Microbial Activity and Carbon Content in the Profiles of Modern and Buried Ancient Chernozems of Different Textures

The distributions of organic matter (C_org), microbial biomass (C_mic), and basal soil respiration (BR) in the profiles of ancient (buried under kurgans 1.2, 1.8, and 4 ka ago) forest-steppe chernozems of different textures and their background modern analogues under pastures were studied in Chelyabinsk, Voronezh, and Samara oblasts. Twelve radiocarbon dates were obtained, including seven dates by the accelerator mass-spectrometry method. In comparison with the background surface soils, the buried soils were characterized by the C_org loss from the upper layer (0–10 cm) that reached 47, 58, and 62% in the clay loamy chernozems buried 1.2, 1.8, and ~4 ka BP, respectively, and 24–40% in the loamy sandy chernozems buried ~4 ka BP. The BR and C_mic values in this layer of modern chernozems were four times higher than in the buried analogues for clay loamy soils and five–seven times higher for loamy sandy soils. In the deeper layer, these differences were leveled out. A decrease in the C_org, BR, and C_mic of the paleosols is related to organic matter mineralization in the absence of the input of new plant residues and the deficit of oxygen and nutrients. In the buried soils, the stratification of С_org and С_mic distribution became less pronounced downward the soil profile. The values of C_mic and BR in the layer of 0–20 cm reached 70% of their total sum in the layer of 0–50 cm in the modern clay loamy chernozems and 50–60% in the modern loamy sandy chernozems and in the buried chernozems. The percent of C_mic in C_org gained maximum in the layer of 0–10 cm in the modern clay loamy chernozems: 1.6–2.9% versus 0.3–1.1% in the other soils. The BR/C_mic ratio was close in the modern and buried soils; it was the highest in the loamy sandy chernozems attesting to a higher deficit of available C_org for microbes. This was also indicated by the fact that the ancient chernozems buried 1.2, 1.8, and ~4 ka BP had close C_mic and BR values to a depth of 60 cm regardless of the soil texture, except for loamy sandy varieties. The coefficients of variation in the upper 10 cm were 9–15% for С_org, 8–19% for BR, and 13–31% for С_mic. They slightly increased down the soil profile. There was a significant correlation between the microbiological parameters and the С_org content (r = 0.54–0.97) in all the studied soils.