13C-NMR Spectroscopy and Elemental Composition of Humic Acids of Brown Forest Soils and Sod-Brownzems of the Southern Vitim Plateau (Russia, Baikal Region)

This study shows that the structural features of humic acids reflect the specific characteristics of organic matter in permafrost soils of the southern Vitim Plateau. The region’s extracontinental climate determines the rate of decomposition, the depth of humification, and the chemical structure of humic acids. Brown forest soils (Haplic Cambisols) and sod-brownzems (Leptic Cambisols Skeletic) contain high amounts of organic carbon and total nitrogen in their upper horizons but differ in their vertical distribution. Brown forest soils are characterized by a sharp decrease in organic carbon content with depth and the presence of humus pockets enriched in carbon and exchangeable bases. Sod-brownzems contain more organic carbon with increase in acidity and base loss with depth. Both soil types retain satisfactory natural fertility. 13C nuclear magnetic resonance spectroscopy data reveal marked differences in the structural maturity of humic acids. Humic acids from the A horizons of brown forest soils contain an equilibrium combination of aliphatic and aromatic structures, a well-developed system of oxygen-containing groups, and moderate condensation, indicating an intermediate stage of humification. Humic acids from humus pockets are more aromatic and highly humified. They reflect an advanced stage of humification and possess high chemical stability. Humic acids from sod-brownzems also exhibit high aromaticity, which facilitates the formation of stable organomineral complexes. A comparison of the samples reveals a consistent increase in aromaticity, condensation, and stability from the A horizons of brown forest soils to the A horizons of sod-brownzems and further to humus pockets. This progression corresponds to an increase in humification and a decrease in the mobility and bioavailability of organic matter. These results confirm that the structural characteristics of humic acids are determined by soil type and formation conditions. Elemental composition revealed that humic acids from brown forest soils are characterized by the highest aromaticity and maturity, while humic acids from HA-brown forest soils-A have a less condensed structure. Humic acids from sod-brownzems occupy an intermediate position, combining high aromatization with a moderate degree of humification. Overall, the obtained elemental composition data are fully consistent with the results of 13C NMR spectroscopy, mutually confirming the identified structural features and the degree of transformation of soil organic matter.