The elemental composition of humic acids in typical virgin chernozem

The working purpose is to study the elemental composition of humic acids (HA), hymatomelanic acids (HMA) and fulvic acids (FA) in typical virgin chernozem of the Kursk Region. Humus acid preparations were obtained by extraction with 0.1 N NaOH solution from a decalcified sample of soil according to the traditional method for our country, purification by centrifugation and electrodialysis. HMA were extracted from the crude gel with ethanol until the solution became colorless. In the resulting preparations of humus acids, the following parameters were determined: ash content by gravimetric method; C, H, N content by an automatic CHNS-varioMicrocube analyzer; O content was found as the difference; degree of oxidation and calorific value – by using empirical formulas. Elemental composition data were expressed in mass and atomic percentages. The highest energy potential is typical of HMA, the heat of combustion is 18.71 kJ/g. The lowest for FA is 10.99 kJ/g. According to the obtained data, chernozem HAs are formed mainly by cyclic compounds and are depleted in nitrogen, as evidenced by the H : C ratio (0.87) and C : N ratio (16.5). The O : C ratio is 0.50, and the oxidation degree is +0.13. HMA and FA are aliphatic in origin – the values of the H : C atomic ratios are 1.26 and 1.57, respectively, which corresponds well to the literature data. FAs are enriched with oxygen- and nitrogen-containing groups, the values of the atomic ratios O : C and C : N are 0.90 and 10.8, the degree of oxidation is +0.24. Among humus acids, HMAs are most depleted in nitrogen-containing groups; the C : N ratio is 22.8. Unlike HA and FA, HMA are under-oxidized compounds, which is confirmed by the O : C ratio of 0.44 and the negative degree of oxidation -0.37. Thus, HMAs have not yet completely passed the humification stage. According to the graph-statistical analysis, their transition to HA is associated with demethylation and oxidation reactions, supplemented by dehydrogenation. The transition from HMA to FA is due to a clearly expressed oxidation process, complemented by weakly manifested hydration. The position of humic acids on the diagram in coordinates of the degree of oxidation (ω) – the value of the H : C ratio indicates that they not only differ in the quality of their structural components, but are also at different stages of humification.

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