Humic Substances of Cambisols of the Southern Vitim Plateau

Cambisols (brown soils, burozems) of the southern Vitim Plateau are characterized by wedge-shaped humus pockets formed by frost cracks. The dark brown humus material filling these pockets is distinguished by bright black stripes, highly contrasting with the surrounding soil profile in color and properties. The polygonal network formed by frost cracks penetrates down the profile up to half a meter. In the soil material of the humus pockets, the content of organic carbon and adsorbed bases is relatively uniformly distributed, whereas these values decrease sharply down the profile in the Cambisols. The elemental compositions of humic and fulvic acids in the soil of cryogenic pockets and in the humus horizon of Cambisols have been compared. The presence of humus in the pockets under conditions different from those in the real humus horizon leads to an increase in the degree of aromaticity, benzenoidity, and oxidation of its molecular structure. An increase of the carbonization of humic acids in the soil of humus pockets is associated with a decrease in nitrogen content. Fulvic acids of the humus pockets are enriched in carbon and carboxyl groups compared to fulvic acids from the humus horizon.