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

This study is dedicated to the soils of one of the least studied mountainous regions of Europe – the Polar Urals, which develop on carbonate rocks under mountain tundra and meadow communities. The objective of this study was to examine the structural and functional parameters of humic acids isolated from different types of carbonate soils in the Polar Urals region and to elucidating their relationship with the environmental conditions that influence soil formation. The molecular composition of humic acids was investigated using modern physicochemical methods of analysis, including 13C NMR spectroscopy, exclusion liquid gel chromatography and gas chromatography. The impact of soil formation on the accumulation of humus and the variation of structural and functional parameters of humic acids in the organogenic and humus horizons of the studied soils has been established. The humic acid content was estimated to range from 0.03 to 6.27% and was found to be related to the aboveground biomass stock. It was demonstrated that humic acid molecules are predominantly formed by aliphatic fragments, which determine the number-average molecular mass. It has been identified that the least condensed structures correspond to humic acids of organogenic horizons of soils formed under shrubby-moss and shrubby-moss communities. This is likely to be related to the deficiency of lignin in the composition of humification precursors. This group of soils corresponds to the highest ratio of carbohydrates to unoxidized aliphatic fragments, indicating a low degree of humification and stability of organic matter.