Soil Microbiota of the Humus Cycle in Gray Soils of Ravine Oak Forests of the Steppe Zone

Current studies of the global carbon cycle cover all environmental components, including the biosphere as a whole. Decomposition and humification of organic residues in the soil result in the formation of a complex system of intermediate compounds, which are simultaneously consumed in the processes of humification and mineralization up to final products. In terrestrial ecosystems, forest ecosystems and their soils represent critical links of the carbon cycle. Despite the fact that forest ecosystems in the steppe part of the Central Chernozem region occur only as islands of ravine oak forests, their ecological functions in arid ecosystems are very important. The soils of ravine oak forests are formed along an elevational series in the following order: typical light-loamy gray soils of the summit of a west-facing slope → stratified humic water-accumulating loamy gray stratozems on proluvial–deluvial deposits of the ravine bottom → typical light-loamy gray soils of the summit of the east-facing slope. The study of the forest soil microbial community of the ravine complex was based on the recording of the abundance of groups involved in carbon turnover, such as actinomycetes, micromycetes, and autochthonous and zymogenous microflora. The results of this study showed that, in the case of arid climatic conditions (HTC = 0.8), the processes of humus formation in the oak forest soils were the most active in spring, whereas humus mineralization processes dominated in autumn. In autumn, the abundance of the zymogenous group differed insignificantly between the elements of the ravine complex (R2 = 0.9956). In spring, the abundance of this group and its contrast between the elements of the ravine complex increased significantly (R2 = 0.7715). The maximum abundance was observed on the west-facing slopes. A contrasting trend of development was observed for this group of microorganisms in the spring period. For all sites studied, the abundance of micromycetes in autumn was lower than in spring, with the maximum value observed in the soil of the ravine bottom. Actinobacteria have different development trends for the autumn and spring periods: in autumn, their quantity gradually decreased from the west- to east-facing slope, while the opposite trend was observed in spring. Thus, the activity of participation of different microbial groups in humus formation processes is highly dynamic and depends on both the seasonality (spring, autumn) and the landscape position (slope aspect, ravine element).