**1. Introduction**

Growing attention is being paid to the proper functioning of ecosystems [1,2]. This is due to numerous initiatives which have been undertaken globally to make the population aware of the need to protect soils and for their sustainable management. One of the first of these initiatives was the announcement of the European Soil Health Card, which has made both the population and policy makers aware of the significant role of the soil environment, and has introduced a new term, 'environmental services', which was later changed to 'ecosystem services' [3]. The concept of soil quality and health, often used interchangeably, is becoming increasingly recognized worldwide [4–6]. Legaz et al. [6] and Bünemann et al. [7] define 'soil quality' as the capability of soil to function in

the framework of ecosystems, and also as land management and promotion of the biodiversity and health of plants and animals. According to Cardoso et al. [5] and Veum et al. [8], the term 'soil health' highlights the fact that soil is a live and dynamic being whose functions are determined by the diversity of living organisms. Therefore, the physical, chemical, and biological properties of soil may change according to biotic and abiotic factors, which consequently affect its functions and ecosystemic services [3,9].

Apart from elements such as profile morphology, physical and chemical properties, and microclimate, the microbiological activity of soil is another principal factor affecting its fertility [10]. Microorganisms and enzymes determine the transformation of organic matter introduced into soil, influence the humification process, and play a key role in the biogeochemical cycles of many macro- and micro- elements [7]. They are key participants in most of the important cycles, including carbon, nitrogen, phosphorus, and sulfur circulation. It is mainly the biogeochemical cycles of these elements that determines the quality of the natural environment, including the soil environment [9]. The role of microbes is invaluable in the transformation of postharvest residues, as well as natural and organic fertilizers [11,12], the detoxication of organic contaminants in soil [13,14], minimizing the prevalence of pests and pathogens [11], and finally, in establishing symbiotic systems with plants [15]. The enormous importance of microorganisms for the soil environment and their high metabolic activity are confirmed by the fact that the microbial biomass of soil accounts for approximately 85% of the total biomass of all living organisms colonizing this environment [11].

The main factors which affect the development of soil microorganisms include the abundance of organic and mineral colloids in the soil, the climate and microclimate, oxidation associated with humidity status [16–19], soil pH [20,21], soil tillage and plant cultivation systems [22–24], and fertilization systems [25]. An inseparable element in this case is soil temperature, which is affected by the climate and microclimate [16,26,27]. Significant are also contamination with heavy metals [28,29], various hydrocarbons [30–32], plant protection agents [33–35], and dioxins, as well as the salinity level [36], all of which determine the proliferation of various microbial communities for which an increase their diversity may reduce soil fertility and, ultimately, influence soil productivity [37].

All the aforementioned parameters determine the development of not only microorganisms, but plants as well. The connection between both of these groups of organisms is of the utmost importance, e.g., in the case of symbioses of certain bacteria and fungi species with plants. Noteworthy are also the microorganisms that live in association with plants and produce growth hormones, bind atmospheric nitrogen, and protect plants against pathogens. An important role is also ascribed to the microbes classified as plant growth-promoting rhizobacteria (PGPR) [38]. On the other hand, apart from bacteriorrhiza, mycrorrhiza, and microbe associations with plants, we cannot observe a beneficial effect of root secretions on microorganisms or a positive impact of root systems on the physical properties of soil, as it serves the function of drainage, which is essential for appropriate soil oxygenation [27,39]. Root secretions, which may contain organic acids, amino acids, carbohydrates, vitamins, and metal ions, modify the microenvironments of the rhizosphere [40]. They release ions, oxygen, water, and carbon-containing compounds [41]. They may both stimulate and inhibit the development of a soil microbiome [42], and act both as repellents [43] and attractants [44]. Volatile organic compounds (VOCs) emitted by soil microorganisms can affect root growth. Bacterial volatile compounds (BVCs) are used as a source of nutrients and information in plant–bacterium interactions [15]. Such interactions are the strongest in the rhizosphere [7,45], which is colonized by 10 to 100 times more microorganisms than the sphere that is distant from plant roots [46]. The beneficial bacteria of the rhizosphere compete with other microorganisms for organic compounds and colonize plant roots [47]. Microorganisms colonizing this microecosystem are affected by the root secretions of particular plant species and form specific microbial communities [48]. The 16S amplicon sequencing analysis, a small subunit ribosomal ribonucleic acid (SSU rRNA), is an important element in biological quality assessments of soil, as it may provide answers about the response of individual taxa to variable factors in different agricultural

ecosystems [49]. Despite the severity of the problem, investigations into the effects of plants on soil bacteria based on next-generation methods are still scarce.

The complex nature of the active rhizosphere has become a premise for undertaking a study aimed at determining the effect of a crop and its cultivation regime on changes in the soil microbiome. Three plant species were used in the study: winter wheat, winter rape, and field pea; these species were cultivated on soil with a similar fraction size. Analyses were performed to determine the structure and diversity of microorganisms and the activity of the soil enzymes participating in the metabolism of carbon, nitrogen, phosphorus, and sulfur. The coupled use of microbiological, biochemical, and physicochemical parameters form the basis for a better understanding of the interactions between the rhizosphere, microorganisms, and plants, which is extremely important from the viewpoint of soil fertility.
