Molecular Structure of Humic Acids from Chestnut Soils of Transbaikalia

Abstract

The bioclimatic conditions of soil formation in the cryoarid climate zone of Western Transbaikalia determine the specific composition of humic substances, but the heterogeneity of these compounds, as well as the widely used classical methods of organic matter analysis, does not allow us to judge with sufficient confidence the structure of high-molecular organic compounds in soils. ¹³C-NMR spectroscopy of humic acid preparations from chestnut soils made it possible to identify the ranges of chemical shifts belonging to carbon atoms of various functional groups and molecular fragments. The peculiarity of the humus formation process in chestnut soils is that, despite the harsh conditions of soil formation (the influence of long-term seasonal permafrost, continental climate), humic acids with a fairly high content of C, N, and O are formed. Macromolecules of humic acids in arable samples of chestnut soils are characterized by a less developed peripheral part and a high benzenoid index compared to humic acids of the virgin variant, which is due to the environmental conditions of soil formation and the component composition of humification precursors. An increase in the proportion of aliphatic structures and a decrease in the proportion of aromatic fragments allow us to talk about a simplification of the structure of humic acid macromolecules. A short growing season and insufficient moisture in combination with a light granulometric composition significantly reduce the rate of transformation of organic matter and lead to an increase in the proportion of unoxidized aliphatic fragments.