INTRODUCTION

Climate change and an increase in global temperature are mainly determined by anthropogenic emission; its decrease requires not only the limitation of greenhouse gas emissions, but also the use of active carbon sequestration strategy. Biological CO2 sequestration and fixation via photosynthetic absorption by terrestrial ecosystems, a significant component of the global carbon cycle, are regarded as the most efficient and reproducible method [51]. Part of plant organic carbon enters the soil in the form of exudate and with plant residues, which are then persistently accumulated in soil organic matter via its microbiota. The formation of stable soil organic carbon compounds depends on the type of vegetation, soil and landscape characteristics of localities, and methods of field management. In a broad sense, the type, productivity, and frequency of particular land uses influence the general balance of carbon losses/stocks in vegetation and soils [40, 46].

As is believed, the degraded ecosystems possess a higher potential of carbon sequestration and an increase in the organic matter stock [51]. The main land-use management approaches with the potential of increased carbon sequestration include no-tillage agriculture, agroforestry, reforestation, and rewetting of wetlands and peatlands. It is known that the restored forests have a high carbon sequestration capacity and are able to compensate for up to 68% of the global CO2 emissions [39]. The rate of carbon sequestration via reforestation depends on climate, soil type, and tree species. A higher intensity of carbon accumulation in biomass is observed in young tree stands, deciduous forests, and humid cold climate. Note that the dynamics of soil carbon pool during reforestation is ambiguous and insufficiently studied. On the other hand, a large-scale forest restoration can well compete with croplands, thereby decreasing the food security because the lands allotted for agriculture are limited and provide the material facility for human life [45]. A spatially oriented approach taking into account the local conditions is required to optimize the strategic management of carbon emissions and preserve the balance between different types of land use [41, 51].

The ecosystems associated with typical zonal soils of the Karelian mid-taiga have been studied to assess the specific regional features of different land use types [8, 9]. Here, we examine the land uses of azonal black soils of the Zaonezhye Peninsula associated with shungite deposits. The umbrella term shungite unites the rocks with a high content of total organic carbon (<10–98 wt %) also containing silica, aluminum oxide, and iron oxide [42, 49]. The region of shungite deposits is associated with the Paleoproterozoic Onega structure in the east of the Fennoscandian Shield. Shungites occur in the regionally metamorphosed deposits of the upper subsuite of the Zaonezhye suite. Up to 60–70% of the total thickness of the Zaonezhye suite is represented by magmatic rocks; however, sedimentary and volcanic-sedimentary rocks are also observed [16, 47]. Characteristic of shungite deposits is mineralogical and petrographic heterogeneity; they contain tufa, siltstone, limestone, and gabbrodolerite [25]. The large-scale carbon accumulation in the Zaonezhye suite is referred to as the Shun’ga Event and is described as a global and synchronous epoch (2.1–1.85 billion years ago) of a large-scale organic matter accumulation in several regions of the Earth [33, 48]. Shungites are a nongraphitizable irregular carbonaceous matter with a globular fullerene-like supramolecular structure and a low H/C ratio [44]. The best-known classification of shungites is based on the carbon content and comprises five classes of the rock [5]. As is accepted, the genesis of shungites is associated with the petrifaction of early Paleozoic oil with an age of approximately 2 billion years [32]. The biogeochemical and isotopic compositions of shungites confirm their biological origin [4, 31, 53].

Soils in the Zaonezhye area are formed on the eluvium and colluvium of shungite shale and shungite moraine. The soil-forming rocks are most diverse in both chemical compositions and texture and the content of shungite material considerably varies, determining a complicated and heterogeneous soil cover [17, 38]. The chemical composition of parent rocks of the Zaonezhye suite is characterized by high concentrations of some elements (As, U, Mo, V, and Ni) [32]. The Quaternary deposits inherit the elements from the parent rocks but the total level of their natural pollution is considerably lower. In general, the soils on shungite rocks differ from the typical zonal soils by an increased content of both macroelements important for plant nutrition and the rare-earth and toxic elements. The concentrations of some elements can exceed the corresponding clarke values [14, 21].

The local soils are long known as fertile; according to historical records, the agricultural use of Zaonezhye lands started before the 14th century and this is one of the main agricultural regions in Karelia [13]. According to some estimates, about 30% of the Peninsula was cultivated and used as croplands and hayfields during its maximum development, while the area of forest stands was considerably reduced [3, 12]. Until now, farming is active there. The goal of this work was to study the effects of different land uses on the properties and functions of the soils formed on Karelian shungite rock and their influence on the structure of total carbon stock.

OBJECTS AND METHODS

The work was performed in the Zaonezhye Peninsula near the Tolvuya village, Medvezh’egorsk district, Karelia. The site is situated in the southern lakeside agroclimatic region, which has the most favorable climatic conditions. The mean temperature of January there varies from –8 to –10°C and that of June is +16°C; the frostless season is 120–130 days; and the sum of effective temperatures over growing season amounts to approximately 1500°C at an annual precipitation of 650 mm [2]. The natural vegetation in the region belongs to the mid-taiga subzone of green moss coniferous forests. Characteristic of the Zaonezhye is a denudation tectonic ridge–mound landscape. As is mentioned above, the soil in a larger part of the Zaonezhye Peninsula is formed on the eluvium and colluvium of shungite shale and shungite moraine, which enhances development of specific dark-colored organo-accumulative soils (Umbrisols).

The key sites (Table 1) were selected on highland landscape elements with the soils of normal moistening state. The agricultural sites (arable land and hayfields) belong to the OAO Sovkhoz Tolvuiskii state farm and are under long-term agricultural use. The young forest stand grows on an abandoned unimproved pasture. The middle-aged forest is a reforested site of an abandoned plowland with well-preserved rows of piled stones. The control forest plot is situated on a bouldery upland with close hard rock. Although the control tree stand is not old, its productivity and quality (pine without any admixture of deciduous species) are typical of the zonal communities growing under similar topographic and ecological conditions. Some selected sites are located in the areas inappropriate for plowing because the overall region in the past was actively plowed [12], and it is continuing now. The examined region lacks arable fields (hayfields) abandoned rather recently and old-aged forest stands, therefore, it was impossible to select some land use types with more adequate soil and vegetation characteristics.

Table 1.   Characterization of examined sites
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In the key sites, botanical description was performed, age and stock of tree stands were measured, and the carbon stock in vegetation was calculated. The main ecosystem carbon stock was determined in five pools, namely, aboveground phytomass (tree stand and ground cover), underground phytomass, debris (standing and fallen deadwood), forest litter, and soil in the 0–100-cm layer. The stock of aboveground phytomass was determined by cutting technique and the stock of tree stand was calculated from the survey measurements according to departmental standard OST 56-69-83 and the wood density characteristics; then, the data on total phytomass were distributed between individual fractions. A coefficient of 0.5 was used for calculating the stock of phytomass into carbon stock for the wood fractions and needles and of 0.45, for leaves and grass.

Soils morphology was described and samples were taken (according to the horizons and from the upper mineral horizons in small pits) as well as the litter in six replicates. The bulk density (ρ) of soils and litters were determined by weighing; the content and stock of organic carbon (Corg) and the carbon of microbial biomass (Cmic) were assessed as well. The differentiation between the lithogenic carbon of shungites and pedogenic carbon is methodologically unresolved [15]; correspondingly, the total carbon in fine earth was determined by high temperature catalytic combustion in a TOC-L CPN analyzer and regarded as the total organic carbon. The carbon of microbial biomass was determined using the substrate-induced respiration, which was assessed according to the rate of initial maximum respiration of microorganisms after soil enrichment with glucose and incubation for 1.5–2 h at a temperature of 22°C. The change in CO2 concentration was recorded in a NDIR SenseAir gas analyzer.

Soil pH (pHKCl) was also measured by potentiometry; the content of total nitrogen (Ntot) using Kjeldahl digestion in a Buchi nitrogen analyzer; the content of available phosphorus (P2O5) with spectrophotometric endpoint in an UV-1800 Shimadzu spectrophotometer and of available potassium (K2O) with atomic emission endpoint in a AA-7000 Shimadzu atomic absorption spectrophotometer according to Kirsanov; total exchangeable bases (S) according to Kappen–Hillkowitz; microbial metabolic coefficient QR and C/N and Cmic/Corg ratios were calculated. See Dubrovina et al. [8] for the details of sampling, analysis of soils and vegetation, and statistical data processing. The soils on shungites are stony; correspondingly, the carbon stock in soil was calculated as

$$Q = {{{\text{C}}h\rho (100-s)} \mathord{\left/ {\vphantom {{{\text{C}}h\rho (100-s)} {100}}} \right. \kern-0em} {100}},$$

where Q is the carbon stock, t C/ha; C, carbon content, %; h, thickness of horizon, cm; ρ, bulk density, g/cm3; and s, content of stones, %.

RESULTS

Morphology of the soil profile. The soils in the examined sites have a sandy loam texture, medium and high amount of stones, and a shortened weakly differentiated profile (Fig. 1). Characteristic of the upper soil horizons is an subangular blocky structure with the prevalent color hues of 7.5YR 3/1–2/1 (dark brown and black) according to the Munsell color chart [50]. The soil of control forest site is diagnosed as gray-humus dark-profile soil (Epileptic Skeletic Umbrisol). Gray-humus horizon with an average thickness of 3.5 cm is directly under the forest litter; it is divided into two subhorizons according to density and is underlain by a hard rock at a depth of 48 cm. An analogous profile is characteristic of the site under middle-aged forest with insignificant signs of postagrogenic transformation in the lower part of the gray-humus horizon underlain by shungite eluvium. The soil there is classified as a postagrogenic gray-humus dark-profile soil (Skeletic Umbrisol). The forest litter under young tree stand is thin, approximately 1.5 cm, and the gray-humus horizon contains a large amount of stones at a depth of 14 cm, residing on the shungite shale fragments replaced by a hard rock at a depth of 40 cm. The soil of this site avoided any considerable cultivation; it is identified as dark-profile gray-humus lithozem (Skeletic Umbric Leptosol). On the contrary, the sites of arable land and hayfield were cultivated and stones were removed. The soils of these sites are referred to as dark-profile agrohumus soils. The soil of the arable site, Mollic Umbrisol (Aric), displays the best-developed profile with a sod cover of up to 5 cm in the upper part of the agrohumus horizon 25 cm thick. (In the year of obseration, goat’s rue seeds mixed with grass seeds were sown; the roots of the latter formed the sod.) Horizon P is divided into two subhorizons according to density, and is underlain by gray-humus horizon merging into shungite moraine. The soil under hayfield, Endoleptic Mollic Umbrisol (Hyperhumic), has a thick sod to 8 cm and agrohumus and gray-humus horizons with the shungite eluvium below, which is replaced by hard rock at a depth of 65 cm.

Fig. 1.
figure 1

Schematic structure of soil profiles: (1) arable land, (2) hayfield, (3) 17-year-old forest, (4) 70-year-old forest, and (5) 70-year-old forest (control).

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Agrophysical and agrochemical characteristics of soils. The examined soils have a low bulk density amounting to 0.65–1.19 g/cm3 in the upper horizons (Fig. 2, Table 2). The density does not change with depth in the control site and young forest stand, whereas it insignificantly increases to 1.14–1.24 g/cm3 in the arable sites and under middle-aged forest. The soil pH in the arable, young forest stand, and middle-aged forest sites is close to neutral, reaching 5.6–5.9. Characteristic of the soil under hayfield is neutral pH (pHKCl 6.3) and of the control forest site, strongly acidic pH (pH 3.7). The pH values in middle-aged and young forests decrease with depth to 4.6–5.1, whereas the pHKCl in the remaining sites is almost constant along the profile.

Fig. 2.
figure 2

Changes in soil properties along the profile (mean, n = 3): (1) arable land, (2) hayfield, (3) 17-year-old forest, (4) 70-year-old forest, and (5) 70-year-old forest (control).

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Table 2.   Characteristics of soil fertility and microbiological activity in the upper soil mineral horizons (n = 6)
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The distribution of organic carbon along the profiles is of a uniform accumulative type in all sites and differs only by the Corg content. The carbon contents in the upper horizons of arable site and the middle-aged and control forests are very close, amounting to 6.3–6.4%. A low Corg content (1.6%) is recorded in the-Leptosol under young forest stand versus the hayfield soil with its maximum carbon content (11.7%). A similar trend is observed for the total nitrogen: its content in the arable soil and the soil under middle-aged and young stands amounts to 0.43–0.45%. The lowest Ntot content is recorded in the soil under young forest stand (0.18%) and the highest, in hayfield (0.87%). The soil under 17-year-old forest displays a narrow C/N ratio (10) and the remaining sites, the close values of 16–17.

The content of available phosphorus varies in a wide range. The P2O5 content in arable land is very high, amounting to 1144–1978 mg/kg versus the soil under young and middle-aged forests (14.5 mg/kg) and control forest stand (34.3 mg/kg). The content of available potassium varies from 532 mg/kg in the hayfield to 84 mg/kg in the soil under control forest. The sum of exchangeable bases (S) is the maximum in arable soil and hayfield (35–46 cmolc/kg), somewhat decreases in the soil under young and middle-aged forests (16–26 cmolc/kg), and drops to its minimum in the soil under control forest (4 cmolc/kg).

Microbiological characteristics of soils. Carbon of the microbial biomass has a regressive accumulative distribution pattern in all sites except for the 17-year-old forest, where the Cmic content decreases more uniformly. The maximum Cmic content is recorded in the upper soil horizons under hayfield (245 mg C/kg) and middle-aged forest (237 mg C/kg). As for the arable soil and control site, the Cmic content is slightly lower (124–184 mg C/kg) and the minimum content is observed in the soil under young forest stand (84 mg C/kg). The share of Cmic in total Corg is minimum in the hayfield and control site (0.22–0.23%) and insignificantly increases in arable soil and middle-aged forest (0.32–0.40%) to reach its maximum under the young forest (0.56%). The values of microbial metabolic coefficient QR are the minimum in agricultural lands (0.12–0.17) and vary in the range of 0.34–0.41 in the forest sites (Table 2).

Analysis of carbon stock structure in different land use types. In the examined land use types, the carbon stock in a soil layer 0–100 cm deep considerably varies: from 16.5 t C/ha in the Leptosol under 17-year-old forest to 250.7 t C/ha in the soil of hayfield (Fig. 3a). In the forest sites, the soil Corg stock amounts to 89.8–103.2 t C/ha and reaches 159.4 t C/ha in arable land. The contribution of the upper 50 cm of soil in the sites under young and control forest stands, where the underlying hard rock is close to the surface, amounts to 100%. As for the remaining sites, the 0–50-cm soil layer contains 70 to 89% of carbon. The Corg stock in litter is rather small and increases from the middle-aged forest (4.4 t C/ha) through young forest (6.6 t C/ha) to the control (13.1 t C/ha). The carbon stock in the litter accounts for 40% of the total soil Corg stock in the young forest, for 5% in the middle-aged forest, and for 13% in the control site. The total Corg stock of the soil and litter reaches its maximum in the control site (116.3 t C/ha), somewhat decreases in middle-aged forest (94.2 t C/ha), and is the lowest in the young forest (23.1 t C/ha).

Fig. 3.
figure 3

Contributions of different layers (%) and total stock of (a) organic carbon and (b) microbial biomass carbon in soil (n = 3) and litter (n = 6); error bars denote the error of the mean.

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The Cmic stock in a soil layer of 0–100 cm amounts to approximately 43 g C/m2 in hayfield and middle-aged forest, slightly decreases in arable land (34.9 g C/m2) and control forest stand (20.7 g C/m2), and is the minimum (6 g C/m2) in 17-year-old forest (Fig. 3b). A large part of Cmic concentrates in the 0–50-cm soil layer (83–93%) and reaches 100% under young and control forest stands. The Cmic stock in the litter is the maximum in the control site (24.3 g C/m2) and is considerably lower under young (15.9 g C/m2) and middle-aged stands (10.8 g C/m2). The Cmic stock in litters exceeds that in soils 2.7-fold in 17-year-old forest and 1.2-fold in the control forest. The total Cmic stock in soil and litter are the maximum under middle-aged forest (53.9 g C/m2), somewhat lower in the control site (45.0 g C/m2), and the minimum under young forest (21.9 g C/m2).

The largest carbon stock in the phytomass of ground vegetation accumulates in hayfield and arable sites (8.3–9.5 t C/ha). As for the remaining types of examined land use, this value is low, amounting to 2.2 t C/ha in young forest and 0.3–0.9 t C/ha under middle-aged and control forest stands (Fig. 4a). The share of the carbon of underground phytomass is the largest in the ground vegetation of 17-year-old forest (91%) as well as hayfield and arable land (64%). As for the carbon of aboveground phytomass, its share is higher in the middle-aged and control forest stands, amounting to 56–67%. The carbon stock in wood phytomass of forest sites increases from 45.4 t C/ha in young forest to 87.2–92.7 t C/ha in middle-aged and control stands (Fig. 4b). The carbon stock in trees mainly increases at the expense of aboveground phytomass, the share of which amounts to 85–88%. The carbon stock in debris of the control site is insignificant, amounting to 1.1 t C/ha, and increases to 3.4–6.9 t C/ha in the sites of young and middle-aged forests. The share of carbon in the dead organic residues is approximately 1% of the total tree stand carbon in the control site and 8% in the remaining sites.

Fig. 4.
figure 4

Structure of the carbon stock in the phytomass of (a) ground vegetation and (b) trees.

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The total carbon stock of the ecosystems varies in a wide range from 74.1 t C/ha in young forest to 259 t C/ha in hayfield (Table 3). The carbon stock in the sequence of arable land–middle-aged forest–control amounts to 168.9–211 t C/ha. The share of soil carbon is 94–97% in cropland, 48–49% in middle-aged and control forest stands, and 22% in 17-year-old forest. As for the share of carbon in plant phytomass, it reaches 65% in young forest and amounts to 44–46% in the remaining sites. The underground phytomass accounts for 2–4% in croplands and 5–12% in forest ecosystems. The shares of the carbon of debris and litter in forest sites are rather small, amounting to 1–4 and 2–9% for debris and litter, respectively.

Table 3.   Total ecosystem carbon stock (t C/ha) and shares of each pool, %
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DISCUSSION

The soils on shungite rock are azonal and untypical of their natural and climatic regions. Shungite rock, sharply inconsistent with the geochemical conditions of podzol and podzolic soil zone, is a specific soil-forming factor. The morphology of examined soils considerably resembles that of mountainous soddy soils because of a characteristic shallow profile, high proportion of gravels, and hard rock occurring at a low depth [1]. Soil reclamation and removal of stones considerably contribute to the change in morphology of shungite soils. The sites situated near of large water bodies with the most fertile soils were the first to be farmed; the thickness of agrohumus P horizons there reaches 20–30 cm and they are mainly used as arable land and hayfields. The cultivation of highly skeletal soils was accompanied by the removal of stones, typically piled in rows, which is an indicator of agricultural development in this area and important element of the Zaonezhye cultural landscape [3]. These stone rows are present in the middle-aged forest, where old arable AYpa horizon can be rather conditionally identified. In both the middle-aged and control forest stands, gray-humus horizons reach a thickness of over 40 cm with two subhorizons separated by density and stone content. The Leptosol under young forest with a thin AY horizon (15 cm) underlain by a stony material and hard rock is the most morphologically “natural” soil.

The examined soils are well structured, and their profiles are weakly differentiated according to some characteristics. A low bulk density of the upper horizons insignificantly changing with depth is typical of the soils. All sites except for the control forest stand have a neutral or close to neutral pH, which almost does not change with depth. This specific feature allows the soils of hayfield and arable land to be diagnosed as Mollic Umbrisols according to the WRB classification [55], which is untypical of the acidic humid condition of Karelia. A strongly acidic pH of the control forest site caused by the coniferous litter without any deciduous species in it [23, 28]. Unlike the majority of zonal soils [8, 9], the distribution of carbon along the profile of shungite soils is uniformly accumulative. Note that farming enhances the accumulation of organic carbon in the upper part of the soil profile, especially in the soil of hayfield. The microbial carbon is concentrated in the upper soil horizons and follows a regressive accumulative distribution pattern, since its content sharply decreases below the depth of 30 cm in all types of land use except for 17-year-old forest stand. Characteristic of Leptosol is a low content of both Corg and Cmic in the profile, which is considerably lower as compared with the soils of the remaining sites. A wide range of the Corg content in the examined soils is explainable with a heterogeneous composition and diversity of soil-forming rocks. Different degree of interlayer ordering is also typical of shungites; consequently, they vary in their resistance to leaching [34]. This specific feature determines different accumulation rates of carbon and other chemical elements in shungite soils.

The contents of organic carbon and total nitrogen in the upper horizons of arable land, middle-aged forest, and control stand match the level of leached chernozems [10, 27]. The Corg and Ntot values in hayfield exceed the average content in chernozems, which is explainable with the initially higher carbon concentration in the rock and active humus-accumulative process determined by meadow vegetation. The studies in different climatic zones demonstrate that grassland ecosystems enhance the Corg accumulation in the upper part of soil profile [20, 37, 43]. Shungite rocks determine high values of exchangeable bases and potassium for different categories of land use as compared with the zonal soils [8, 9]; however, a low content of phosphorus is recorded in forest sites. Land farming enhances the accumulation of exchangeable bases, available potassium, and phosphorus (its content in agrocenoses is very high). In general, the control forest site has low pH and the values of main agrochemical characteristics except for Corg and Ntot. Characteristic of the remaining land use types are high functions of biomass production and nutrient cycling except for P2O5 in forests. Close values of C/N ratio, matching the agricultural lands on Al–Fe humus soils [9], are recorded in the examined soils of all sites except for the young forest stand, where the C/N ratio is even narrower. This specific feature suggests an equally high mineralization rate of organic matter in both agrocenoses and forest communities. In general, a change in land use has a less pronounced effect on the morphological, physical, and chemical properties of the soils on shungite rock as compared with the zonal soils [7, 24].

The Cmic content in the examined soils falls into the range characteristic of the zonal soils in Karelia [8, 9]. Note that the content of microbial biomass positively correlates with the content of soil organic carbon [54] and the Cmic content in the soils with a relatively high Corg content is considerably higher as compared with the soils on shungites [26, 57]. On the background of a high Corg content in soils, the share of microbial carbon in the total organic carbon is rather small, being the minimum in arable and control sites. Presumably, the low values of Cmic/Corg in shungite soils reflect poor availability of the substrate because of a lithogenic origin of carbon. A low Cmic content may also result from the specific features in rock and soil compositions of the Zaonezhye Peninsula, containing such elements, as Co, Ni, Cu, Zn, and Cr, at concentrations exceeding the corresponding TLVs [21, 30]. Heavy metals (HMs) are known to influence the soil microbiome and to decrease Cmic content [6, 52, 57]. However, the presence of HM in soils with a high Corg content can have no effect on the functional diversity of microorganisms unlike the Cmic content, which is a sensitive indicator [29]. It is known that the toxicity of HMs has a stronger negative effect on the population of fungi as compared with the other functional microbial groups, thereby decreasing the fungi to bacteria ratio [56]. This pattern was observed in shungite soils by Zagural’skaya and Morozova [11]. Note that shungite rocks also contain the rare earth ultratrace elements—lanthanides, with pronounced antibacterial properties [14, 18, 19]. Although the observed microbiological activity in the examined soils is relatively low, very low values of microbial metabolic coefficient QR were recorded there, especially in the arable land and hayfield, indicating the ecological wellbeing and stability of microbial cenoses. The totality of microbiological characteristics suggests that the transformation function of shungite soils to a higher degree depends on the endogenous properties of soil-forming rock rather than on the types of land use, as is characteristic of zonal soils. The structure of Corg and Cmic stocks is to a considerable degree determined by the specific features of soil genesis and full-scale reclamation (Fig. 3). In general, the 0–50-cm layer is an important contributor to the Corg and Cmic stocks, which accounts for 70–83 to 100% depending on the presence of underlying bedrock. The absence of cultivation, high content of stones, shallow bedrock, and insignificant soil Corg content explain a low carbon stock (16.5 t C/ha) in the Leptosol under young forest stand. On the contrary, the cultivated farming sites display the maximum Corg stock, amounting to 159.4–250.7 t C/ha, which is comparable to the soil Corg stock in the chernozem zone [22, 36]. A considerable carbon stock in hayfield despite a rather thin soil layer is determined by a relatively high Corg content in the rock and pronounced humus-accumulative process. The soil Corg stocks in middle-aged and control forest stands are rather close and amount to 89.8–103.2 t C/ha, which is by 20–30% higher as compared with podzolic soils and by 42–56% higher as compared with Al–Fe humus soils. However, the Cmic stock in shungite soils is small, being on the average two–threefold lower as compared with zonal soils [8, 9] because of a rather low content of microbial carbon and high content of stones.

On the contrary, a combination of low Corg stock with a high microbiological activity is observed in the forest litter. The Corg stock in forest litter is on the average 1.5–2-fold less as compared with the sites associated with zonal soils because of a high rate of organic matter mineralization in forest soils on shungites. The stock of microbial carbon in litters is comparable to the stock in the forests on podzolic and Al–Fe humus soils [8, 9]. The Cmic stock in the litters of young and control forest stands exceeds the Cmic stock in soils and significantly contributes to the stock of microbial carbon in the sites under forests.

The structure of the total ecosystem carbon stock is untypical of the taiga zone and is determined by a large stock of soil Corg. The carbon stock is the maximum in hayfield (259 t C/ha), which is by 50–70 t C/ha larger as compared with the middle-aged and control forest stands. The sites of these forests and arable land have comparable carbon stocks. The minimum carbon stock is recorded in 17-year-old forest stand (74.1 t C/ha), which is close to the carbon stock in hayfield sites with podzolic and Al–Fe humus soils. The share of soil carbon stock in arable land is traditionally high and amounts to 94–97%. As for middle-aged and control forests, the soil Corg accounts for 48–49% of the total carbon stock. This ratio is considerably larger as compared with the sites of analogous land uses with zonal soils and is characteristic of the ecosystems of broadleaved forests [35]. A linear dependence of the phytomass stock on the age of tree stand is observed in forests on shungite soils. The carbon stocks in vegetation, debris, and litter are somewhat lower as compared with the values recorded in zonal soils [8, 9] because the density of tree stands is low and the local population actively utilizes trees for economic needs. This decreases the competition intensity and falloff in the growing part of tree stand and determines the low stock of debris, especially in the control tree stand. In general, all sites except for the young forest on Leptosol display comparably high carbon sequestration function, which is provided mainly by soil in arable lands and equally by soil and aboveground phytomass in forest ecosystems.

CONCLUSIONS

The shungite rock of Southern Karelia enhances the development of azonal organo-accumulative soil, characteristic of which is a highly skeletal and weakly differentiated shallow profile. The transformation of shungite soils resulting from a change in land use type is less pronounced as compared with zonal soils. Cultivation and stone removal, which leads to formation of arable horizons and development of the humus-accumulative process, considerably contribute to the change in soil profile morphology. Characteristic of these soils are low bulk density and neutral pH of the upper horizons, which insignificantly change with depth. A heterogeneous composition of the soil-forming rock and different degrees of shungite resistance to leaching determine a wide range of the soil Corg content from an insignificant value in the Leptosol under young forest stand to the level of leached chernozems in other sites. Shungite rocks determine a relatively high content of exchangeable bases and potassium in soils; note that farming enhances the accumulation of these elements and phosphorus. The examined soils display close values and rather narrow range of C/N ratio irrespectively of the type of land use, suggesting a high rate of organic matter mineralization. Characteristic of these soils are high functions of biomass production and nutrient cycling (except for some characteristics, such as pH and P2O5 in mature forests). On the background of a high Corg content and low values of microbial metabolic coefficient QR, the share of soil microbial carbon and Cmic stock are insignificant. This specific feature is explainable with both a poor availability of the substrate determined by a lithogenic nature of Corg and elevated concentrations of HMs and lanthanides in soils. Correspondingly, the transformation function of soils to a greater degree depends on rock-inherited properties of shungite soil rather than on the types of land use. The structure of Corg and Cmic stocks is to a considerable degree determined by the specific features of their genesis and full-scale reclamation. In farmland sites, the carbon stock is the maximum and is comparable to the stock in the soils of the chernozem zone. A high content of stones, shallow hard rock, and insignificant carbon content are the factors that contributing to a decrease in carbon stock. The structure of the total ecosystem carbon stock is untypical of the taiga zone and varies in a wide range from 74.1 t C/ha in young forest stand to 259 t C/ha in hayfield. The carbon stock in arable land, young stand, and control forest falls into the range of 168.9–211 t C/ha. The share of soil Corg in the total carbon stock in middle-aged and control forest stands amounts to 48–49%, which is characteristic of the ecosystems of broadleaved forests. The share of soil carbon stock in farmland and hayfield sites is traditionally high and amounts to 94–97%. The function of carbon sequestration is provided mainly by soil in farmlands and equally by soil and aboveground phytomass pools in forest ecosystems.