1. Introduction

In connection with the growth of urbanization, which is inextricably connected with the development of the temps of industry, with an increase in the territories allocated for the construction of residential areas, with the rapid growth of vehicles, the analysis of urban soils is a paramount and important task today [12], [16].

Baku is a large developing metropolis, which is polluted with waste from the enterprises of the machine-building, food, oil and gas, petrochemical and oil industries. In the coming decades, it is planned to increase the area of ​​the city, however, to expand the transport infrastructure and for the construction of residential and industrial facilities, new “untouched open areas” are allocated, which leads to a violation of the soil profile, the death of microflora and soil animals, and the deterioration of soil properties.

The accumulation of pollutants in the soil is explained by the fact that, unlike the atmosphere, the soil is a sedentary medium and the process of migration of harmful substances proceeds much more slowly [5]. The accumulated toxic harmful substances seep into the groundwater. The rate of their entry into groundwater directly depends on the mechanical composition and physicochemical properties of soils, as well as on the geochemical characteristics of pollutants.

 Taking into account the fact that within the city there are large oil territories, saline soils, oil producing and oil refineries, machine-building plants, as well as other large industrial facilities, the city is subject to severe technogenic pollution [8].

The problem of soil pollution with a large number of pollutants, among which heavy metals, which are of paramount importance in terms of the scale of pollution and the impact on biological objects, has been the subject of many scientific works and studies. However, due to the accelerated temp of economic growth, unregulated territorial development and population growth in the capital city of Baku, a long-term plan for the development of the city has been developed and the environmental and geochemical assessment of urban soils is in great demand. [9].

According to literary sources, heavy metals play an important role for the normal course of vital processes in all living organisms, but on the other hand, when the permissible concentrations are exceeded, they are toxic and can kill all living things [11], [14]. Most of the heavy metals emitted into the atmosphere as waste products from petrochemical and oil refineries and other enterprises are deposited on the surface of 0–10 cm of urbanized and 0–20 cm of arable soils.

The factors influencing the uneven distribution of heavy metals (aluminum, cadmium, chromium, copper, lead, nickel, silver, vanadium, zinc and others) in the soils of the Apsheron Peninsula include the following: population density, meteorological features, landscape conditions, geochemical factors , features of pollution sources (production of petroleum products), the introduction of organic and phosphorus fertilizers rich in cadmium and containing impurities of uranium and lead into the soil, the use of pesticides [1].

An increase in the concentration of heavy metals in the soil leads to a rupture of natural ecological bonds, an acceleration of the processes of mineralization of soil humus, a change in acidity and alkalinity, etc. Heavy metals that pose a danger to the environment include more than 40 metals from the periodic system of D. I. Mendeleev, and among them, xenobiotics, i.e., metals that are not part of biomolecules, pose the greatest danger to humans. It is these metals and their compounds that play a primary role in increasing the number of genetic mutations, cancers and various pathologies [3], [7].

The results of scientific research on the identification of amounts of heavy metals in the soils of Baku and the Absheron Peninsula as a whole allow us to conclude that the zones with the maximum technogenic load, i.e., the zones where industrial facilities are located, or these zones are most affected by heavy metals located along major highways, in the zone of toxic landfills (there are 7 of them in the territory of Baku) and municipal solid waste (fig. 1).

Fig.1. Scheme of pollution of soils of the Absheron Peninsula with heavy metals (according to the Ministry of Ecology of Azerbaijan, 2000):

1-uncontaminated land, 2-minimal pollution, 3-light pollution, 4-medium pollution, 5-heavy pollution.

According to the literature data, due to the technogenic impact, the soils of the city of Baku are heavily polluted with heavy metals [10]. It is known that the central and industrial zone, as well as the oil production zone, are predominantly polluted with substances of the 1st and 2nd hazard class. Thus, the maximum concentration of lead exceeds the maximum permissible concentration (MPC) by 18 times, cadmium, tin and molybdenum by 2–5 times, nickel, chromium, manganese by 1–5 times. Soils in Baku are also contaminated with mercury, which comes from industrial plants in Sumgayit.

The results of studies of the impact of heavy metals on phytotoxic activity in the soil should be the foundation for the development of preventive measures aimed at preventing the negative consequences of pollution [17], [6], [18], [2].

The purpose of the work is to study the influence of soils contaminated with heavy metals on the physicochemical and biological indicators of soils, as well as on the phytotoxic activity of soils in different functional zones of the city of Baku.

In accordance with the given goal, the following tasks were set.

1. To study the scientific research works of domestic and foreign authors, whose work is aimed at studying the effect of heavy metals on the indicators and phytotoxicity of soils;
2. Conduct a comparative assessment of the indicators of soils contaminated with heavy metals taken from different functional zones of an urbanized area;
3. To identify the degree of contamination of the studied soil samples through biotesting;
4. Determine the average values ​​of each indicator of soil biota activity according to IIBS of soils for the functional zones of the city.

2. Materials and methods

Studies to determine the physicochemical parameters of soils and biotesting were carried out in the field and laboratory conditions [20]. In particular, the soils of the city were studied in the following locations: a thermal power plant, along highways, soils of areas designated for wasteland, soils of recreational areas. Soil samples were taken and prepared for analysis in accordance with state standard 17.4.4.02–84.

Samples of open soil were taken from a depth of 0–10 cm, 10–20 cm.

To determine the ecological state of soils in urban areas, the chemical indicators of soils (determination of humus and actual acidity) and the biological activity of soils (determination of catalase activity and soil phytotoxicity) were studied.

Sampling sites include the following areas.

Crossroads is one of the busiest highways in the Narimanov region. The constant traffic flow of Tabriz and Aga Neymatulla streets (near the Nariman Narimanov metro station) is the cause of the anthropogenic load on the nearest ecosystems (and in particular on the soil, fig. 2).

Fig.2. Crossroads of Tabriz and Aga Neymatulla streets

Wasteland along the street. Samedbek Mehmandarov and st. Abbas Fatullayeva (fig. 3). The object is fenced along the perimeter, there are no buildings, no cars and green spaces were noted.

Fig.3. Wasteland along Samad bey Mehmandarov street and Abbas Fatullayev street Baku TPP, (coordinates: 40°22′25″ N 49°55′11″ E) (Fig. 4)

Fig.4. «Azerbaijan» Thermal Power Station — Only Power Station in Country Capable of Operating on Fuel Oil

Dede Gorgud Park. The park is surrounded on three sides by highways, on which auto traffic is constantly boiling, an artificial lake with an area of ​​​​three hectares and a waterfall have been created (fig. 5).

Fig.5. Dede Gorgud Park

An initial assessment of soil quality was carried out at the sampling site. During the initial assessment, such positions as the presence of vegetation, anthropogenic inclusions in the soil, hardness, stonyness, and clutter were considered. The soil samples studied by us are classified as naturally anthropogenic, superficially altered. Traces of anthropogenic load are found at all sampling points in the form of inclusions of construction and household waste. Most of the emissions of pollutants into the urban environment are concentrated on the soil surface, where they, gradually accumulating, can lead to a change in the physical and physico-chemical properties of the substrate.

Sampling was carried out under comparatively similar weather conditions.

Conclusions about soil phytotoxicity were made on the basis of seed germination data (length of roots and green sprouts). The seeds of watercress, which are highly sensitive to the presence of toxic substances in the soil, were used as a biotest.

3. Results and Discussion

The average indicators of biochemical indicators of soil samples in areas with varying degrees of technogenic load were analyzed. Environmental conditions significantly affect the metabolic processes occurring in the soil. So, as can be seen from table. 2, the optimum pH values ​​for catalases are in the range of 5,8 to 7,92. Changes in soil pH are accompanied by a reversible process, the essence of which is the ionization of acidic or basic groups in the active center of the enzyme. According to literary sources, the primary task to overcome all negative processes (dehumification of cultivated soils, low enzymatic activity, etc.) is to increase the biological activity of the soil and the content of organic matter through the application of organic fertilizers, the use of bacterial fertilizers, the use of green manure crops and green spaces [2]. The high rate of catalase activity (0,76) in the soil sample from the intersection of Tabriz and Aga Neymatulla streets suggests that this soil is contaminated with carbonate crushed stones, which are used in the construction of highways, is more saline compared to other samples due to the use of anti-icing mixtures and other factors.

An analysis of the tabular data leads to the conclusion that the low content of humus, ranging from 0,9 to 1,4, is an important indicator of their ecological potential, characterizing the degree of soil resistance to heavy metal pollution. The low content of humus activates the rate of organic matter mineralization processes, which leads to a deterioration in soil quality, and, consequently, fertility.

The catalase enzyme is a measure of the soil’s ability to process hydrogen peroxide. The catalase activity index varies from 0,58 to 0,76. The low indicator of catalase activity in soil samples from the territory of the Baku TPP and the high one in samples from the Dede Gorgud park is explained by less disturbance of the soil cover in parks than in industrial zones. The catalase activity of the soil from the territory of the Baku TPP is characterized on a scale as very weak, in three other territories — the catalase activity is weak. This fact only confirms that the city of Baku is highly prone to technogenic pollution, so there is an urgent need to reduce the intensity of heavy metal migration in the ecosystem.

Phytotoxicity is the ability of polluted soil to slow down plant growth, leading to disruption of physiological processes. Phytotoxicity is the most informative and reliable indicator of technogenic transformation and resistance of soils in urban areas to heavy metal pollution [21] The assessment of the level of soil phytotoxicity was carried out according to the method described in work [13]. The level of soil phytotoxicity was carried out according to four levels:

• < 10% — ecologically clean soil;
• from 10 to 30% – weak phytotoxicity;
• from 30 to 50% — average phytotoxicity;
• 50% — high degree of phytotoxicity.

In table 1–4 show the lengths of the root and sprout on soil samples on days 3, 5, and 7 of research, mm.

Table 1

Morphometric indicators of the length of the root and sprout on days 3,5,7 of the study (mm) for a soil sample taken from the intersection of Tabriz and Aga Neymatulla streets

Table 2

Morphometric indicators of the length of the root and sprout on the 3rd, 5th, 7th day of the study (mm) for a soil sample taken from the wasteland along Samad bey Mehmandarov street and Abbas Fatullayev street

Table 3

Morphometric indicators of the length of the root and sprout on the 3rd, 5th, 7th day of the study (mm) for a soil sample taken from the territory of the Baku TPP

Table 4

Morphometric indicators of the length of the root and sprout on the 3rd, 5th, 7th day of the study (mm) for a soil sample taken from the territory of Dede Gorgud Park

According to the results of laboratory studies, the length of the watercress sprout is higher than this control indicator at the sites: “the intersection of Tabriz and Aga Neymatulla streets”and «Baku TPP» by 1,5 times, at the site «Wasteland along the street. Samad bey Mehmandarov and Abbas Fatullayev street” by 1,3 times.

The root of a plant is the organ that is in direct contact with the soil, therefore, by the rate of its growth and development, one can judge the degree of soil contamination or, in other words, its exposure to anthropogenic impact. With an excessive amount of heavy metals in the environment, the protective autoregulatory mechanisms of the plant weaken, excess ions enter both the root and the aerial part of the plants.

The ecological state of the territory was determined by biotesting with watercress. We have noted that for watercress, the length index of the main root, which characterizes the phytotoxicity of the soil, can vary from 12,2 cm in the control (distilled water) to 17,2 cm at the sites “crossroads of Tabriz and Aga Neymatulla streets”; «Baku TPP» and «Wasteland along the street. Samad bey Mehmandarov and Abbas Fatullayev st. Heavy metals also penetrate into plants through the root system. At the same time, such mechanisms can be involved in admission, through which the concentration of incoming heavy metals can be significantly minimized. For example, when using growth regulators of natural origin, the level of heavy metals in the plant is significantly reduced and the degree of dependence of their concentration on fruit weight is noticeably reduced [19].

The difference between the length of the roots of plants germinated in aqueous extracts of the control sample (distilled water) and the test samples averaged 2–4 cm, and the length of the main root in the samples of the control sample varied from 24,0 cm to 27,7 cm.

When comparing the length of the root of the sample «Wasteland along the street. Samedbek Mehmandarov and Abbas Fatullayev Street” and two other samples “CHP” and “Intersection of Tabriz and Aga Neymatulla Streets” there was a slight increase in the first sample and a decrease in this indicator in two other cases. The lowest growth rate of watercress sprouts (42,5 cm), when compared with the control sample, is from the Baku TPP site, and the highest (50 cm) from the intersection of Tabriz and Aga Neymatulla streets. According to the studies, the length of the watercress sprout is 1,3 times higher than this indicator of the control sample from the section “crossroads of Tabriz and Aga Neymatulla streets”, 1,2 times from the site “Wasteland along the street. Samedbek Mehmandarov and Abbas Fatullayev Street” and 1,1 times from the section “Dede Gorgud Park”.

 The highest rate of root growth (1.4 times) was observed in watercress from the «Dede Gorgud Park» site, the smallest — «Crossroads of Tabriz and Aga Neymatulla» and «Baku Thermal Power Plant». The calculated indicators of phytotoxicity of soil samples along the length of the root and along the length of the stem are presented in table. 5.

Table 5

Indicators of phytotoxicity of soil samples along the length of the root and sprout

Biotesting of soil samples revealed a low phytotoxic effect on the third day. For watercress, soil phytotoxicity manifested itself in stimulating the development of the root system and sprout. According to the calculations, a decrease in the length of watercress roots was noted in test samples of all territories, with the exception of the soil sample from Dede Gorgud Park, which indicates a low degree of soil phytotoxicity. The watercress sprout length indicators revealed a weak degree of phytotoxicity of all experimental soil samples, with the exception of the Baku TPP experimental site, where this indicator was 40%.

Based on the obtained data on the biochemical parameters of soils, we carried out an integrated approach to determine the integral indicator of the ecological state of soils in order to generalize data on soils in different functional zones of Baku.

In table 6 shows the generalized results of determining biochemical parameters with the calculated value of IIBS.

Table 6

Generalized results of the determination of biological indicators

4. Conclusions

It has been established that the main source of heavy metals in the soils of the city of Baku is motor transport and the oil industry. The influence of heavy metal pollution on the phytotoxicity of soils taken from different functional zones of the city of Baku was studied. Based on the generalization of the calculated averaged values, it can be argued that all districts and zones of the city of Baku, as well as recreational and residential areas, are characterized by a high soil IIBS. At the same time, it should be noted that the average value of the maximum value of the IPBS of the recreational zone is 3 times higher than this indicator of the industrial zone. This fact confirms the high technogenic impact of industrial facilities on urbanized soil. Low indicators of soil samples taken from the industrial zones of Baku were recorded. In the aggregate, all soil samples taken from different functional zones have low biochemical parameters, but at the same time, all samples are characterized by high soil IIBS. The results of the study can be used in assessing the state of contaminated soils and, as well as in the urban planning of the city of Baku.