Features of the chemical structure of different organic matter pools in Haplic Chernozem of the Streletskaya steppe: 13C MAS NMR study

The aim of the current research was to study structural features of four organic matter (OM) pools isolated by the modified method of granulo-densimetric fractionation from two Chernozems. We purposed to relate these features to the OM allocation and the transformation processes. The pools included: 1) free light fraction located in an inter-aggregate space, 2) light fraction occluded inside the microaggregates, 3) OM bound with clay particles, and 4) OM bound with a residual heavy fraction left after light fractions and clay separation. Soils of contrasting land uses: steppe and long-term permanent bare fallow were selected to assess changes that occur in soil OM during the degradation. We used C-13 CP/MAS NMR spectroscopy controlling the quantitativeness of spectra with the aid of C-13 DP/MAS NMR. The obtained spectra of the studied fractions clearly differed in the proportion of main functional groups. The occluded OM is more aromatic compared to the free OM. The structural changes observed at transition from the free light fraction to the occluded one indicate an active decomposition of lignin and carbohydrates in the latter fraction. This provides in the occluded OM appropriate conditions for the formation of "young HA": high local concentration of substrates and spatial proximity of enzymes. At C-deficiency (bare fallow) the chemical structure of the occluded OM is very close to that of humic acids. The chemical structure of the clay bound OM reflects high content of products of microbial origin. OM of the residual heavy fraction differs from that of the clay: the proportions of main functional groups in it are more close to that of the free light fraction, but with higher carboxyl content. Heavy fractions also differ under acid treatment: the residue losses less carbon. The above-mentioned differences show that the division of heavy fractions into two components is reasonable. Various acid hydrolyzability indicates a predominance of strong chemical bonds in the occluded OM and the significant contribution of weak bonds in the clay OM, i.e., the occluded OM is highly condensed, in contrast, components of the clay OM are largely interconnected by hydrogen-, coordinate, hydrophobic and other relatively weak bonds. Soil degradation under extreme land use leads not only to OM scarcity, but also to its greater hydrolyzability, strong enrichment with aromatic fragments and depletion of carbohydrate and aliphatic fragments in all studied fractions. Degradation changes in the occluded OM are most pronounced. Our results demonstrate that the applied fractionation scheme coupled with quantitative C-13 CP/ MAS NMR spectroscopy is a very promising approach for evaluating processes of soil OM transformation and degradation.