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.

This paper present results on soil contamination with petroleum hydrocarbons (PHs) on soil sorptivity and hydrophobicity under different soil moistures. The micro-infiltrometer method was used in laboratory experiment to determine the soil water repellency index (R) and the water drop penetration time (WDPT) test. The increase in PHs contamination contributed to soil repellency and caused a decrease in water sorptivity. The negative effect of contamination with PHs on soil sorptivity depended on soil moisture and was marked especially clearly after exceeding the critical moisture threshold. However, contamination by PHs did not reveal significant changes when ethanol was used instead of water. The R index and the WDPT test revealed a similar trend, inversely related to the level of soil contamination with PHs. The total amount of water available to plants in non-contaminated soil was 19.04%, while contamination equal to 100 g kg -1 caused a decrease to 6.36%. Hydrophobization of water-conducting pore surfaces by petroleum hydrocarbons severely reduced infiltration and destroyed the existing hydrological system of naturally hydrophilic soil. The almost three-fold decrease in total amount of water has a fundamental influence on increasing the risk of soil drought. The soil water repellency causes a decrease in resistance to droughts and slows the alimentation process of soil water retention. The results obtained indicated that the interrelations presented between the level of PHs contamination, soil sorptivity, water repellency, and soil moisture are key to predicting the environmental effects of contamination and effective soil remediation.