Elemental Composition and Structural Features of Humic Acids from Floodplain Soils of the Selenga River Delta

Abstract

The study of the elemental composition and structure of humic acids of floodplain soils of the delta of the Selenga River (Western Transbaikalia) was carried out. The research area belongs to the delta meadowmarsh and foreststeppe region, which is an area where deposits of sandy and silty fractions are actively carried out from the entire basin. Here, soils and vegetation are natural biofilters. Depending on the bioclimatic conditions, groundwater regimes, and sediment composition, soils are formed that differ in the composition of humus and humic acids (HA). The objects of research were alluvial (meadow, meadowmarsh, meadow saline) soils of the Selenga delta, according to the classification WRB – Fluvisols. The analysis of 13C-NMR spectra of HA preparations of the studied floodplain soils made it possible to identify ranges of chemical shifts belonging to carbon atoms of various functional groups and molecular fragments. NMR spectroscopy data showed that humic acids of alluvial meadow soil are more enriched in aromatic fragments than humic acids of meadowmarsh and meadow saline soils. A slightly higher degree of aromaticity in alluvial meadow and meadowmarsh soil is associated with environmental conditions and the component composition of the precursors of humification. An increase in the proportion of aliphatic structures and a decrease in the proportion of aromatic fragments in the composition of preparations of meadow salt marsh soil make it possible to judge the simplification of the structure of humic acid. Soil moisture in combination with salinization significantly reduces the rate of transformation of soil organic matter and leads to an increase in the proportion of non-oxidized aliphatic fragments. The use of NMR spectroscopy in the study of soils has significantly expanded the understanding of its composition and structure, deepened the understanding of the mechanisms of humification and transformation of soils of various genesis.