Chemical Structure of the Organic Matter of Water-Stable Structural Units in Haplic Chernozem under Contrasting Land Uses: Solid-State CP-MAS 13 C-NMR Spectroscopy

The chemical structure of different soil organic matter (OM) pools in water-stable macro- and free microaggregates isolated from the surface horizons of Haplic Chernozem in contrasting variants of land use (steppe and long-term bare fallow) is studied using solid-state 13 C-NMR spectroscopy. It is observed that the degree of OM protection increases with a decrease in the size of structural units, which is confirmed by integral characteristics of OM chemical structure, such as lower degrees of OM decomposition, aromaticity, and hydrophobicity of microaggregates as compared with macroaggregates. A negative effect of the long-term bare fallow regime causes a sharp increase in the integral indicators of the chemical structure in all studied OM pools, being more pronounced in free microaggregates. The proposed multiple linear regression models for the C/N value prediction ( R_adj^2 = 0.993, P < 0.05 for discrete (free and occluded) OM and R_adj^2 = 0.996, P < 0.05 for the clay-bound OM and the residue fraction) clearly explain its increase in the bare fallow, reflecting the maximum nitrogen losses in OM, which in its status approaches the degraded one. According to the proposed model, the size of clay particles is determined by a set of factors. The best model with all its parameters being significant ( R_adj^2 = 0.997, P < 0.05) clearly demonstrates that the average diameter of clay particles increases with a decrease in the amount of OM hydrophobic fragments, the share of smectites in clay, and increase in the content of the clay-bound carbon.