Abstract <p>Biological activity in the transition between light crusty solonetz (Haplic Solonetz) and chestnut nonsolonetzic soil (Каstanozem) is studied. The research objects are located on natural pastures in dry steppe zone in the south of the East European Plain on the northern macroslope of the Sal-Manych Ridge. We have determined the microbial biomass by the content of soil phospholipids, the ratio between microorganisms of different trophic groups, and enzymatic activity in fluorogenically labeled substrates. Microbial biomass decreases with depth and is the greatest in the upper layer of the crusty solonetz located in the erosional micro-depression. This parameter is half as much in the upper horizon of other soils. Phosphatase activity dominates in the chestnut soil, and the content of leucine-aminopeptidase and arylsulphatase increases with depth in the solonetz. The upper horizon of the light crusty solonetz is the closest to the upper horizon of the chestnut soil by the biological activity and differs from other solonetzes. The pool of extracellular enzymes is active in solonetzic horizons. The transitional soil between Haplic Solonetz and Каstanozem is pronouncedly distinguished by the complex of biological parameters. It is proposed to reflect the effect of the solonetz process on the biological activity of soils by coefficients of biological differentiation of the soil profile, which shows the ratio of biological parameters in the illuvial and the eluvial horizons. According to it, the activity of leucine-aminopeptidase and arylsulphatase, as well as the number of CFU of cellulolytic fungi, are closely related to the intensity of the solonetz process.</p>