Elemental composition and structural features of humic acids from floodplain soils of the Selenga River delta (Transbaikalia, Russia)

Humic acids (HA) play a multifunctional role in the environment, controlling the biogeochemical carbon cycle, the cycle of macro- and microelements, providing nutrients and biostimulants for plant growth, are responsible for soil structure formation, and reduce the effect of toxic substances. Due to the structural heterogeneity of soil organic matter, it is very difficult to determine the composition and properties of humic acids Currently, NMR spectroscopy is the most effective analytical method for obtaining information on the chemical composition and structure of OM. Thus, the problem of studying the composition and structural features of humic acids isolated from the floodplain soils of the Selenga River delta, which are the youngest and most vulnerable areas of the land, is very relevant and practically not reflected in publications. The study was conducted in the delta of the Selenga River (the world's largest freshwater delta) located in the central zone of the southeastern coast of Lake Baikal. It is a foothill tectonic depression with an area of 1,120 km2 filled with deltaic, alluvial and dealluvial sediments. Its territory is located within the Kabansky district of the Republic of Buryatia and is characterized by a significant diversity of environmental conditions due to the complexity of the geomorphological structure and different degrees of hydromorphism. Extraction of HA preparations and their purification were performed by standard methods by extraction with 0.1 n sodium hydroxide solution after preliminary decalcification. Elemental analysis of isolated HA preparations was determined on automatic elemental analyzer "CHNS/O-2400 series II" PerkinElmer (USA). Nuclear magnetic resonance spectra 13C-NMR were taken on spectrometer "Avance 300 MHz" Brucker (Germany) with working frequency 100.53 MHz using solid-phase CP-MAS technique. The humus horizons of the floodplain soils of the Selenga (meadow-marsh, meadow, meadow-saline) according to the WRB classification - Fluvisols, were used as research objects. The average elemental composition of humic acids of the studied soils is rather similar. The carbon content varies in the range 37.4-43.8 mol %, hydrogen 32.3-35.0, oxygen 21.1-25.0, and nitrogen 2.2-2.6. The increase of carbon content and decrease of hydrogen amount in the studied HAs in the transition from Mollic Fluvisols (Salic) to Mollic Fluvisols is explained by the increase of condensation degree, which corresponds to the ecological conditions of soil formation and hydromorphic degree. In general, the HAs of Mollic Fluvisols are more enriched in aromatic fragments than those of Gleyic Fluvisols and Mollic Fluvisols (Salic). It is a more "mature" product of humification. Lower degree of aromaticity as expected in HA Mollic Fluvisols (Salic). This result is supported by the H/C values, which are obtained from elemental composition data and indicates a higher content of less decomposed hydrophilic structures. Studies have shown that general principles of soil structure and content of elements (С, H, O, and N) in soils do not change under the influence of bioclimatic conditions, groundwater regimes, and sediment composition. But when the soil is exposed to groundwater combined with salinization, the rate of OM transformation decreases and leads to an increase in the proportion of unoxidized aliphatic fragments. The article contains 2 Figures, 3 Table and 51 References. The Authors declare no conflict of interest.

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