Changes in the Carbonate Status of Chernozems of Azov Region upon Their Conversion from Cropland to Long-Term Fallow

Properties of chernozems of different-aged abandoned plots are compared with their analogues within cropland in the Rostov oblast (Russia). The main attention is focused on the carbonate status of chernozems. The field data on the morphology of soil profiles, results of meso- and micromorphological examination, and physicochemical properties of soils are analyzed; the carbonates were dated by radiocarbon method both in the carbonate-accumulative horizon and in the horizons transitional to the parent rock. Decompaction of the former plowed and lower horizons to a depth of 50 cm is observed in the abandoned plots. The organic carbon is accumulated; carbonates are partially leached; and pH values decrease. When the cropland is conversed to an abandoned plot, hard carbonate nodules in the lower part of the profile dissolve and disappear, whereas carbonate migrational forms appear in the upper part (ABca and BCAnc horizons; 40–80 cm). The carbonates in chernozems under the plots abandoned about 30 years ago acquire a new quasi-stable state different from that in the cropland. Vegetation that develops on abandoned plots also has a considerable effect on the transformation of the carbonate status of the studied chernozems.