Soil carbon stock in forest ecosystems of the Tom-Yaya interfluve (Western Siberia)

In the context of climate instability and anthropogenic pressure, terrestrial ecosystems play a crucial role in maintaining biosphere stability by accumulating significant carbon stocks and providing numerous vital ecosystem services. The lowland dark coniferous forests of Western Siberia, located in the subtaiga zone, exhibit an insular distribution pattern, typically persisting in remote or protected areas. These forests are predominantly in various stages of successional restoration, a consequence of the increasing degradation rate of coniferous forests. Therefore, when assessing the forest carbon balance, it is essential to account for the high spatiotemporal heterogeneity of forest ecosystems. Forest soils are an integral component of boreal landscapes, playing a central role in the biogeochemical carbon cycle and containing up to 40% of the planet’s total carbon stocks. However, insufficient attention has been given to analyzing this issue. Accordingly, this study aims to assess the quantitative indicators of variability in carbon content and its reserves in Luvic Retic Greyzemic Phaeozems (Loamic) of the dark coniferous forests of Western Siberia, located at the southern boundary of their distribution. District Forestry (Tomsk Oblast, Tomsk District, near the village of Arkashevo; 56.44° N, 85.26° E) (See Fig. 1). The area is located on the western slope of the Tom-Yaya interfluve and is a flat, gently dissected plain. The predominant parent material is loess-like loam underlain by clays. According to climate zoning, the Tom-Yaya interfluve is part of the Tom-Chulym climatic district. The climate is humid continental cyclonic, with moderately warm summers and snowy winters. The average temperature in July is 18.1°C, while in January it is -19.2°C. Precipitation is unevenly distributed throughout the year, ranging from 550 to 630 mm. The vegetation cover of the Tom-Yaya interfluve consists of a complex of plant communities that combine southern taiga and subtaiga. In the study area, 11 soil profiles were placed in different forest (See Table 1). The soils in the area are slightly frozen, which promotes high biological activity on the surface and rapid decomposition of leaf litter. Air-dried samples were analyzed for actual acidity, loss on ignition, total carbon content (CHNS-O, VELP Scientifica, Italy), and particle size distribution. Carbon stocks were calculated for the 0-20 cm, 0-50 cm, and 0-100 cm layers based on total carbon (TC) content and bulk density. The soil cover has common morphological features and soil profile structures. The humus profile is extended in the form of a series of transitional horizons. The thickness of the humus horizons ranges from 20-45 cm, depending on the sampling location. Based on particle size distribution, the soils are classified as middle loamy soils and, less often as silty clay (See Fig. 1). The silt fraction predominates in all soil profiles (42-74%), which is typical of soils formed on loess-like deposits. The pH of the soil in the upper horizons is slightly acidic. The pH values gradually increase to pH = 6-6.8 towards the illuvial horizons and parent material. In the presence of carbonates or signs of residual carbonate content (P3, P8, P9, P11), the pH shifts to alkaline, ranging from 7.1 to 8.2. The soils are characterized by a pronounced accumulation of organic matter in the humus-accumulative horizons. In the upper 10 cm, the TC content reaches 5-9%, while in the illuvial horizons, it drops to 0.3-1%. Compared to all other soils, the upper humus horizon in the Luvic Retic Stagnic Greyzemic Phaeozems (Loamic) under the tall-forb aspen forest has the lowest TC values (2.4-4.8%) (See Fig. 3). In turn, the highest carbon stocks are observed in soils under small-forb Siberian stone pine forest (127.87 ± 30.46 t/ha in a 30-cm layer) (See Fig. 4). The soil under the tall-forb aspen forest is characterized by the smallest reserves of total carbon (87.38 t/ha in a 30-cm layer), which is primarily due to the higher rate of biological turnover and increased mineralization of organic matter annually entering the soil surface. The heterogeneity of the species and age structure of the forest ecosystem stand, as well as the overall microclimatic differences in habitat conditions, are reflected in the activity of the humus-accumulative process, which in turn affects the variation in the content and soil carbon stocks. The study revealed the heterogeneity of carbon spatial distribution and established that its content depends on the composition of the plant community, which must be considered when monitoring carbon balance processes. The article contains 5 Figures, 1 Table, 48 References. The Authors declare no conflict of interest.

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