A number of key sites for soil sampling were selected in the southern part of European Russia. The sites were located in arid and semiarid environments and represented the transition from forest-steppe through steppe to dry steppe landscapes. MAP shifts from ~360 to ~600 mm, MAT changes from ~ 4.6 to 9.5^oC. Carbonate pedofeatures (CP) were sampled in a variety of modern soils (Mollisols, Vertisols, saline and vertic intergrades). CP were represented by a variety of morphologies including veins, pseudomycelium, soft masses, impregnations, and hard nodules; the last three forms were investigated in the lab. The aim of this research was to obtain the stable isotopic compositions of carbon and oxygen in CP of the representative key-sites, which have  variable modern physiography and different evolution . Additional laboratory methods such as meso- and micromorphology, SEM, TDA were used on selected samples for better interpretation of isotopic variations.

Stable isotopic compositions in CP vary from -12.6 to -7.1 for carbon and from -10.8 to -6.3 for oxygen. This indicates an isotopic shift of approximately 5^o/_oo for carbon and 4^o/_oo for oxygen. Such important isotopic variations can be explained primarily by the decrease in moisture content and an increase in MAT, aridity and C4 plants in the different pedoenvironments during formation. Meanwhile the shifts about 2-4 ^o/_oo for C, and up to 3-3.5 ^o/_oo for O were found in CP of the same site. This is likely due to different carbonate morphologies: the formation of hard and soft carbonates may occur under different T and moisture. Also to note that soils may have different age and evolution depending the position of the key-sites related to the glaciation boundary. Some of the CP were identified as relict on a base of their ¹⁴C-age up to 12-22 Kyrs.B.P.