## 1 Introduction  
Sustainable functioning of natural and anthropogenic ecosystems includes an assessment of the soil ecological state that meets the requirements of FAO [1, 2, 3]. This balance affects the performance of ecological and agricultural functions by the soil. Such soil functions are particularly sensitive when polluted with heavy metals [4, 5]. Sources of soil pollution with heavy metals because of anthropogenic activities are the burning of fossil fuels for electricity production, operation of metallurgical, oil refining, and cement plants, mining and ore processing, the use of bottom sediments as fertilizers, and the use of pesticides. It is because of such activities that such elements as silver (Ag), bismuth (Bi), tellurium (Te), and thallium (TI) enter the environment [6, 7, 8]. The result of silver, bismuth, tellurium, and thallium entry into the environment is the accumulation of increased concentrations of elements in soils. At the same time, silver content varies from 8 to 35.9 mg/kg [9, 10], and in ore mining sites up to 7000 mg/kg [11]. The range of bismuth concentrations in soil is from 0.15 to 1891 mg/kg [12, 13], thallium-7 mg/kg [14, 15], 19 mg/kg [16], and near coal mines up to 20,000 mg/kg [17]. The recorded tellurium content is from 0.166 to 11 mg/kg [7, 18], and near ore deposits it is up to 290 mg [19]. The presence of high concentrations of silver, bismuth, tellurium, and thallium in the soil has a direct effect on the activity of soil enzymes, the abundance and diversity of soil bacteria, and plant growth and development. The effect of silver soil pollution has been studied on a wide range of biological indicators [20, 21]: decrease of soil enzyme activity [22; 23], reduction of plant root length [24], reduction of species diversity, and abundance of soil bacteria [25; 26]. Under the influence of bismuth soil pollution, the activity of dehydrogenases in soil ecosystems is inhibited [27]. Some studies have noted the accumulation of silver [10], bismuth [28; 29], tellurium [30; 31], and thallium [32; 33] in plants. Higher ecotoxicity of silver and thallium for plants was revealed compared to bismuth [29]. Ecotoxicity of tellurium for microorganisms begins at low concentrations in the order of 1 mg/kg [34], nevertheless, it is tellurium that has been least studied and almost all studies have been conducted in vitro. According to the authors, with sufficiently high antibacterial activity [35], tellurium is less toxic than silver [36]. Separate studies have been conducted to assess silver ecotoxicity [37; 38; 39], bismuth [40; 41], tellurium [42], and thallium [43] on the quantitative content of soil bacteria, changes in the activity of soil enzymes and indicators of the intensity of initial growth and development of seeds. Previously, the activity of seven enzymes was studied in silver, bismuth, tellurium, and thallium contamination [44]. However, a comprehensive study on the comparative assessment of the ecotoxicity of silver, bismuth, tellurium, and thallium based on the spectrum of informative and sensitive biological indicators of soils was conducted for the first time. Haplic Chernozem is a common type of soil in Europe [45; 46; 47]. Thermal power plants operating on coal and to some extent on fuel oil. For example, in the south of Russia, Novocherkassk Power Station is a source of pollution of chernozems [48; 49]. Emissions from power plants include products from the incomplete combustion of coal including heavy metals like silver, bismuth, tellurium, and thallium. Trace elements bound in coal are released and redistributed into bottom ash, fly ash, and gas phase during combustion [50; 51]. Heavy metals (HM) are 0.1-1.0 \(\upmu\)m in size; they are adsorbed on large soot particles and transported by air over long distances from the emission source, forming technogenic anomalies.