Humic substances of Cambisols of the southern Vitim Plateau

As a result of cryogenic processes in pockets, carbon sequestration occurs, the initially formed humus in the upper soil layers, mechanically entering the illuvial horizon, is deposited at depth, preserving information about its main components. Therefore, we consider it relevant to determine the scale of carbon conservation in humus pocket reservoirs. The aim of the work was to study the humus substances of brown soils in the south of the Vitim Plateau: functional groups, electronic absorption spectra and elemental composition of humic and fulvic acids. A feature of the morphological structure of podzolized brown soils in the south of the Vitim Plateau is the presence of frost cracks filled with humified soil material, where the humus content is significantly higher than in the enclosing layer, which contributes to the redistribution and accumulation of organic carbon in the soil profile. The dark brown humified material filling the pockets stands out with bright black stripes, creating a contrast with the surrounding soil profile in color and properties. The polygonal network formed by frost cracks penetrates down the profile to half a meter. In the soil material of the humus pockets, the content of organic carbon and absorbed bases is distributed relatively evenly, whereas in the soil layer of brown soils the indicators sharply decrease down the profile. A comparative analysis of the elemental composition of humic and fulvic acids in the soil material of cryogenic pockets and in the humus horizon of brown soils was carried out. The presence of humus in pockets under conditions different from those in the humus horizon itself leads to an increase in the degree of aromaticity, benzenoidity, and oxidation of its molecular structure. With an increase in the carbonization of humic acids in the soil material of humus pockets, a decrease in the amount of nitrogen is noted. Fulvic acids from the soil material of humus pockets are enriched with carbon and carboxyl groups compared to fulvic acids from the humus horizon. It was revealed that more mature carbonized molecules of humic acids with a high degree of benzenoidity and reduced nitrogen and hydrogen contents are formed in the soil material of pockets. It was determined that in the soil material of pockets, a high content of acidic functional groups and a large absorption capacity of humic acids determine the high reactivity and adsorption properties of soils. Different levels of biochemical activity in soils cause differences between the characteristics of fulvic acids of the humus horizon and pockets. The optical density and carbon content of fulvic acids from humus pockets of brown soils significantly exceed similar indicators for fulvic acids of the humus horizon. The increase in carbonization and carboxylation of fulvic acids of brown soils is due to the fact that humic acids of brown soils are less stable and more susceptible to decomposition. Thus, it can be concluded that the stability and preservation of the humus fund of soils in the cryolithozone of Transbaikalia is ensured by the accumulation and fixation of humus in the soil material of brown soil pockets. This occurs through the formation of more stable and low-mobility compounds that contribute to the improvement of the soil condition.