today it is increasing at a rate of 3.3 Pg C/year [56]. With the high population growth on the planet and the increase in agricultural land, arable land contributes significantly to the planet’s climate. Cropland occupies about 37% of the land and is a major source of agricultural carbon dioxide [57]. This is due to afforestation, drainage of wetlands, and plowing. Economic activities have reduced soil organic matter content by 3% since the 1990s [58]. The greatest effects of agriculture are found in Brazil, India, and Australia due to extensive farming. However, arable soils can potentially be a carbon sink for conservation tillage practices [59]. In Europe, for example, from 1993 to 1997, 519 thousand hectares were afforested for agriculture [60]. As a result of deforestation and agriculture aimed at fixing carbon in soils in European countries, the organic matter content increases, but the above-ground biomass carbon content decreases [61]. Forest biomass contains, on average, 2–4 times more carbon per unit area than cropland, but a closer look at forest and cropland shows that cropland has an important role in climate mitigation. However, this depends on climatic parameters as well as the type of biome on which crops are cultivated. Thus, carbon transformation is more active in warm and humid conditions, while in cold and humid conditions, this process is slower.
When peatlands, which in the undisturbed state are a carbon sink (on average 0.1–0.3 t C ha−1 year−1), are plowed, they become a source of carbon (2.2–5.4 t C ha−1 year−1), this is due to the loss of carbon from deep plowing and intense mechanical impacts, but the CH4 emission almost completely disappears, the average emission in meadows under peatlands is 3.3–6.5 t C ha−1 year−1; in croplands under peatlands, the highest emission is 3.8–9.5 t C ha−1 year−1 [61]. Deep plowing negatively affects the carbon sequestration of peat soils and can have a significant impact on climate change. Different farming practices have different effects on soil carbon sequestration, with the highest level of atmospheric carbon sequestration occurring in conservation agriculture in the tropics (0.3–0.8 t C ha−1 year−1), as well as improved crop production and erosion control (0.05–0.76 t C ha−1 year−1) on the global average [62]. Analysis of a no-till system in Canada showed relatively low rates of carbon sequestration (0.16 t C ha−1 year−1); this is primarily due to the cold type of climate; the highest rates of sequestration are observed in humid and warm conditions [63]. Even with conservation agriculture techniques in drylands, the sequestration rate decreased significantly to 0.15–0.3 t C ha−1 year−1 [62]. For the boreal zone, the involvement of land in agriculture in the initial stages leads to increased carbon emissions from the soil, as it is often associated with the fact that in cold conditions, it takes more time to develop the soil carbon pool than in the tropical zone [61]. Taking into account local lithological conditions, in loamy and clay soil, compared with sandy and sandy loam soils, carbon accumulates much faster, due to the formation of stable organo-mineral complexes [30,55]. For European countries, the most suitable method of farming is organic farming using animal manure, which allows to increase the rate of sequestration up to 4.6 t C ha−1 year−1. Conversion of arable land to forestry has a positive effect on sequestration rates; with this approach, soils can sequester up to 0.6 t C ha−1 year−1; the greatest effect of agricultural land conversion is observed when pastures are formed in their place (1.2–1.4 t C ha−1 year−1). Self-restoration of fallow lands in Northwest Russia will inevitably be connected with the loss of carbon in the initial stages of the landscape transformation, which is associated with drainage of the territory, as well as with plowing. However, long-term farming using techniques aimed at increasing the carbon content of the soil will allow the accumulation of a significant amount of carbon, which is a necessary mechanism for climate mitigation on the planet.
Over the past 30 years, a significant amount of organic carbon has been accumulated as a result of post-agrogenic succession; according to various estimates more than 60.5 MtC has been accumulated, which can fully compensate the annual national carbon losses from forest fires [64]. Despite the active accumulation of carbon as a result of post-agrogenic succession, even after 120 years, SOCs are not restored to natural levels. The active growth of carbon sequestration rates is directly related to the change in vegetation cover as a result of succession and occurs during the first 7–8 years. The average rate of carbon sequestration by fallow lands in Russia is 70 gC/m2, which is equivalent to 35% of the sequestration recorded by forests in the European part of Russia. Revegetation of all fallow lands in the European part of Russia could result in the emission of more than 400 TgC. Thus, reclamation of all lands may result in the loss of natural carbon sinks. Therefore, it is necessary to consider the carbon sequestration potential of existing post-agrogenic systems and their future contribution to climate change mitigation [46].
Land conversion to a fallow state has a significant impact on carbon stocks; so in Russia, where the largest amount of land in the world has been converted to a fallow state (up to 706 thousand km2), on average about 0.46 t C/ha is lost annually. Change in land use status is considered the second largest cause of carbon dioxide emissions from cropland after fuel consumption [65]. International organizations such as the FAO and IPCC recommend the involvement of natural and fallow soils in agriculture, as the effective management of such areas will better realize the sequestration potential of the territory and mitigate the effects of climate change [66].
Conversion of fallow lands to modern agriculture is one of the biggest problems in the world; every year a significant amount of land is subjected to self-restoration, and dehumification, along with this change in physical and chemical parameters of soil, vegetation cover, erosion, soil microbiome, etc. For the renewal of agriculture in such territories, it is necessary to carry out a wide complex of works aimed at the introduction of biogenic elements and erosion control, as well as drainage.

4. Conclusions

The analysis of the recent literature data established the features of the distribution of soils and their humus state in Northwest Russia. The main processes affecting the transformation of soil organic carbon and its evolution as a result of the conversion of soils to a fallow state are revealed. The analysis of carbon sequestration during the conversion of soils to the fallow state and the main techniques aimed at fixing and increasing the carbon in the soil are presented. As a result of this study, the following conclusions have been made:
(1)
Fallow soils, according to the lithological composition of parent materials, are characterized by the development of a wide range of problems associated with degradation of the soil cover (sodding, dehumification, etc.), so it is necessary to develop specific actions aimed to reclaim fallow lands for the purpose of their effective reintegration into crop rotation.
(2)
As a result of the different age conversion of soils to the fallow state and different lithological composition of rocks, there are significant differences in the formation of soil carbon pools. Soils formed on sandy and sandy loamy limnoglacial sediments are characterized by the accumulation of active carbon pool, and during plowing are able to lose a significant amount of carbon content, while organic matter in soils formed on clays and carbonate parent materials is in a passive carbon pool as a part of organo-mineral complexes and are less prone to degradation processes, which leads to a low degree of transformation of organo-accumulative horizons.
(3)
The conversion of agricultural soils to the fallow state is accompanied by a change in vegetation cover, and there is a common change of communities in the direction of zonal species.
(4)
During soil conversion to the fallow state, the process of carbon sequestration is activated; this is due to self-restoration and the accumulation of biomass. However, during land restoration, which is accompanied by plowing, drainage, and deforestation of the territory, there is a loss of carbon from the ecosystem. Arable soils are capable of accumulating a significant amount of carbon, this is due to the formation of stable organo-mineral complexes, and depends on agricultural techniques.

Author Contributions

Conceptualization, V.P. and E.A.; methodology, V.P. and E.A.; validation, V.P. and E.A.; formal analysis, V.P. and E.A.; investigation, V.P. and E.A.; data curation, V.P. and E.A.; writing—original draft preparation, V.P. and E.A.; writing—review and editing, V.P. and E.A.; visualization, V.P. and E.A.; supervision, V.P. and E.A. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the Russian Scientific Foundation in accordance with agreement from 20 April 2023 No. 23-16-20003 and Saint Petersburg Scientific Foundation in accordance with agreement from 5 May 2023 No. 23-16-20003.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Acknowledgments

This research was partially supported by scientific equipment from the Scientific Park of Saint Petersburg State University, Chemical Analysis and Materials Research Centre and Environmental Safety Observatory.

Conflicts of Interest

The authors declare no conflict of interest.

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Atmosphere 14 01370 g001
Figure 1. The territory of Northwest Russia.
Figure 1. The territory of Northwest Russia.
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Figure 2. Background forest (birch pine forest with bilberry); fallow land, 120 years (alder–birch forest on fallow soils); fallow lands, 80 years (grassy pine forest underbrush); fallow land, 40 years (meadow with pines up to 6 m high). Leningrad region.
Figure 2. Background forest (birch pine forest with bilberry); fallow land, 120 years (alder–birch forest on fallow soils); fallow lands, 80 years (grassy pine forest underbrush); fallow land, 40 years (meadow with pines up to 6 m high). Leningrad region.
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Figure 3. (A)—background Retisol, (B)—fallow soil (Plaggic Retisol), 20 years, (C)—Anthrosol, Leningrad region. (D)—fallow soil (Plaggic Retisol), Novgorod region, (E)—fallow soil (Umbric Podzol), 30 years, Leningrad region, (F)—fallow soil (Umbric Podzol), 70 years, Leningrad region, (G)—background Podzol, (H)—fallow soil, (Plaggic Podzol), 120 years, (I)—fallow soil, (Plaggic Podzol) 40 years, Leningrad region.
Figure 3. (A)—background Retisol, (B)—fallow soil (Plaggic Retisol), 20 years, (C)—Anthrosol, Leningrad region. (D)—fallow soil (Plaggic Retisol), Novgorod region, (E)—fallow soil (Umbric Podzol), 30 years, Leningrad region, (F)—fallow soil (Umbric Podzol), 70 years, Leningrad region, (G)—background Podzol, (H)—fallow soil, (Plaggic Podzol), 120 years, (I)—fallow soil, (Plaggic Podzol) 40 years, Leningrad region.
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Figure 4. Carbon stores in background forest and different ages of fallow lands (based on Kalinina et al. [21]).
Figure 4. Carbon stores in background forest and different ages of fallow lands (based on Kalinina et al. [21]).
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Table 1. The humus state of background soil on massive crystalline rocks (based on Gagarina et al. [16] and Kuznetsova et al. [17]).
Table 1. The humus state of background soil on massive crystalline rocks (based on Gagarina et al. [16] and Kuznetsova et al. [17]).
Soil NameHorizonpHContent of Humus, %Stocks of Humus, kg × m−2
Entic PodzolOe4.487.3763.3
Ah4.610.90
Bs14.68.30
Bs25.52.41
B/C5.71.98
Entic PodzolOe4.371.8942.58
E4.41.54
B5.41.54
Table 2. The humus state of background soil on local moraines (based on Gagarina et al. [16]).
Table 2. The humus state of background soil on local moraines (based on Gagarina et al. [16]).
Soil NameHorizonpHContent of Humus, %Stocks of Humus, kg × m−2
Stagnic Retisol on moraine with Cambrian clay sedimentsOe5.816035.9
Ah5.555.58
E6.453.95
B/C6.501.7
Cg7.250.5
Retisol on moraine with Devonian sandy–silty sedimentsOe4.4058.3431.4
Ah4.035.30
E5.101.4
Bt5.500.35
Entic Retisol on moraine with Devonian sandy–silty sedimentsAh7.883.0512.3
B/C7.981.09
C7.700.36
RendzinAh3.604.4816.3
E4.302.6
Bt4.402
B/C4.000.45
Table 3. The humus state of background soil on lake-glacial clays (based on Gagarina et al. [16] and Aparin et al. [26]).
Table 3. The humus state of background soil on lake-glacial clays (based on Gagarina et al. [16] and Aparin et al. [26]).
Soil NameHorizonpHContent of Humus, %Stocks of Humus, kg × m−2
Typical RetisolAh4.76.2926.1
E4.31.71
Bt5.10.86
B/C7.10.53
C7.70.51
Stagnic RetisolAh4.47.6933.1
Bts4.21.55
B/Cs5.80.46
Cs7.10.31
Typical RetisolAY4.16.2018.5
E4.33.01
B4.20.48
B/C6.10.22
C6.60.22
Table 4. The humus state of background soil on water-glacial sands (based on Gagarina et al. [16] and Abakumov et al. [28]).
Table 4. The humus state of background soil on water-glacial sands (based on Gagarina et al. [16] and Abakumov et al. [28]).
Soil NameHorizonpHContent of Humus, %Stocks of Humus, kg × m−2
Histic PodzolHi4.489.754.7
He3.878.6
Ha3.914.63
E4.30.67
E/B4.40.67
Bs4.51.55
B/C5.50.43
Folic PodzolOe4.965.534.6
Oa4.64.17
E4.80.53
Bs5.83.4
B/C5.20.23
Folic Stagnic PodzolOe6.01223.168.1
E4.443.2
Bs15.351.6
Bs25.661.1
G5.990.9
Table 5. The humus state of background Histosol (based on Polyakov et al. [29]).
Table 5. The humus state of background Histosol (based on Polyakov et al. [29]).
Soil NameHorizonpHContent of Humus, %Stocks of Humus, kg × m−2
Fibric HistosolHi15.1666.05108.8
Hi25.2872.17
Hi34.779.72
He14.0575.33
He23.6669.64
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Polyakov, V.; Abakumov, E. Estimation of Carbon Stocks and Carbon Sequestration Rates in Abandoned Agricultural Soils of Northwest Russia. Atmosphere 2023, 14, 1370. https://doi.org/10.3390/atmos14091370

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Polyakov V, Abakumov E. Estimation of Carbon Stocks and Carbon Sequestration Rates in Abandoned Agricultural Soils of Northwest Russia. Atmosphere. 2023; 14(9):1370. https://doi.org/10.3390/atmos14091370

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Polyakov, Vyacheslav, and Evgeny Abakumov. 2023. "Estimation of Carbon Stocks and Carbon Sequestration Rates in Abandoned Agricultural Soils of Northwest Russia" Atmosphere 14, no. 9: 1370. https://doi.org/10.3390/atmos14091370

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