Accumulation of heavy metals in soil and litter of roadside plantations in Western Polissia of Ukraine

Environmental pollution by harmful emissions from road transport is currently a global problem. The usage of road transport for various purposes is growing every year all over the world, which has a corresponding effect on the level of environmental pollution, especially air pollution. This problem is especially relevant in densely populated cities, where congestion is much higher compared to suburban areas (Arkhipova et al. 2009).

The transport and road complex of Ukraine is one of the largest sources of environmental pollution (noise pollution and heat are the most common). There is also a steady increase in the usage of road transport for various purposes despite the crisis and the declining population in the country. This leads to congestion of the road network of urban areas and exacerbates the socioeconomic, sanitary and technical problems associated with human health and traffic organisation (Baliuk et al. 2004). The vast majority of highways pass through settlements, which lead to restrictions the speed of road transport and they do not meet the requirements for international transport corridors.

The impact of road transport on the sanitary condition of soils along the roads is significant and determined by the increased content of heavy metals on the soils adjacent to the roads. In particular, French scientists have proposed, based on the operation of roads that have undergone a 20-year service cycle, a set of specific indicators to assess the impact of road transport (Jullien and François 2006).

Quality control of areas adjacent to highways and the environment is an extremely important task, as urban areas, fields and many other facilities are vulnerable to pollution. There are more than 100 different components in the harmful emissions (exhaust gases) of automobile engines, the vast majority of which are toxic (Jovic et al. 2012). These include soot and carcinogens, aerosols, nitrogen oxides, sulphur and lead compounds and many others. The consequences of the impact of such emissions are pollution of the atmosphere and water bodies, changes in the chemical composition of soils, creation of high levels of noise and vibration, increased morbidity of humans and animals, and so on.

Heavy metals formed by traffic and road maintenance cause chronic pollution in water runoff and roadside soil. Some of the pollutants can be sprayed into the atmosphere or deposited on the soil by wind. Sediments affect the quality of roadside soil and their content in the soil gradually decreases, reaching the background level at a distance of up to 25 m from the road. Galvanised road barriers can be a source of pollution, in addition to cars (Legret and Pagotto 2006).

Heavy metals are given considerable attention in the research of various components of harmful emissions from road transport. It should be noted that heavy metals include elements of the periodic table with metallic properties, which have an atomic mass of more than 40 atomic units and do not belong to the radioactive elements. As trace elements, they are actively involved in important biochemical and physiological processes in plants and their high concentrations can cause toxic effects.

Heavy metals are characterised by high toxicity, the ability to accumulate, sedentary nature and high stability in the environment in comparison with other pollutants. Harmful effects of such substances reduce the natural stability of biological objects (Deurer and Bottche 2007; Olaniran et al. 2013). The accumulation of heavy metals in living organisms can lead to activation of various chemical processes and can cause damage to cells, tissues or individual organs and parts of the plant, which can adversely affect the state of the whole organism (Hazrat et al. 2019).

Heavy metals are also considered in the agro-ecological aspect because under the action of air currents, pollutants fall on the adjacent field lands and are characterised by the property to accumulate in crops. Consumption of such crops by humans can lead to various diseases (Foy et al. 1978; Reimers 1992; Grigalavicienė et al. 2005). Numerous studies have shown that due to accumulation in the environment, chemical elements such as lead (Pb), cadmium (Cd), zinc (Zn) and copper (Cu) pose a significant danger based on their high biological activity and toxic properties (Khan et al. 2004; Almeida et al. 2007).

It should be also noted that copper and zinc are biogenic metals (bimetals) and they are necessary to ensure the viability of plants. Such elements are concentrated in the chloroplasts of plant leaves and are directly related to the process of photosynthesis. Normal amounts of copper and zinc have a positive effect on the biological resistance of plants to low and high temperatures and soil salinity. Copper can, to some extent, affect the uptake of nitrogen by plants, and zinc enhances the growth and development of the root system and is important for plant fruiting processes (Skarlyna-Ufimtseva 1980; Barbieri 2016; Iacoban et al. 2019).

Thus, copper and zinc are part of living organisms and are directly involved in metabolic processes, but excess amounts of these cause toxic reactions and may increase the impact of other harmful substances (Titov et al. 2007). Zinc is characterised by low toxicity, but becomes toxic with increasing concentrations, which manifests itself in the form of interveinal chlorosis, marginal chlorosis and leaf necrosis. Excess copper inhibits the growth processes of the root system and the formation of shoots, and the leaf surface becomes dark green (Akpor and Muchie 2010).

Lead and cadmium have carcinogenic properties and are among the most toxic pollutants (Seregin and Ivanov 2001). They are among the most dangerous pollutants due to intensive and widespread use in industry and transport, and their half-life is 10–30 years.

The presence of lead compounds in the air and soil along the roads is due to harmful emissions from gasoline engines running on leaded gasoline. Ethyl liquid is burned in the combustion chambers of engines and inorganic compounds such as oxides and salts are formed, which are released into the atmosphere in the form of aerosols. Suspended particles are sprayed into the environment. The vast majority of lead compounds settle on the surface near highways. Aerosols can enter the human body during respiration, through the skin or with food. They lead to dysfunction of the digestive system, neuromuscular system and brain. In plants, excess lead causes inhibition of growth processes, reduced intensity of photosynthesis, physical deformation of leaves and brown colouration of root systems. Excess cadmium causes development of chlorosis in plants, deformation and discolouration of leaves and inhibition of root growth (Arkhipova et al. 2009; Fateeva and Pashchenko 2003).

In order to protect the areas adjacent to the roads from harmful phenomena, as well as for the aesthetic design of the area, roadside forest plantations are created. In this context, the harmful phenomena include the following: snow and soil during snowstorms and strong winds, water erosion of the soil and contamination of the territory with heavy metals, dust, weathering products, acoustic impact, etc. (Agroforestry 2010). Such plantations should be formed from biologically stable and durable species of woody plants; they resist snowbreaks, should fully retain within the calculated width of the land strip blizzard, retain soil and dust during strong winds and dust storms, accumulate harmful emissions from road transport and reduce noise pressure (Spiegel 2002).

In this context, the sanitary-ecological function of roadside forest plantations is important, which is, first of all, retention and absorption of hazardous substances by tree species as well as prevention of their further spread. Such plantations perform the functions of chemical pollution accumulators and have high phytomeliorative properties in the conditions of heavy metal pollution (Järup 2003; Ali et al. 2013).

The purpose of the research was to determine the level of soil and forest litter contamination with heavy metals, which is mainly due to the harmful emissions from road transport in the area protected by roadside forest plantations of different species composition.

It is known that the entry of heavy metals into the soil determines the possibility of their further migration into groundwater and availability to plants, and it poses a potential threat to living organisms. At the same time, soil is one of the important protective biochemical barriers for a number of compounds during their migration to groundwater and plants. The content of mobile forms of heavy metals in the soil, depending on the composition of roadside belt plantations and the place of sampling is given. The concentrations of heavy metals in terms of test areas are shown in Figure 2.

Figure 2

The data of Figure 2(A) and (B) show that linden plantations accumulated lead and cadmium the most and maple forest belt retained the least. At the same time, hornbeam plantation most effectively retained zinc and copper, as evidenced by the data of Figure 2(C) and (D), respectively. The same metals were accumulated less by the birch forest belt on the E373 highway. Figure 2(C) and (D) shows that there was a significant increase in the concentration of heavy metals in birch plantation (test area 4) compared to other plantings, but the permissible level was not exceeded. It can be assumed that this trend was caused directly by the functioning of linear plantations themselves.

Based on the data shown in Figure 2, there is a dependence on the fact that the content of heavy metals from the edge of the belt on the side of the road to the opposite edge decreased in most measurements. This dependence is explained by the partial influence of the distance factor from the source of pollution, as well as the direct influence of protective forest belts.

The content of heavy metals was higher on the highways of international importance than on the national roads, which is associated with a decrease in traffic on the latter. This trend is confirmed by the studies of Degórski and Zawiska (2015), Clemens (2006) and others.

Polish researchers studied the content of heavy metals (Ni, Pb, Cu, Zn, Cd and Cr) in soils at the places along roads with different traffic intensity, different capacity and smoothness of road transport. Samples were taken along transects perpendicular to the road edge, at a distance of 1, 3, 5, 10, 20, 50 and 100 m from the road edge. Scientists have determined that the content of heavy metals in soils between the individual analysed segments can reach a ratio of more than 100 times. It was found that the content of heavy metals in soils decreased with distance from the road and was characterised by a more regular distribution on the sections along national roads and a less regular distribution on the sections along highways (Degórski and Zawiska 2015).

Since soil is a multicomponent system and can be considered from different points of view, regulation of the content of pollutants in it and plants is quite a difficult task. This is due to the inability to fully take into account all the environmental factors that directly affect the migration and accumulation of pollutants. In particular, the issue of rationing the content of heavy metals in the soil is quite difficult to solve, and the basis of this problem should be recognition of the diverse functions of soil ecosystems.

The establishment of levels of man-made contamination of soils and forest litter with heavy metals and comparison of the results can also be influenced by a number of objective factors, namely, local background content of individual elements, traffic intensity, climatic conditions, vegetation period, etc. (Fateeva and Pashchenko 2003).

To assess the degree of danger of the pollutant element in the research, we used the concentration coefficient – the ratio of pollutant content in the soil (average of all sampling sites) to its maximum allowable concentration. The following values of the maximum allowable concentrations of mobile forms of heavy metals in the soil were taken according to the general sanitary limiting indicator: Cu – 3.0 mg · kg^-1, Pb – 6.0 mg · kg^-1, Cd – 0.7 mg · kg^-1 and Zn – 23.0 mg · kg^-1 (Fateeva and Pashchenko 2003). The average content of heavy metals and the coefficient of their concentration in the context of the experimental plots are given in Table 2.

The obtained values of the coefficient of concentration of heavy metals in the soil of roadside forest belts are given in Figures 3 and 4.

Figure 3

Figure 4

The average content of heavy metals in soil samples selected in roadside plantations along roads of international importance showed the accumulation of zinc (3.70 mg · kg^-1) and copper (0.85 mg · kg^-1) in birch belts. The greater concentrations of lead (1.14 mg · kg^-1) and cadmium (0.15 mg · kg^-1) were fixed in maple plantations. In other roadside belts, the pollutant concentrations occupied an intermediate position.

Studying the content of heavy metals in soil from the roadside forest belts of the Vilnius–Klaipeda highway, Grigalavicienė et al. (2005) found that the content of Pb, Cu and Cd in the soil behind the forest belt was 7.9, 1.32 and 1.19 mg · kg^-1, respectively. Such high concentrations of heavy metals are due to heavy traffic on the highway.

The content of heavy metals in the soil under roadside forest belts along the roads of national importance had the following values: lead – 0.73–0.81 mg · kg^-1, cadmium – 0.05–0.07 mg · kg^-1, zinc – 1.12–1.39 mg · kg^-1 and copper – 0.29–0.37 mg · kg^-1. We did not find a clear relationship between the composition of plantations and the content of pollutants. Thus, in the soil of maple plantations, there was a higher content of zinc and copper, and in the soil of poplar plantations, lead and cadmium were high in content.

General analysis of the coefficient of concentration of heavy metals in the soil of roadside forest belts of different species composition along roads of international importance showed that its value did not exceed the maximum allowable concentrations and was on average within 0.10–0.20 of these indicators.

The coefficient of lead concentration had approximately the same value (0.12–0.14) for roadside forest belts of hornbeam, linden and birch.

The concentration coefficient in the soil of cadmium was characterised by a value of 0.10 for roadside belts of linden and birch and 0.16 for roadside belts of hornbeam. A slight increase in the concentration coefficient in the maple plantation to 0.21 was also observed.

The zinc concentration coefficient was lower than that of other metals. Its value was in the range of 0.05–0.09, and in the conditions of the roadside belt of birch, it was 0.16. An almost similar pattern could be traced for the coefficient of copper concentration. Mainly, this indicator was 0.08–0.11, and in the conditions of the birch belt, it was 0.28.

As mentioned above, roads of national importance are characterized by lower traffic intensity, which in turn leads to a slight decrease in the concentration of heavy metals in the soil of roadside forest belts. The belts of maple and poplar were selected for comparative analysis. The distance from the road to the first row of belts was 8–10 m (Table 1). The results of the research show that the coefficient of lead concentration was in the range of 0.12–0.14, cadmium – 0.07–0.10, zinc – 0.05–0.06 and copper – 0.10–0.12. Based on the obtained data, it can be noted that the composition of roadside plantations, their width and structural features do not have a significant impact on the value of the studied indicator.

Forest litter is known to be formed as a result of accumulation of a layer of vegetation and litterfall on the soil surface, which is at different stages of decomposition. In the process of annual accumulation of plant litterfall and its gradual decomposition, the litter forms a layered structure. Since the maximum permissible concentrations of chemicals have not been set for forest litter, we will limit ourselves only to the analysis of heavy metal content in this element of forest plantations (Tab. 3).

Analysis of the content of heavy metals in the forest litter of roadside forest belts along highways of international importance showed a very significant difference in their values compared to the soil. Thus, the concentration of zinc was at the level of 13.20–15.09 mg · kg^-1, copper – 2.24–3.62 mg · kg^-1, lead – 0.35–0.97 mg · kg^-1 a nd cadmium – 0.02–0.04 mg · kg^-1. Forest litter of birch stands was characterised by a higher content of lead, cadmium and copper and that of hornbeam by zinc.

This difference in performance that depends upon the composition of plantations is quite significant. For example, the lead content in the forest litter of the roadside belt of hornbeam and birch was 0.35 and 0.97 mg · kg^-1, respectively. A similar pattern, but in a smaller ratio, was observed for the content of copper (in hornbeam plantations 2.24 mg · kg^-1 and in birch 3.62 mg · kg^-1) and cadmium (0.02 and 0.04 mg · kg^-1, respectively). Forest litter of roadside belts of maple and linden occupied an intermediate place according to these indicators.

Since forest litter is formed mainly from the leaf litter of woody plant species, it can be assumed that the leaves of birch have a greater ability to accumulate heavy metals compared to other species. Hornbeam leaves were found to have the lowest accumulative capacity in this respect. The results of the analysis allow us to conclude that the content of heavy metals in forest litter largely depends on the species composition of roadside forest belts.

The content of heavy metals in the forest floor of roadside forest belts along the roads of national importance has the following values: lead – 0.65–0.85 mg · kg^-1, cadmium – 0.02–0.03 mg · kg^-1, zinc – 13.15–14.01 mg · kg^-1 and copper – 2.38–2.66 mg · kg^-1 (Tab. 3). It should be noted that the forest litter of gray poplar is characterised by a slightly higher content of lead, cadmium and copper and of maple by zinc.

Comparative analysis of the content of heavy metals in these components of the plantation is important from the point of view of the interaction of soil and forest litter. Thus, there is no clear dependence on the lead content: in some plantations (hornbeam, maple), its content is higher in the soil, and in others, it is almost the same. The content of cadmium is 2–5 times higher than in forest litter. The content of zinc and copper is inversely related. The content of zinc in 4–12 times and copper is 4–10 times more in forest litter, compared to soil. One of the reasons for this ratio, in our opinion, is fixation of heavy metals in the organic matter of this substrate due to the formation of complex organomineral compounds – chelates.

Thus, it can be stated that the studied roadside forest belts in the conditions of Western Polissia of Ukraine perform important functions regarding the accumulation of heavy metals in the soil and forest litter. Cadmium has the lowest concentration and zinc the highest (especially in forest litter), of all the pollutants studied. Lead and copper occupy an intermediate position in this indicator. Despite the different species composition of plantations, the coefficient of concentration of heavy metals in the soil does not exceed the maximum allowable concentrations and is on average in the range of 0.10–0.20 of these indicators.

The sanitary-ecological function of roadside forest plantations is important, which consists, first of all, of retention and absorption of hazardous substances by the tree species of plants, as well as preventing their further spread. Such plantations perform the functions of accumulators of chemical pollution and have high phytomeliorative properties in conditions of environmental pollution by heavy metals.

In most measurements, there is a dependence on the fact that the content of heavy metals from the edge of the belt from the side of the road to the opposite edge decreases. This dependence is explained by the partial influence of the distance factor from the source of pollution, as well as the direct influence of protective forest belts. At the same time, the content of heavy metals decreases accordingly in terms of the values of highways, which is caused by the higher intensity of traffic on roads of international importance.

General analysis of the coefficient of concentration of heavy metals in the soil of roadside forest belts of different species composition along roads of international importance showed that its value does not exceed the maximum allowable concentrations and is on average within 0.10–0.20.

The content of heavy metals in the forest litter of roadside forest belts along highways of international importance showed a significant difference in values. Thus, the concentration of zinc was at the level of 13.20–15.09 mg · kg^-1, copper – 2.24–3.62 mg · kg^-1, lead – 0.35–0.97 mg · kg^-1 and cadmium – 0.02–0.04 mg · kg^-1. Forest litter of birch plantations was characterised by a higher content of lead, cadmium and copper and hornbeam plantations by zinc.