Sustainable agriculture is only possible if the agroecological services of the soil are preserved. Soil contamination with rare elements such as silver (Ag), bismuth (Bi), tellurium (Te), and thallium (Tl) is less studied, but their toxicity is no less high than in other heavy metals. Activity of soil enzymes is of great importance for the healthy functioning of soils, agroecosystem services, and their fertility. It is necessary to assess the ecological state of black soil using the most sensitive and informative indicators of the state of soils—their enzymatic activity. The objective of this research was to evaluate changes in activity of five priority soil enzymes (catalase, dehydrogenases, invertase, phosphatase, and urease) when contaminated with oxides and nitrates of Ag, Bi, Te, and Tl in a laboratory model experiment. The integral toxicity of nitrates and oxides of Ag, Bi, Te, and Tl was assessed by the integrated index of soil enzymatic activity. A comparison of the toxicity of oxides and nitrates of each element, according to the integrated index of soil enzymatic activity, allowed us to establish that Ag oxide is more toxic than Ag nitrate; Bi oxide is equivalent in its toxicity to Bi nitrate; and Tl and Te oxides are less toxic than Tl and Te nitrates. When contaminated with oxides, the most informative indicators are activity of invertase (Ag), urease (Bi, Tl), and phosphatase (Te). When contaminated with nitrates, the most informative indicators are activity of phosphatase (Ag) and invertase (Bi, Tl, and Te). Activity of phosphatase and catalase are the most sensitive to contamination by oxides and nitrates of Ag, Bi, Tl, and Te, and dehydrogenases, invertase, and urease are the least sensitive.

Soil health is the basis of ecological and food security for humanity. Among the informative indicators of soil health are microbiological indicators based on the intensity of the carbon dioxide release from the soil. The reaction of the microbial community of Haplic Chernozem Loamic, Haplic Arenosols Eutric, and Haplic Cambisols Eutric to contamination with oxides and nitrates of Ag, Bi, Tl, and Te at doses of 0.5, 1, 3, 10, and 30 derived specific permissible concentrations (SPC) was analyzed in the conditions of a vegetation experiment (the exposure period was 10 days). One derived concentration is assumed to be equal to three background concentrations of the element in the soil. The carbon content of microbial biomass in Haplic Chernozem varied between the experimental options from 6 to 218 mg/kg of soil; in Haplic Arenosols, from 3 to 349 mg/kg of soil; and in Haplic Cambisols, from 7 to 294 mg/kg of soil. Microbial biomass was a more sensitive indicator of contamination by the studied pollutants than basal soil respiration. A decrease in specific microbial respiration was found when Haplic Cambisols were contaminated with Ag, Bi, Te, and Tl oxides. Te and Tl nitrates had a significant toxic effect on each type of soil. At the maximum dose of Tl and Te nitrate, a decrease in basal soil respiration of 56–96% relative to the control and an increase in the metabolic coefficient by 4–6 times was found. The toxicity series of heavy metals averaged for all types of soils in terms of microbiological activity was established: Bi > Ag > Te > Tl (oxides) and Te > Tl > Ag > Bi (nitrates). Nitrates of the elements were more toxic than oxides. Soil toxicity due to Ag, Bi, Tl, and Te contamination was dependent on soil particle size distribution, organic matter content, and soil structure. A series of soil sensitivity to changes in microbial biomass and basal soil respiration when contaminated with the studied pollutants: Haplic Arenosols > Haplic Chernozems > Haplic Cambisols. When diagnosing and assessing the health of soils contaminated with Ag, Bi, Tl, and Te, it is advisable to use indicators of soil microbiological activity.

Soil contamination with such rare heavy metals as silver (Ag), bismuth (Bi), tellurium (Te), and thallium (Tl) leads to disruption of its agricultural and ecological functions. Each of these rare heavy metals has a different level of soil toxicity, which affects the ecological state of the soil and its recovery degree estimated by biological indicators. The work objective is to assess the ecotoxicity of oxides and nitrates of silver, bismuth, tellurium, and thallium by biological indicators of Haplic Chernozem. Under the conditions of a laboratory simulation experiment, silver, bismuth, tellurium, and thallium were introduced into the samples of soil. The ecological state of the soil for each incubation period was assessed by the activity of soil enzymes (catalase and dehydrogenases), the intensity of seed development in polluted soil (the length of shoots and roots of winter wheat), and microbiological indicators (the total number of bacteria and Azotobacter sp. abundance). For 90 days, when contaminated with oxides of silver, bismuth, tellurium, and thallium, the most sensitive biological indicator was the length of wheat roots; when contaminated with nitrates, the total number of bacteria was the most sensitive biological indicator. The most informative biological indicator for contamination with both chemical forms of rare elements (silver, bismuth, tellurium, and thallium) was the Azotobacter sp. abundance. The most ecotoxic elements among those studied were thallium and tellurium, both in the form of oxides and nitrates. The results of the study may be useful for biomonitoring and diagnostics of the state of soils contaminated with silver, bismuth, tellurium, and thallium.

Thallium ecotoxicity was assessed in laboratory model experiments by changing in microbiological, biochemical, and phytotoxic properties of soils in the South of Russia: ordinary chernozem (Haplic Chernozem (Loamic)), seropesok (Eutric Arenosol), and slightly unsaturated brown forest soil (Eutric Cambisol), differing in texture, pH, and organic matter content. There was usually a direct relationship between Tl concentration and the deterioration rate of the studied soil properties. The ecotoxicity of Tl nitrate was higher as compared to Tl oxide. The ecotoxic effect of Tl was the strongest for chernozem and seropesok within 10 days and for brown forest soil within 30 days after contamination. Restoration of biological soil properties was recorded on the 90th day. The resistance to Tl contamination was the greatest for ordinary chernozem and the smallest for seropesok. The results obtained indicate a high ecotoxicity of Tl.