The change in carbonate state of arable and abandoned soils in the south of the forest-steppe zone of the Central Russian Upland (Nature Reserve "Les-na-Vorskle")

With agricultural use of soils, their functioning changes. This causes a transformation of their composition and properties. Dehumification, compaction, acidification (decalcification), carbonatization and alkalinization can occur in soils (Phaeozems and Chernozems) of the southern part of the forest-steppe zone of the Central Russian Upland in the process of agricultural use. Pedogenic carbon in humus and carbonates undergoes changes, too. The aim of the research was to study changes in the properties of arable and abandoned soils in comparison with virgin Phaeozems of the forest-steppe zone. Particular attention was paid to their carbonate state. The objects of the study were the soils of the "Belogorye" reserve at the "Les-na-Vorskle" site. We studied two key sites. Both sites have similar geomorphological and lithological conditions. The first site includes the following soil profiles: LnV5-13, LnV6-13 and LnV8-13. The second site includes LnV9-13 and LnV10-13 (See Fig. 1). We studied soils of 40-year-old abandoned lands (at the sites with forest and meadow renewal), arable and virgin soils. During field work, we carried out morphological description of the investigated soil profiles and determined the classification position according to the "Classification and diagnostics of soils in Russia". The bulk density was determined by the ring method in triplicate. The grain size composition of the profiles was determined by the pipette method with sodium pyrophosphate dispersion, the soil organic content was determined by the Tyurin method of wet combustion and the carbonate content calculated manometrically. We estimated the stocks of these substances. We collected undisturbed soil samples from the main soil horizons of all the studied profiles and prepared thin sections. Micromorphological analysis of the thin sections was carried out and radiocarbon dating of carbonates from the upper depth of their occurrence in the studied profiles was carried out. We showed that the arable soil differs from the virgin one in morphology. The arable soil has a specific layered microstructure in the plowed horizon. Clearly expressed grains of quartz appeared in the sub-eluvial horizon. In the texture horizon of the arable soil there are no ferruginous-humus-clay films that were found in the virgin soil (See Figures). The traces of activity of large animals and soil mesofauna shifted from humus and sub-eluvial horizons in the virgin soil into the textured horizons up to the BCt horizon in the arable soil. The bulk density of the plowed horizon in the abandoned and arable soils is higher than that of the humus horizon of the virgin soil. The bulk density of the subsurface horizon in the abandoned soils under meadow still shows increased values. The bulk density of the subsurface horizon of the abandoned soil under forest was almost equal to that of the virgin soil (See Figures). In the arable soil, the bulk density of the texture horizon increases in comparison with the virgin soil. The content of clay particles of the deep layers in the arable soil shows increased values as compared with the virgin soil. The enrichment of clay fraction is also observed in the arable horizons of the abandoned and arable soils in comparison with the virgin one. The content of organic carbon in the former arable horizon and its stocks in the 0-50 cm and 50-100 cm layers of the abandoned soils is lower than in the virgin soil. On the other hand, in the arable soil the content of organic carbon and its stocks in the 0-50 cm layer is higher than in the virgin soil, as noted previously by other researchers. The arable soil differs from the virgin one by a lesser depth of occurrence of carbonates in the profile and their substantially higher ¹⁴C-age (See Table). For 40 years in the abandoned state, the carbonates are gradually leached out. The stocks of carbonates are still higher than in the virgin soil, but the equalization of the radiocarbon age of carbonates in them clearly indicates the processes of carbonate re-crystallization and leaching. The main role in changing arable Phaeozems in the process of their abandonment is played by vegetation, which affects the redistribution of moisture, the accumulation of organic and carbonate carbon, and the bulk density and processes of transformation of the soil structure of the former arable horizons. The paper contains 5 Figures, 1 Table and 26 References.

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