Natural 13C Abundance and Chemical Structure of Organic Matter of Haplic Chernozem under Contrasting Land Uses

Natural 13C abundance and chemical structure of different organic matter (OM) pools of Haplic Chernozem in contrasting land use variants (steppe and long-term bare fallow) are described. The sequence of OM decomposition in soils is traced. The pattern of changes in OM chemical structure of virgin chernozems under steppe from free (LFfr) to occluded states (LFocc) follows the pattern characteristic of the initial stages in decomposition, namely, the degree of OM aromaticity and hydrophobicity increases as well as the degree of its microbial degradation. The products of LFocc microbial decomposition adsorb on clay particles of the corresponding fraction. The clay-bound OM has a pronounced aliphatic nature with a considerable contribution of long-chain alkyls; it displays the highest degree of decomposition and the maximum contribution of polypeptides, which is consistent with its “heaviest” isotope signature among all studied OM pools. The chemical structure of the residue fraction is in many respects similar to that of the clay fraction and is enriched in short-chain alkyls of predominantly microbial origin. An almost complete absence of fresh organic material input to the soil of a bare fallow over a long period (52 years) causes a sharp increase in the degree of microbial decomposition of the available organic material in all studied OM pools, which is confirmed by their “heavier” isotope signature. The amount of the most easily available and energetically attractive fragments of organic material (O-Alk) for microbial community decreases, whereas the degrees of OM decomposition and hydrophobicity increase. An increase in the share of aromatic fragments and the degree of OM aromaticity suggests a selective accumulation of the most chemically stable aromatic compounds.