XII Международная студенческая научная конференция Студенческий научный форум - 2020

Introduction

As a result of the destructive anthropogenic impact on the soil cover, the total area of ​​cultivated land resources is steadily decreasing from year to year [1], which cannot but affect the deterioration in the food supply of the Earth’s population. Being the most important biogeochemical barrier, the soil is exposed to the negative effects of heavy metals (HM) that fall into it as a result of the activities of industrial enterprises, the operation of vehicles, and the influx of municipal sewage. To date, HMs occupy leading positions in the scale of soil pollution, the consequence of which is a violation of their fertility [2]. An important contribution to its formation is made by soil enzymes participating in the processes of transformation of organic substances in the soil and providing living organisms with available nutrients [3]. As is known, different types of soils respond differently to pollution [4]. High buffering properties determine their best resistance to the effect of pollutants [5].

The reasons for the negative impact of HM on the biological properties of soils are that HM, binding to sulfohydryl groups of proteins, inhibit the synthesis of proteins, including enzymes, and change the permeability of biological membranes. Under the influence of HM, disturbances occur in the structure of soil microbiocenosis, which changes the level of enzymatic activity of the soil [6].

The behavior of the most common soil enzymes in soil contamination with heavy metals can serve as an indicator of the degree of soil contamination. Moreover, such changes in enzyme activity serve as an indicator of early diagnosis of negative changes in soil properties [7].

Catalase. The most important soil enzyme from the class of oxidoreductases is catalase. It catalyzes the decomposition of hydrogen peroxide, which is formed during the respiration of plants and as a result of the biochemical oxidation of organic substances in soil, into water and molecular oxygen. Soil catalase activity is usually considered not only as an indicator of the functional activity of microflora, but also the preservation of enzymes in post-mortal plant material [8]. The study of the relationship between the level of pollution and the maintenance of TM is one of the urgent tasks for using catalase as a bioindicator.

Urease.Urease is one of the most studied soil enzymes. It plays an important role in the conversion of soil nitrogen. The presence of urease in bacteria enables them to use urea as a source of ammonia, since urease catalyzes its hydrolysis [9].

Anthropogenic pressure on urban soils is also manifested in the influx of heavy metals into them due to the intensification of automobile traffic, the development of industry, and public utilities, which is one of the factors of soil degradation in urban conditions [10]. The most dangerous heavy metals include cadmium, lead, nickel, chromium, mercury, etc. [11]. Some metals vital for the mineral nutrition of plants are also classified as heavy (zinc, iron, copper), and at high concentrations they become dangerous [12].

An important role in the formation of soil fertility is played by soil enzymes. HMs inhibit their activity even in soils with high buffer capacity. In this regard, it is of interest to study the activity of hydrolytic enzymes in anthropogenically modified soils.

The aim of the work is to study the effect of heavy metals (Pb, Zn, Cd) on the enzymatic activity of catalase and urease when vermicompost, sulfur-perlite-containing waste, and their mixture are introduced into the serozem soil..

Objects and research methods.

To study the nature and degree of change in the activity of soil catalase and urease from the amount of lead, zinc and cadmium, experimental experiments were performed. Vermicompost, sulfur perlite-containing waste, and mixtures thereof were introduced into gray earth soils contaminated with heavy metals, as well as uncontaminated (control).

Heavy metals are introduced in the form of readily soluble acetates in doses: 1) control (without making); 2) 0.5 MAC; 3) 2.5 MAC; 4) 5 maximum concentration limits. Calculation of metal concentrations was carried out on the basis of MAC values. The enzyme activity of urease was determined in soil samples. All analyzes were performed in triplicate. For the determination of metals, the voltammetric method using the TA-Lab instrument was used.

Catalase activity was determined by the method of A.Sh. Galstyan described by F.Kh. Khaziev [20]. Urease activity was determined by the method of I.N. Romeiko and S.M. Malinsky [10]. The method is based on the photometric measurement of the amount of ammonia (ammonium nitrogen) formed during the hydrolysis of urea under the action of urease by the formation of colored complexes with Nessler's reagent.

Research results and discussion.

Catalase plays an important role in the processes of neutralizing hydrogen peroxide, toxic for soil living organisms, which enters the soil as a result of their high physiological activity during a period of favorable living conditions. The presence of heavy metals in the soil may affect the rate of decomposition of hydrogen peroxide by catalase.

As the results of our studies showed, when introduced into the gray earth soil, the intensive decomposition of hydrogen peroxide depends on the concentrations of lead, cadmium and zinc. With an increase in the content of the studied metals in the soil system, a decrease in the activity of catalase is observed and, accordingly, the rate of decomposition of hydrogen peroxide is reduced.

As can be seen from table 1 of all studied ranges of concentrations of Pb, Zn, Cd, a decrease in catalase activity is observed. The greatest decrease in catalase activity was found for Cd, the smallest - for Zn.

Table 1 - Change in the activity of catalase when fertilizer-ameliorants are added to serozem contaminated with heavy metals (Pb, Zn, Cd)

Catalase activity when adding 5 MAC Cd decreases by 42% compared with the control, with 5 MAC Pb - by 38%, with 5 MAC Zb - by 29%. The results obtained, indicating a sharp decrease in the catalase content with an increase in the content of HM in the soil system, are explained by the toxic effect of HM on microorganisms, accompanying the death of a significant part of them. A decrease in the quantitative content of microorganisms that are sources of catalase enzyme production, respectively, leads to inhibition of the decomposition of hydrogen peroxide.

Thus, the research results show that disturbances in the structure of soil communities of microorganisms occurring under the influence of heavy metals lead to a decrease in the level of enzymatic activity of catalase.

The results of the experimental data obtained by studying the effects of the content of HM (Pb, Zn, Cd) and the presence of vermicompost (VK), sulfur perlite-containing waste (SPW) on the enzymatic activity of urease are presented in table 2.

Table 2 shows an increase in the enzymatic activity of soils with the addition of vermicompost, sulfur-perlite-containing waste and their mixture, which indicates an increase in soil fertility. Moreover, this is observed for all concentrations of HM (Pb, Zn, Cd) introduced into the soil system.

Table 2 - Change activity of urease when fertilizer-ameliorants are added to serozem contaminated with heavy metals (Pb, Zn, Cd)

The presence of Zn and Cd inhibits the enzymatic activity of urease. With an increase in their content, a decrease in the activity of this enzyme is clearly manifested, which leads to an inhibition of the reaction rate of urea decomposition.

Thus, the enzymatic activity of urease, regardless of the presence of all the studied metals, increases when vermicompost and sulfur perlite-containing waste are introduced into the soil system, i.e., the presence of HM does not cause a significant change in the level of soil fertility.

Conclusion

Based on experimental studies, a change in some indicators of the biological activity of the soil, i.e. catalase and urease activity from the content in the soil of heavy metals (lead, zinc, cadmium).

HM, depending on their nature, have a different effect on the activity of catalase and urease in the soil system. In the case of the presence of Pb in the soil, an increase in the concentration of urease is observed with increasing metal concentration, i.e. Pb activates the decomposition of urea. Catalase activity decreases with a high concentration of Pb. Because lead slows down the decomposition of hydrogen peroxide in the soil system. The presence of heavy metals such as Zn and Cd decreases the enzymatic activity of catalase and urease, which indicates an inhibition of the formation of ammonia, carbon dioxide and the decomposition of hydrogen peroxide.

References

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