This paper provides new information on molecular properties of the humin fraction (HM) isolated from the mollic horizons of Phaeozem and Chernozem soils developed from different parent materials in the temperate climatic zone of Poland. In our work we did not extract this fraction by dissolution but isolated it by exhaustive extraction of humic and fulvic acids with NaOH and then removing the mineral fraction with HF/HCl treatment. Obtained HM reflects the properties of this fraction that naturally occurres in the soil environment. The assessment of the structural properties of the HM was carried out using the spectroscopic methods ( 13C CP MAS NMR, FTIR, EPR), HPLC, SEM-EDX, and elemental composition analyses. The 13C CP MAS NMR spectra of the HM showed the advantage of aromatic structures over other organic components. The FTIR spectra confirmed that the content of aromatic structures dominated over the aliphatic ones, ranging from 7.05 to 10.32%. The EPR study indicated evidence of an unpaired electron situated on the condensed aromatic moieties. The HPLC investigation revealed the dominance of hydrophobic fractions, ranging from 77.41 to 80.83%. The ash content, ranging from 22.89 to 54.50%, as well as SEM-EDS study indicate that the isolated HM is not a pure organic fraction but constituted strongly bound organo-mineral compounds, resistant to further treatment with an alkali and HF/HCl mixture. This fraction represents the most resistant pool of the SOM that plays a crucial role in soil carbon sequestration and due to high functional group content contributes to the maintenance of soil and its ecosystem services.

Clear-cutting means forest removing (stem only) and is the most common type of forest harvesting but undoubtedly has a negative impact on the C budget in soils. This work aimed to describe responses of soil organic matter in the forest soils to forest removing under temperate climate conditions of lowland and mountain regions in south-western Poland. Using advanced instrumental analysis, like EPR, 1H NMR and FT-IR spectroscopy it has been found that clear-cutting, alters C cycling and accelerates decomposition in the forest floor leading to loss of humic fractions in the investigated soils. In the mountain forests the more labile, low-molecular fulvic fraction decreased as the effect of harvesting practice. The transformation of organic matter after clear-cutting resulted in the loss of less humified organic matter containing humic substances of less polymerised molecules. Analysis of the semiquinone radical structures and concentrations showed a decrease in radical concentration observed for HA from mountain clear-cut areas compare to the undisturbed forest. Results presented in this paper have proved less aliphatic character of humic acid molecules from the lowlands, compared to the mountain forest as the effect of clear-cutting. Harvesting practices in mountain regions should be approached with particular care due to the risk of erosion of exposed surfaces and soils containing less humified and less stable organic matter than in the lowlands. Humic fractions of higher solubility, less stability and tendency to migrate through the soil profile may favour the leaching of nutrients and consequently cause the eutrophication of waters.