1. Introduction

One of the reasons for the deterioration of the state of ecosystems since the end of the last century is the process of urbanization. More than half of the world’s population today lives in cities, and the trend of urban population growth is steadily growing. According to UN data, in 2030 about 8.3 billion people will live on the planet Earth, of which about 5 billion will live in cities (the share of the planet’s urban population can grow to 60%) [ 17].

The object we are studying is the soils of Baku. Today, Baku is the largest metropolis in the country and the core of the Baku agglomeration. More than half of the country’s population lives here, most of the socio-cultural and educational institutions function, and the industrial complexes of Azerbaijan are located. The presence of more than 70% of the country’s industrial potential in the Apsheron economic region is explained solely by the historical position of the city and the Apsheron Peninsula. Baku is a large city with an area of ​​2430 km2 with a highly developed highway system. In addition, Baku, being the center of industry in Azerbaijan, is steadily growing and developing, and its network of transport infrastructure is also expanding.

One of the important points of the General Plan «Development Concept «Azerbaijan — 2020: a look into the future»» is the development of the country’s integrated transport system. Achieving environmentally sustainable socio-economic development of Azerbaijan is one of the main goals of the concept. Measures taken to protect biodiversity, reduce and neutralize the negative impacts of existing industrial complexes on the environment, eliminate pollution of the Caspian Sea and its coastal zones and protect them, restore and plant green areas and effectively protect all existing resources have been implemented and will be implemented in the future. “Improvement of transport infrastructure” (one of the points of this program) provides for the introduction of comprehensive measures for the reconstruction of road, rail, water, air and underground (metro) transport.

Bringing the country’s road transport infrastructure in line with international standards dictates the need to expand highways in the direction of the East-West and North-South transport corridors, complete projects for the reconstruction and construction of roads nationwide. To do this, it is planned to replace gravel roads with hard-surface roads, reconstruct and expand the network of roads of republican significance, ensure the transition to international environmental standards, build road junctions and bridges with high environmental safety, carry out work to modernize existing roads. All these works will be carried out on the principle of ensuring the safety of the population. Along with the work to improve the relevant infrastructure, intelligent transport control systems will be integrated in large cities, the transport system of the capital and other cities, districts, villages will be developed.

The above measures should contribute to the optimization of the functioning of vehicles and the development of a network of environmentally sound modes of transport. However, today it cannot be denied that vehicles serve as a source of pollution not only of air and water, but also of the soil cover of highways and adjacent territories [13,18]. Highways are laid both within the city and through agricultural lands, covering people, flora and fauna with a veil of pollutants. Relatively untouched territories are allotted for the construction of transport infrastructure. In addition, the city is actually surrounded on all sides by operating oil fields, some of which are located within the city itself.

During the construction of new and repair of existing roads, the road dust released at the same time, the components of the exhaust gases of vehicles, negatively affect the land fund. When asphalting, the soil cover is covered with water- and air-tight materials, which negatively affects the properties of the soil [6, 9]. Most of the harmful substances emitted with the exhaust gases of cars intensively settle at a distance of up to 30 m from the main road. So, at a low speed, a car running on a gasoline engine emits 0.05% of hydrocarbons (of the total emission) into the environment, and at low speed, the emission of pollutants is 0.98%, carbon monoxide corresponds to directly – 5.1% and 13.8% [16].

Salt and other chemicals that are a component of anti-icing mixtures have a negative impact on the ecological state of highways (their concentration causes drying of green spaces along the roads). The ecological state of soils is also aggravated by the climatic conditions of the region: high temperatures in the warm season and a high dose of ultraviolet radiation cause intensive decomposition of petroleum products in soils. If we take into account the fact that 60-65% of the automobile transport of Azerbaijan is concentrated in Baku and the number of cars in the republic is steadily growing, then we can guess the consequences of pollution with harmful emissions of the environment [1].

The situation is aggravated by the fact that the share of vehicles that use less environmentally friendly gas fuel is only 5%. The European Economic Commission of the United Nations approved an environmental standard that reflects the need to comply with established standards for the content of various hydrocarbons and other harmful substances in automobile exhaust gases. To further reduce soil pollution in the Republic of Azerbaijan, the national standard “Road transport. Ecological classes. According to this standard, depending on the level of pollution, motor transport was divided into 6 classes, and from April 1, 2014, all vehicles operating on the territory of the republic must use fuel corresponding to the Euro-4 standard. Restrictions do not apply only to individual brands of cars.

The importation of a large number of vehicles into the country, the construction of filling stations and parking lots, which are the cause of the violation of the hydrological and geochemical regime of landscapes, the emissions of exhaust gases and oils into the environment by motor vehicles, have acutely posed the problem of preserving the city’s ecology. The tension of the geoecological state of the Apsheron landscapes, expressed by the degradation of the soil and vegetation cover, is also a consequence of such factors as climate aridity, wind erosion, desertification, a significant excess of evaporation over precipitation, a high level of radiation, non-leaching regime of soils [5].

On the Absheron Peninsula, mostly gray-brown soils. These soils are characterized by a grayish-brown color, granulometric composition — mainly clay, loam, lumpy structure. As a result of asphalting, the natural factors of soil formation change, for example, vegetation, relief, and climate change. Prior to laying asphalt-concrete pavements, preliminary work is carried out, such as filling the top layer of soil for surface uniformity or strengthening with concrete. These actions to some extent reduce the amount of natural urban soils, as soils are mixed, polluted with organic and inorganic substances (mainly construction waste and household waste). Soils are also contaminated with heavy metals and oil products. These soils are distinguished by low moisture capacity, stoniness, neutrality of the soil solution. The physicochemical parameters of gray-brown soils under the conditions of the Apsheron Peninsula depend on the type of soil cover structures (SCS) (Table 1) [8].

Table 1

Physical and chemical properties of gray-brown soils of the Apsheron Peninsula

The soil cover of urban areas can be considered a mirror image of the qualitative state of the city’s ecology, because soil properties affect the state of all other elements of the city’s ecosystem. The soil cover of the city of Baku, laid down in the climatic conditions of the arid region, is represented mainly by varieties of gray-brown soils, characterized by a low content of humus (1,2–1,8%), an alkaline reaction of the environment, low capacity absorption (about 20 meq. per 100 g of soil). Today, given the importance and relevance of the problem of protecting land and the urban environment, there is a need to monitor indicators of the soil cover of the territory along the roads and to study international experience in improving the ecology of urban areas.

2. Materials and methods

The main goal of the work is to study the ecological state of soils along the main transport routes of Baku (Nizami district) (fig. 1). The Nizami district of Baku city covers an area of ​​20 km²  with a population of over 182 thousand people, the total area of ​​green spaces is 40.0 ha.

Figure 1. Map of highways of Nizami district of Baku city

When conducting research, field and laboratory methods were used to study the soils of urbanized territories [15]. Soil sampling for the determination of heavy metals was carried out at sites located close to communications; if possible, samples were taken at the same temperatures of 25–280C [15]. The upper soil layer (0–10 cm) was studied. In the soil around the highway there is an accumulation of metals such as cobalt, nickel, copper, zinc, and lead. It has been established that the degree of transport pollution depends on the location of the soil relative to the road. At a distance of up to 25 m (zone 1) — the greatest accumulation of metals. The second zone (within 25-100 m) — at this distance, the accumulation of metals weakens due to the large dispersion of the air flow. Heavy metals were also found in the third zone (200 m from the highway) (table 2).

Table 2

Metal content in soils (mg/kg)

It is possible to inactivate heavy metals in the soil or to significantly reduce their toxic effect by different methods [2]. All these methods are mainly based on the use of lime and phosphate substances in combination with organic substances. For this purpose, along the main major highways of the Nizami district1, at a distance of 3–5 m from the road, soil samples were taken for research from depths of 0–10 cm. Soil sampling for analysis was carried out according to approved methods [15]. To identify the ecological state of the territories adjacent to highways, the chemical indicators of soils (determination of humus and actual acidity) and the biological activity of soils (determination of urease activity and catalase activity) were studied. Soil samples taken for research from the territories of 4 main highways in Baku differ in their degree of contamination.

Under conditions of technogenic impact on the urban environment, a distinctive humus is formed, the composition of which differs markedly from the humus of soils that are not subject to anthropogenic impact. To determine the degree of resistance of soils along highways to the impact of the emission of harmful and toxic substances, the value of humus was determined. Determination of the amount of humus, which is an important indicator of the ecological potential of soils, was carried out according to the method of I.V. Tyurin. To do this, a soil sample was sifted through a sieve with 0,25 mm holes and the soil sample was weighed on an analytical balance to 0,5 g. in a 1:1 ratio with diluted sulfuric acid. The contents of the flask were boiled for 5 min, then cooled and, after adding 10 drops of phenylanthranilic acid, titrated with 0,2 N solution of Mohr’s salt until the solution turned dirty green.

 In urbanized conditions, as a result of anthropogenic impact on the environment, there is a change in the indicator that determines important stages in the conversion of nitrogen-containing compounds in the soil [3]. As a result, there is a violation of all soil functions. Therefore, the study of urease activity, which makes it possible to assess the biochemical potential of soils, as well as their self-cleaning ability, is a very important step in identifying the ecological state of the soil. In this work, we use an express method for the determination of urease, which is advisable to carry out to assess the violation of the stages of the conversion of nitrogen-containing compounds in samples of a large volume of urban soil covers.

 For analysis in laboratory conditions, the following were used: technical scales, Petri dishes (diameter ≈10 cm), indicator paper, urea, watches. A 50 g sample of soil and 0,5 g of urea preliminarily diluted in a small amount of distilled water were placed in a Petri dish. The cup was covered with a lid (leaving a gap between them for the reaction to proceed) with a strip of filter paper impregnated with the indicator solution attached to the inside. The data resulting from the decomposition of urea, accompanied by the formation of ammonia, causing a change in the color of the indicator paper (the exposure time of the cups was 3,5 hours, the samples were taken in May), are listed in table. 3.

Table 3

Biochemical indicators of soils along the main thoroughfares of the city of Baku

Changes in soil properties also have a significant impact on such a biochemical indicator as catalase activity [14]. The activity of catalase depends on the redox processes occurring in the soil and its granulometric composition. Determination of catalase activity was carried out by the gasometric method described in [15]. The gas volume was measured after 1 min.

Intensive pollution of roadside soils of motorways causes changes in the chemical composition of soils, leading to the processes of transformation of the components of the mineral and organic parts of the soil, as a result of which a change in actual acidity occurs. For better dispersion of the soil in the aqueous solution, an electric stirrer was used while shaking the flasks. Soil acidity was determined using a pH meter.

3. Results and Discussion

As can be seen from the obtained data (Table 3), the content of humus in the soils of highways in Baku varied from 0,9 to 1,2%. This difference in the humus index is explained by the fact that a gas station is located along Rustam Rustamov Street and samples were taken close to it. The reason for the low content of humus in the soil is caused by the lack of nutrients in urbanized soils, which, in turn, leads to an acceleration of the processes of humus mineralization. The low quality of soil fertility is caused by high rates of mineralization of organic matter.

When comparing the obtained data on the activity of the urease enzyme in soils near highways, a low activity of the urease enzyme was revealed. This fact indicates that the studied soils are characterized by a high number of bacterial microflora and this, in turn, is accompanied by a low activity of soil enzymes. In the study of soil along highways with heavy traffic (Babek Avenue and Heydar Aliyev Avenue) with grass, exposed to pedestrians, exhaust gases and other anthropogenic and technogenic factors, a significant weakening of soil enzymes, which play an important biogeochemical role, was revealed. Enzymes carry out functional connections between the soil and the microorganisms inhabiting it and thereby contribute to maintaining the integrity of the ecosystem. Microorganisms, in turn, support the processes of transformation of substances that take place with the participation of various groups of enzymes.

Metabolic processes occurring in the soil are determined to a large extent by environmental conditions. Thus, as can be seen (table 3), all soil samples are characterized by a certain pH optimum. The optimal pH values ​​for ureases and catalases are in the range from 6,3 to 7,2. Changes in soil pH lead to a decrease in urease activity as a result of a reversible process representing the ionization (deionization) of acidic (basic) groups in the active site of the enzyme. It is known that enzymatic activity decreases with increasing soil salinity [7]. High levels of urease and catalase activity in the soil sample from Kara Karaev Avenue suggest that this soil is more saline than other samples.

Very high rates of catalase activity in soils along the highways of Baku (25–28) ml /1 min/1 g of soil make it possible to judge the intensity of microbial-biochemical processes, the rate of transformation of organo-mineral compounds, and, as can be seen (table 2 ), an increase in the lead content in the first zone (26 mg/kg). Thus, the conducted studies showed that the emissions of road transport, settling along the road as a whole, increased the activity of the studied soil redox enzymes. The manifestation of a high indicator of catalase activity at a distance of 5 m along the road is a consequence of the sedimentation of pollutants on the surface of the soil cover.

The high value of active acidity is the result of the multiplication of Azotobacter microorganisms. The limiting factor in the presence of alkali-resistant Azotobacter microorganisms in the soil cover may be the presence of a large number of toxic substances in it. Alkalinization of the soil along highways is usually the result of the penetration of calcium and magnesium chlorides into the soil, which are an integral component of the means sprinkled on the soil along sidewalks and roads in the winter season to prevent highways from being covered with ice. Considering that concrete and asphalt concrete are considered conditionally impervious coatings, and with poor-quality road surface, harmful components, being washed out, seep into the depths of the soil cover. In addition, soil alkalinization occurs due to lime dust settling when roads are covered with cement, which also contains calcium.

4. Measures to reduce and prevent soil pollution along highways

There is a direct relationship between the speed of a car and the emission of harmful substances into the environment: the higher the speed of a car, the more toxicants are emitted onto the roads (Table 4). Consequently, highways with high-speed traffic are considered the most polluted. The problem of pollution of highways with harmful emissions (changes in various soil properties, migration of pollutants, pollution of groundwater, etc.) is relevant for specialists in various fields of knowledge and encourages them to look for methods to solve it [19]. To date, effective biological products are being developed to increase the biological activity of soils at the microbiological and enzymatic levels [10].

Table 4

Approximate amount (in%) of components of exhaust gases under various modes of operation of vehicles

According to the recommendations of experts, the most convenient, promising and economical way is to divide the highway into separate sections, taking into account the objects adjacent to the highways. Technological solutions used in Germany and Austria (accumulative treatment facilities with deep wastewater treatment, flow-through facilities for oil products, sand and large debris, filtering treatment facilities to prevent soil erosion and clean up surface runoff) for wastewater treatment help prevent environmental pollution from highways and nearby territories [9]. Therefore, when designing new highways, building and expanding them, such technical solutions for treating main sewage should be adopted. However, all these methods require large capital costs.

Particular attention should be paid to sections of highways with light soil. It is from these areas that the unhindered penetration of toxicants into soil and underground horizons occurs, which are almost impossible to clean. A decrease in the intensity of pollutant migration in the soil environment can be embodied by an increase in the buffer capacity of soils. Given the threatening environmental conditions on the highways of Baku, it is recommended to apply organic and mineral fertilizers to improve soil fertility.

Preservation of soil ecology along highways is possible by improving the development of road transport [4]. One of the options for this kind of action can be the improvement of internal combustion engines of vehicles without high-speed and expensive re-equipment of the enterprises producing them, as well as the transition of vehicles from a fuel-based option to an electric one, which excludes the release of hydrocarbon combustion products into the environment. Such shortcomings of cars as a short range, lack of charging are being solved today.

Another option is to improve transport infrastructure. A renovation solution for improving the transport infrastructure of Baku is the development of an environmentally friendly way of traveling by bicycle and tram, which are widely used abroad. For example, as a result of work on organizing lines of environmentally friendly modes of transport in Canada by 2031, it is expected to increase the passenger flow of cycling by 2,4 times compared to 2011 [12]. An example of the introduction of lines of environmentally friendly modes of transport is the organization in 1985 of the green corridor of the tram line in Nantes in France, the construction of a multimodal station in Melbourne in Australia and the construction of multimodal traffic lanes in Mannheim in Germany. The initial stage of the development of the tram network in Baku is the construction of a tram line along the embankment, the construction of railway lines to the Baku International Airport. Heydar Aliyev and the tourist centers of Shahdag and Gabala [12]. The successful implementation of street improvement (the concept of the development of multifunctional streets) can also have a positive environmental effect [11].

The environmental problem can also be solved by reducing the weight of the car. There are more than 1 million vehicles of various types in the Republic of Azerbaijan. If we take into account that each of them during the year is a source of pollutants emitted into the urban environment, equal to their weight, and the weight of one vehicle, for approximate comparison, is taken as 300 kg, then over 300,000 tons of pollutants are emitted into the atmosphere of Baku per year . Reducing the weight of the car, and as a result of this, less fuel consumed are conditions for reducing the environmental damage caused to the environment. All of the above predictive solutions, combined with the successful implementation of the concept for the development of multifunctional streets, will help improve the environmental situation in general and, in particular, improve the condition of the soil cover of one of the most beautiful cities — Baku.

5. Conclusions

A certain dependence of the biochemical parameters of soils on their location along the highways of Baku has been established. Experimental data on the study of the effect of heavy road traffic on the activity of soil enzymes revealed high levels of urease and catalase activity in soil samples from the studied zones of the Nizami district of Baku and, as a result, relatively low biological activity. Foreign experience in the implementation of a set of measures to improve the ecology of urban areas has been studied. Studies of international experience and analysis of the results of the studies made it possible to determine the degree of importance and the need for the speedy implementation of measures to improve the ecology of soils in Baku.