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).