Molecular characteristics of humin fraction isolated from soils of temperate climate: a study on Chernozems and Phaeozems in Poland

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 ( C CP MAS NMR, FTIR, EPR), HPLC, SEM-EDX, and elemental composition analyses. The C 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.