4. Conclusions

The studied HM that originated from mollic horizons of different Chernozems and Phaeozems derived in temperate climatic conditions of Poland indicated a condensed structure with a predominance of aromatic components. The predominance of aromatic structures reflects hydrophobic moieties of the HM, which usually positively affect the stability of soil aggregates.
The presented results showed a high similarity of the studied HM, suggesting that the properties of the soil parent material slightly affected their structures. However, the HM, which shows the greatest resistance to decay and plays a key role in carbon sequestration, requires further research in soils derived under differentiated conditions.
The HM isolated with the adopted method was not a pure organic fraction but consisted of strongly bound organo-mineral compounds, resistant to further treatment with an alkali and HF/HCl mixture. The reason for this is that the remaining organo-mineral bonds are too strong to be destroyed by exhaustive extraction of HA and FA. Because the organo-mineral bonds also protect the mineral fraction from destruction, the separation of the pure organic humin fraction portion would require the destruction of the organo-mineral bonds by alternating NaOH and HF/HCl treatment. The HM samples obtained in this way would be useful for basic research to better understand the structure of the obtained mineral-free HM, but it seems that investigation of HM existing as strong organo-mineral complexes characterized in this work better reflects the properties of this fraction, naturally occurring in the soil environment. It is necessary to answer the question of whether the HM is insoluble in alkaline solutions during the exhaustive extraction of humic and fulvic acids – according to the classical definition of this fraction – or also after the destruction of the organo-mineral bonds present in the soil. The isolation procedure used in this work led to the separation of the HM, which represents a highly resistant SOM fraction strongly bound to minerals, and the properties well reflect the role of this fraction in the soil system.
Funding This research has been supported by a grant from the National Science Centre, Poland (project 2018/31/B/ST10/00677).