Microbiomes of the Soils of Solonetzic Complex with Contrasting Salinization on the Volga–Ural Interfluve

The analysis of ribosomal genes has been applied to study microbiomes of two soils of the solonetzic soil complex in the northern Caspian region. These soils—solonetz and quasigleyic chestnut soil—drastically differ in their salinity characteristics. The specificity of the vertical distribution of prokaryotes by the genetic soil horizons from the surface to the depth of 120 cm in these soils is discussed. The differences in the structure of microbiomes in the upper soil horizons can be related to the differences in the vegetation cover of the two soils, whereas the differentiation of microbiomes along the soil profiles is affected by the soil salinization. The solonetz is characterized by a much sharper decrease in the abundance and diversity of microorganisms down the soil profile in comparison with the leached quasigleyic chestnut soil. The total number of prokaryotes is mainly limited by the organic carbon content. In the upper soil horizons, Archaea from the phylum Thaumarchaeota are relatively abundant; their percentage decreases down the soil profiles. In the lower horizons of the solonetz, the genes of Marinobacter bacteria, which are considered marine inhabitants, have been found. It is probable that they persist in the soil since the previous transgression of the Caspian Sea.