Nonhydrolyzable Part of Soil Organic Matter in Buried and Modern Soils

Abstract

Results of the study of humus composition in about 200 modern soils of different genesis and more than 100 buried Pleistocene and Holocene paleosols have been collected and analyzed. The content of the nonhydrolyzable part of soil organic matter (NH OM) in the paleosols is comparable with that in the modern soils (50 ± 15% and 54 ± 21%, respectively). Thus, NH OM does not accumulate in the buried soils during their diagenesis and is not the inert fraction of the soil organic matter (SOM). The NH OM content in the natural organomineral soil complexes–clay fractions of the soils—is lower than that in the whole soil mass. Data on the solid-state ¹³C NMR spectroscopy of the whole soil samples and the NH OM indicate that the latter are enriched in O-alkyls, including C of acetal groups. The whole soil mass is richer in C of aromatic structures and carboxyl groups. The degree of organic matter humification estimated from the alkyls/O-alkyls ratio for the NH OM is considerably lower than that for the whole SOM. The existence of negative correlation between the contents of NH OM and humic acids in the modern soils (R = –0.86) and in the buried paleosols (R = –0.83) has been shown. The NH OM content in the clay fraction in comparison with the whole soil mass is lower (36 ± 21% and 60 ± 15%, respectively). This reflects the accumulation of weakly decomposed plant residues in this part of the SOM. It is argued that the NH OM is heterogeneous and includes at least two groups of substances: clay-bound organic matter of organomineral complexes (nonhydrolyzable because of strong chemical bonds) and weakly decomposed plant residues resistant to acid and alkaline hydrolysis. The absence of definite relationships between the contents of NH OM and fulvic acids indicates that genetic links between the NH OM and humic acids are stronger than those between the NH OM and fulvic acids. ¹³C NMR spectroscopy of kerogen from the Carboniferous shale attests to the principal difference between the NH OM of soils and kerogen, in which C of alkyl groups predominates.