Investigating the Effect of Air Pollution on the Accumulation of Heavy Metals in Plant Species in Arid and Desert Areas (Case Study: Acacia, Prosopis and Ziziphus C, in Kahouristan, Hormozgan Province)

Acacia trees, whose leaves and fruits are used for livestock, are a bio accumulator of heavy metals and although they play an important role in absorbing heavy metals from the soil, their fruits are used for livestock consumption. Prosopis trees with their long and strong roots have the ability to absorb heavy metals from the environment and researchers use them as biomarkers for heavy metals as well as phytoremediation. Although phytoremediation is a good way to remove heavy metals from the environment, it cannot be a good way to protect the environment if the plant is a source of nutrition for living organisms. In arid and semi-arid regions of Hormozgan province, the plant organs of Prosopis, Acacia and Ziziphus trees, especially their leaves and fruits, are considered as the food chain of local and wild cattle in the region. If these plants are contaminated with heavy metals and their toxicity is high, they endanger living organisms. Nevertheless, the amount of heavy metal contamination in these plants has not been identified and experts do not have information in this field to be able to develop a plan to control it. If these trees are contaminated with heavy metals, it is not clear how the contaminant accumulated in the plant and the sources of its spread are not known. For this reason, the present study tries to answer these leading questions and it is assumed that air pollution, especially desert dust and road dust is the main cause of heavy metal accumulation in trees. Therefore, the main purpose of this study was to detect the concentration of heavy metals in the leaves of tree species through atmospheric dust and the intended experiments were carried out in the natural forests of Kahouristan region.

In order to achieve the objectives of the study on identifying the concentration of heavy metals in tree species in arid areas of Bandar Abbas, first the location and population of the experiment was determined. The test site is in Kahouristan region -90 km northwest of Bandar Abbas- in Hormozgan province with a geographical position of 27 degrees and 17 minutes north latitude and 55 degrees and 21 minutes east longitude with an altitude range of 250 to 320 meters above sea level. The experiment was conducted in the spring and summer of 2020. The experimental population was selected in a randomized complete block design. The community contains 180 trees, including the species Acacia, Ziziphus and Prosopis. 60 trees were considered from each tree species. On these trees, three treatments were considered in such a way that each treatment included 60 pieces, of which 20 trees were considered for each treatment: 1-Treatment of road dust and desert dust in Acacia, Ziziphus and Prosopis trees located near the Bandar Abbas-Lar transportation road and solid particles emitted from the exhaust of vehicles are deposited on the leaves and soil substrate of the plant and absorb the roots of the trees. 2- Treatment of desert dust that is far from the road and only desert dust settles on the soil and leaves of trees. 3- Control treatment which in terms of distance is far from the road, after the treatment of desert dust and the trees in this treatment are located in the mountain shelter and away from the path of desert dust. Each treatment contained 60 pieces, of which 20 were related to Acacia, 20 pieces of Ziziphus and 20 pieces of Prosopis. Therefore, 180 specimens containing three tree species (blocks) and three treatments formed the experimental community. After introducing the experimental population and identifying the treatments and tree species, the concentrations of heavy metals in the leaves of the experimental treatments and trees were measured. The measurements were taken in two seasons, spring and summer, when desert dust occurs. From each tree, 5 leaves were sampled and transferred to the laboratory after labeling. In the laboratory, the concentrations of heavy metals in leaves including lead (Pb), cadmium (Cd), zinc (Zn) and manganese (Mn) were measured.

 Statistical analysis of heavy metal concentrations in the experimental population shows that the average concentration of lead, manganese and cadmium in the experimental population is higher than the standard limits in the plant, but the concentration of zinc is less than the allowable limit. The concentration of zinc in the experimental community is such that the average is 209 mg/kg, the standard in the plant is 100 to 400 mg/kg, but its distribution in the experimental community is normal. The maximum accumulation in the leaves of dust treatment is due to road transport and the fume that is emitted from car exhaust and its average in this treatment is between 300 to 400 mg/kg, which is close to the upper limit of the standard of this element in a plant. In the treatment of desert dust, the concentration of zinc is between 180 to 220 mg/kg, which in honeysuckle species has a higher concentration than acacia and Ziziphus. The lowest concentration of zinc in the control treatment was between 70 and 80 mg/kg and even lower than the plant standard. Statistical analysis of lead concentration in the experimental community showed that its average was 35 mg/kg, which is higher than the permissible limit in the plant. The highest concentration of lead was in the trees near the road and in the treatment of dust caused by road transport and its deposition on the leaves and soil substrate of plant species and its concentration was between 50 to 60 mg/kg. The maximum was in the species of Prosopis, which has accumulated more lead metal than acacia and Ziziphus. In the treatment of desert dust and its deposition on the surface of leaves and soil of trees, the concentration of lead was between 25 to 40 mg/kg and was in the second place, and among the tree species, Prosopis has a greater ability to withstand the adsorption and accumulation of lead metal compared to acacia and Ziziphus. The average of manganese metal is 115 mg/kg, which is higher than the permissible limit in the plant (15 to 100 mg kg). The highest concentration of manganese in the dust treatment was due to road transport and its concentration was between 145 to 155 mg/kg, the maximum of which was in the species of Prosopis, which is more concentrated than acacia and Ziziphus, manganese metal. Manganese concentrations are close to each other between road dust treatment and desert dust, but their differences with the control are very noticeable. The standard range of cadmium is between 0.2 and 0.8 mg/kg, the average of which in the whole study population is 2.93 mg/kg, with a maximum of 13.5 mg/kg, and at least 0.02 mg/kg in control treatment. Comparison of the mean of treatments showed that the highest concentration of heavy metals in the dust treatment was due to road transport and the concentration of metals in this treatment was different from other treatments and was in the first category. Trees affected by desert dust were in the second category in terms of heavy metal concentrations and the difference with other treatments was significant. Finally, the control trees have the lowest concentration of heavy metals and are located in the last floor. In the treatment of road dust particles, the concentration of all heavy metals was higher than the standard range of metals in the plant. In the treatment of desert dust, the metals of lead, manganese and cadmium were higher than the standard; however, in control trees, the concentration of lead was higher than the standard and the concentration of other heavy metals was less than the standard. The highest concentration of heavy metals was in the Prosopis tree species, but in Acacia and Ziziphus tree species, the concentrations of heavy metals were not statistically different and were very close to each other.

 The results of this study showed that the trees along the Bandar Abbas-Lar Road have been exposed to toxic dust from vehicles. Farther from the road, trees, although far from road dust, are exposed to atmospheric dust from industry as well as dust storms. The concentration of heavy metals in the leaves of roadside trees and then exposed to fine dust is much higher than the standard. Also, among the species, Prosopis trees, due to their long and strong roots, have the ability to absorb heavy metals from the soil and in the leaves of these trees, especially on the roadside and exposed to fine dust, cadmium concentration, lead and manganese are very high and cause heavy metals to be removed from the environment, but the leaves and fruits of Ziziphus, Prosopis and Acacia trees are one of the important food sources for cattle and animals and even birds of Kahouristan region. The accumulation of heavy metals in the limbs of these trees and their transfer from plants to animals and eventually humans is a threat to ecology. Therefore, it can be concluded that toxic road dusts and atmospheric dust have caused the deposition of heavy metals in the soil bed of annual and herbaceous plants and trees of the region, which are absorbed through the roots of the plant and thus enter the food cycle. It becomes a trap for the existing living and nonliving species and human beings. The results of this study showed heavy metal pollution in the dry ecosystems of Kahouristan area and the results can be made available to environmental and health experts. In addition, it is suggested that the concentration of heavy metals in annual herbaceous plants, which are the food of local livestock in the region, be evaluated in order to find a way to control their spread and contamination of environment.