Chemical structure of soil organic matter and its role in aggregate formation in Haplic Chernozem under the contrasting land use variants.

• OM in the bare fallow is close to degraded. • OM in the bare fallow is depleted in O-Alkyl fragments. • OM in the bare fallow is enriched with aromatic fragments. • OM in the bare fallow is characterized by a high degree of hydrophobicity. • The quality of OM significantly contributes to the size of the structural units. The different soil organic matter (SOM) pools were isolated from the surface horizons of Haplic Chernozem in contrast variants of land use: (a) free particulate OM, (b) occluded particulate OM, (c) clay-associated OM, and (d) OM of the fraction of the Residue (Res). The features of the chemical structure studied by solid-state 13C NMR spectroscopy of these SOM pools in virgin Chernozem were used to demonstrate the changes that solid plant residues undergo during decomposition when they are incorporated into microaggregates. The extent of decomposition of organic materials in soils follows a continuum from fresh and partially decomposed organic materials in the discrete OM (Free and Occluded) to significantly/completely processed in the mineral-associated OM (Clay and Res). We studied the alteration of the chemical structure in the SOM pools under the influence of the bare fallow (52-yr duration) and revealed some features of the OM quality that can contribute to the size of the structural units in the C-depleted Haplic Chernozem. It was revealed that the functioning of Haplic Chernozem in the bare fallow regime for 52 years caused sharp increases in the decomposition and aromaticity degrees of the organic matter, as well as the hydrophobicity degree. A sharp increase in its hydrophobicity indicates a sharp decrease in its adhesive ability. This can be the reason for the almost complete destruction of one of the most valuable agronomic aggregate-size classes (2–1 mm) in the bare fallow, and the redistribution of the water-stable aggregate-size classes towards a decrease in their dimension. The decline in the aggregate size was indirectly confirmed by the decline in the average effective diameter of the clay particles (building blocks of aggregates) in the bare fallow compared to that in the steppe (382.7 vs. 443.8 nm, respectively). [ABSTRACT FROM AUTHOR]