Structural and Functional Parameters of Humic Acids in Soils on Carbonate Rocks of the Polar Urals

The soils developing on carbonate rocks under mountainous tundra and meadows in one of the least studied mountainous regions of Europe—the Polar Urals—are considered. The aim of this study is to examine the structural and functional parameters of humic acids (HAs) isolated from different types of carbonate soils in the Polar Urals and to elucidate their relationship with the environmental conditions affecting soil formation. The molecular composition of HAs has been studied using modern physicochemical methods of analysis: 13C NMR spectroscopy, exclusion liquid gel chromatography, and gas chromatography. The influence of environmental conditions on the rate of humus accumulation and on the variability of structural and functional parameters of HAs in the organic and humus horizons of the studied soils has been established. The content of HAs varies from 0.03 to 6.27% and is tightly related to the aboveground biomass stock. The molecules of humic acids are mainly formed by fragments of aliphatic nature, which specify the moderate molecular mass. It is shown that the least condensed structures are characteristic of HAs in organic horizons of soils formed under dwarf shrub–moss and dwarf shrub communities. This is obviously related to the deficiency of lignin in the composition of humification precursors. This group of soils is characterized by the highest ratio of carbohydrates to unoxidized aliphatic fragments, which indicates a low degree of humification and an instability of organic matter.