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The study of the elemental composition of labile humus substances passing into 0.1 M NaOH extract from typical chernozem under different cenoses indicated that, judging from the H : C ratio varying within 1.55–1.91, aliphatic structures predominate over cyclic components. Under the agrogenic impact, the processes demethylation, dehydrogenation, and oxidation of organic substances are developed in the chernozem. As a result, labile humic substances lose their aliphatic components and become enriched with oxidized nitrogen-containing cyclic structures. The intensity of this process increases in the series: permanent winter wheat > permanent corn > permanent bare fallow. At the same time, labile humic substances of the chernozem under permanent bare fallow are the least energy-intensive compounds as judged from the values of the heat of combustion found on the basis of elemental analysis data. Over the 23 years that have passed since the transfer of the permanent bare fallow to the unmanaged fallow overgrown by herbs, there have been no significant changes in the elemental composition of labile humus substances of typical chernozem, except for a tendency of their enrichment in newly formed aliphatic nitrogen-containing reduced and more energy-intensive compounds. [ABSTRACT FROM AUTHOR]