Soils play an important role in the biogeochemical carbon cycle; therefore, in connection with the problem of global
climate change, special attention is paid to the assessment of soil organic matter reserves. The level of accumulation
of organic matter in the soil is the result of the interaction of many factors that regulate the processes of its transformations, which determines its high spatial variability. The degree of soil moisture is one of the leading factors that
aff ect the reserves of organic carbon in them. The moisture factor is of particular importance when assessing carbon stocks in the soils of the forest zone, which accounts for 70% of the area of all semihydromorphic soils in Russia According to the literature data, the organic carbon reserves in semihydromorphic forest soils are several times higher than in automorphic ones. To refi ne regional estimates of organic carbon stocks in soils in the forest zone of Russia, information is needed on the variability of carbon stocks in forest soils and its dependence on moisture conditions. In this paper, we discuss the results of a comparative statistical analysis of the variability of organic carbon stocks in automorphic and semihydromorphic forest soils. To estimate the stocks of soil carbon and calculate the quantitative indicators of their variability, we used information from two sources: the database «Organic carbon of soils of forest ecosystems in the European territory
of Russia» and the soil-geographical database of Russia [4]. The information was collected into a single array, which
includes the characteristics of 289 soil profiles of forest ecosystems. Of these, 201 sections characterize automorphic
soils and 88 sections characterize semihydromorphic soils. The results of statistical analysis showed that organic
carbon stocks (including litter) in semihydromorphic soils are characterized by higher variability, as evidenced by a larger range of changes, interquartile range, and higher standard deviations. The organogenic horizon makes the greatest contribution to the total variability of carbon stocks in soils of forest ecosystems under increased moisture. For it, the values of the standard deviation in semihydromorphic soils are 7 times higher than in automorphic soils, while the diff erences for a meter thick soil are fourfold.

Soils are the largest terrestrial reservoir of organic carbon, so even small changes in soil carbon stocks can have significant eff ects on the atmosphere and climate. To select effective strategies to mitigate climate change, predictions of how soils will respond to future changes in climate and land use are needed. Achieving meaningful predictions requires a deep understanding of the highly complex, open, multicomponent soil organic matter system. One of the most effective methods for predicting the dynamics of soil organic matter is mathematical modeling. Process-oriented (physically based) models make it possible to present the basic concepts about the mechanisms that determine the behavior of this system in a mathematically formalized form and conduct a quantitative analysis. The uncertainty of the forecasts depends on the level of development of the theory explaining the dynamics of soil organic matter, the models representing it and their experimental support. This review examines the achievements of the last decade in modeling the role of microorganisms in the stabilization of soil organic matter, the concept of soil saturation with organic carbon, temperature control, as well as the development of reactive transport models describing the dynamics of organic carbon in the soil profile, and the representation of the dynamics of soil organic matter in global climate models. Unsolved problems associated with the high variability in the structure of new generation soil organic matter dynamics models are discussed.