Elemental Composition of Humic Acids of Granulometric Fractions in Typical Virgin Chernozem

The stabilization of the humus profile of soils is largely due to the interaction of organic and mineral substances and the nature of the organo-mineral derivatives formed. Since humic acids form the basis of the humus of chernozems, the study of their organo-mineral derivatives is of great theoretical and practical importance. The purpose of the work is to study the elemental composition of humic acids, various granulometric fractions (coarse dust, medium dust, fine dust, silty fraction) of the typical virgin chernozem. The granulometric fractions were isolated from a soil sample pretreated with ultrasound for 20 min at 25 kHz, using the elutriation method, withh a ratio of soil to distilled water equal to 1:100. Extraction of humic acids from the granulometric fractions was carried out with 0.1 n. NaOH solution after preliminary decalcification of samples, purification from impurities by centrifugation and dialysis. In humic acid preparations, the ash content was determined by the weight method, the content of C, H, N was determined on the CHNS-vario Microcube automatic analyzer, the amount of O was determined by the difference, the oxidation level and the heat of combustion were calculated using empirical formulae. According to the data obtained, carbon dominates in the elemental composition of humic acids, the content of which was 38.3-40.0 at.%. They contain the least nitrogen (2.2-2.5 at.%), the amount of hydrogen is 36.5-37.9 at.%, and that of oxygen is 21.3-21.7 at.%. Judging by the values of the H:C and C:N atomic ratios, which consistently change from 1.62 to 1.74 and from 18.2 to 15.3, respectively, at the transition from the humic acids of the coarse-dust fraction to the humic acids of the silt fraction, the smaller the size of the granulometric fractions, the more the humic acids associated with them are enriched with aliphatic nitrogen-containing components. At the same time, humic acids of all the granulometric fractions are oxidized compounds, as evidenced by their oxidation level, which varies in the range +0.12-+0.17.

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