*4.2. Counts and Diversity of Bacteria*

The physicochemical properties of the soil from winter wheat cultivation should, theoretically, be more favorable to the development of organotrophic bacteria and actinobacteria than the soil used for winter rape and field pea cultivation. This is due to, among other things, more favorable fraction size distribution, a higher sorption capacity, and lower acidification of the soil. However, the experimental data also pointed to some other dependencies influenced by the species of the cultivated plant. In spite of the fact that the physicochemical properties of the soil sown with field pea were far from being the most favorable, it was its rhizosphere that was the most abundant in organotrophs and actinobacteria. This can be explained by the difference in the root system morphology between winter wheat and field pea, in favor of the latter, as well as by the capability of field pea roots for symbiosis with atmospheric nitrogen-binding bacteria from the genus *Rhizobium*, and by the difference in the chemical composition of root secretions. The aforementioned factors contributed to a more dynamic development of organotrophic bacteria of r strategy than of k strategy, which was indicated by the values of their colony development index (CD). Usually, the greater inflow of nutrients to the natural environment aids the development of fast-growing microorganisms [34]. The k strategists, i.e., the slow-growing bacteria, are more stable in this respect. They are responsible for maintaining soil homeostasis, and are also resistant to the adverse effects of environmental conditions [67], as well as being typical of soil ecosystems [15,48].

Generally, organotrophic bacteria proliferate faster in the soil than actinobacteria as a result of organic matter inflow to the natural environment [13]. Such a response of microorganisms is not always associated with their ecological diversity [48]. In the present study, the highest value of the ecophysiological diversity index (EP) of organotrophic bacteria was determined in the winter wheat rhizosphere (0.877) and that of actinobacteria in the field pea rhizosphere. In the case of the latter microorganisms, the EP values reached 0.843 in the soil from pea field cultivation, 0.791 in the soil from winter wheat cultivation, and 0.684 in the soil from winter rape cultivation.

Under natural conditions, microbiome stability is largely affected by the species of growing plant and, especially, by its root system [41,48]. By providing water-soluble compounds to plants, including organic acids, sugars, or amino acids, the roots of plants stimulate the microbiological activity of soil [68,69], whereas plant growth promoting bacteria (PGPR) colonize roots and increase the root system biomass [40,70]. The PGPR modify the root architecture through the production of phytohormones, siderophores, and hydrogen cyanide, as well as by nitrogen uptake and mechanisms of phosphates stabilization [15,47,71]. Therefore, the rhizosphere is characterized by greater diversity of the population of microorganisms than the soil distant from the root system of plants [42,46,72].

Another important factor which determines soil health, and thus, soil quality and productivity, is the structure of microbial communities [73], because these communities affect the stability of the soil ecosystem [7,11,74]. There is a strong correlation between the soil microbiome and the plant microbiome [75,76]. It is the soil bacteriome that often determines the quality features of cultivable plants [77]. According to Bakker et al. [78] and Xu et al. [76], the prevailing phyla in the arable soil include *Proteobacteria, Actinobacteria, Bacteroidetes, Firmicutes*, and *Acidobacteria.* According to Xu et al. [76], proteobacteria comprised 40% and 43% in the pot and the field experiments, respectively. Proteobacteria can quickly respond to nutrient changes in the rhizosphere. Maron et al. [67] and Pascault et al. [79] have emphasized that *Acidobacteria, Actinobacteria, Planctomycetes, Chloroflexi*, and *Gemmatimonadetes* are classified among slow-growing microorganisms (k-strategists), whereas *Proteobacteria* (mainly those

from classes *Alphaproteobacteria* and *Gammaproteobacteria*) and *Firmicutes* are among the fast-growing ones (r-strategists).

The dependency between a plant species and the microbiological communities in the rhizosphere that was observed in the present study was also reported by Huang et al. [80]. According to Maron et al. [67], diminished diversity of microorganisms retards the degradation of both autochthonous and allochthonous carbon sources, thereby reducing the global emission of CO2 by as much as 40%, whereas the importance of the diversity effect increases along with increasing the availability of nutrients. Bacteria from various families differ significantly in their genetics, which determine their functions in the soil environment [81]. For instance, bacteria representing the families *Solibacteraceae* and *Acidobacteriaceae* play an active part in protein and carbohydrate mineralization; those from the family *Baciliaceae* degrade chitin and cellulose, and participate in the biosynthesis of plant growth hormones and secondary metabolites [82]; bacteria from the family *Burkholderiaceae* are active participants of bioplastics biodegradation [83]; and *Streptomycetaceae*, *Pseudonocardiaceae*, and *Promicromonosporaceae* exhibit robust activity against carboxymethyl cellulose, xylan, chitin, and pectin substrates [84].

According to Chaparro et al. [66], the secretions of the roots of various species or even ecotypes of plants, which determine the soil microbiome, differ in their chemical compositions. They manifest plant responses to the chemical signals emitted by soil microorganisms. Their secretion process can proceed both at the passive and active transport pathways. Root secretions determine interactions between plants and the soil microbiome, and therefore, regardless of the soil type, the crop and its cultivation regime is the key determinant of the soil microbiome [64]. The above factors contribute to a more favorable structure in the bacteriome of the soil from winter wheat cultivation compared to the soils from winter rape and field pea cultivation.
