Introduction: Today, the production of plastic in the world is more than 400 million tons per year. This massive volume of plastic, in the form of various products from kitchen appliances to industrial and agricultural products, is growing rapidly. According to the statistics presented in 2020, about 9% of the plastics produced in the world are recycled. Therefore, most plastics remain persistent in the soil or in the seas as plastic waste. Microplastics (MPs) particles less than 5 mm in diameter originate from the breakdown of larger plastic debris such as plastic bags, bottles, and packaging materials. MPs, as an emerging contaminant, have gained worldwide attention. Unfortunately, current recycling methods have failed to provide a comprehensive solution to plastic waste disposal. However, in the past decade, most research on the effects of MPs pollution has focused on marine ecosystems, while very limited research has focused on terrestrial ecosystems. Soil ecosystems, especially agricultural lands, are known as the main reservoir of MPs but the effects of MPs on soil ecosystems remain largely unknown.  Soil acts as a significant reservoir for MPs and can have substantial impacts on soil quality and fertility. Upon entering soil, MPs  can potentially threaten soil health. MPs can interact with soil particles and organic matter and affect soil structure, nutrient cycling, and microbial communities. Studies have shown that MPs can alter soil microbial communities, potentially leading to changes in ecosystem functioning. MPs in the soil act as a means of absorbing and transporting pollutants. They transport agricultural chemicals, heavy metals and pathogens deep into the soil. The impact of MPs on soil organisms and ecosystems is not yet fully understood, and more research is needed to assess the extent of the problem and its potential consequences. They can negatively impact soil function and fertility by disrupting the physical, chemical, and biological properties of soil. A deeper understanding of how MPs  enter, distribute, and accumulate in soils, as well as their impacts on various soil functions, is essential for developing effective strategies to manage and mitigate MPs  soil pollution. Therefore, the purpose of this research was to determine the distribution of MPs in the deep soil and to identify the structure of MPs and the extent of their effects on some chemical and biological properties in Ahvaz urban waste landfill.
Materials and Methods: To investigate the impact of MPs on some chemical and biological properties of soils in a municipal landfill, a factorial experiment was conducted in a randomized complete block design. The experiment consisted of 18 experimental soil units with three replications. Soil samples were collected from three depths (0-10, 10-20, and 20-30 cm) from each of five municipal landfills using a hand auger. Additionally, soil samples from the same depths were collected from a control area without any waste landfill. Chemical and biological characteristics of soils, including soil salinity, soil acidity, soil organic matter, soluble cations and anions, total nitrogen, available phosphorus, soil lime content, microbial respiration, soil microbial biomass, extraction and identification of MPs based on standard laboratory methods were measured. Identification of MPs using FTIR analysis was considered as a crucial step in this study. The experimental design consisted of two factors: landfill area (landfill vs. control) and soil depth (three levels). The experimental design was done in a factorial form in a randomized complete block. Comparison of average data was also done using Tokay’s 5% probability level test, data results were statistically analyzed with SAS software and graphs were drawn in Excel.
Results and Discussion: The findings of the present study revealed a significant positive correlation between landfill areas and the control area. This indicates that landfill activities have a substantial impact on the concentration of MPs in their surrounding environment. In the soils near the municipal landfills, extremely high levels of MPs were encountered, with up to 4300 MPs pieces per kilogram of soil. This level of MPs contamination indicates severe soil pollution in these areas. Further analysis of the identified MPs revealed that two polymers, polyethylene (PE) and polypropylene (PP), accounted for a major portion of this contamination. Specifically, 71.81% of the MPs studied were PE, 17.15% were PP, 3.11% were polystyrene (PS), and 8.21% were polyvinyl chloride (PVC). This suggests that plastic materials, particularly plastic bags, bottles, and other items made of PE and PP, play a significant role in MPs soil pollution. In addition to physical effects, MPs can also have detrimental consequences for the biological and chemical properties of soil. The results showed that there is an inverse relationship between soil pH and the number of MPs, which means that as the number of MPs increases, the pH level decreases. But this relationship is not the same in all regions and at all depths. An increase or decrease in soil pH is probably due to the release of alkaline or acidic components from MPs. Soil salinity in areas one to five is higher than the control area. In some areas, at the same time as the number of MPs decreases with increasing depth, soil salinity increases and in others it decreases. The amount of soil organic matter in the areas with MPs pollution was significantly higher than the control area. The results showed that the presence of MPs in the soil is associated with a significant increase in the amount of total nitrogen in the soil. The reason for this increase can be related to the effects of MPs on microbial activity and biochemical processes in the soil. The average microbial respiration in the control area is 261 mg C-CO₂/Kg soil, which is 38% lower than the average microbial respiration in areas contaminated with MPs. The average microbial biomass in the control area was 73.7 mg C/Kg soil, which is 51% less than the contaminated areas. A significant increase in microbial biomass in soils contaminated with MPs indicates an increase in the population of microbes, which can be due to the efforts of microbes to decompose MPs. This study demonstrated that MPs , as a major source of pollution in municipal landfills, can lead to significant changes in the chemical and biological characteristics of soils. These changes can negatively impact soil fertility, biodiversity, and the health of soil organisms. MPs might sorb (adhere) to nutrients and organic matter, altering their availability to plants and soil microbes. Additionally, the breakdown of MPs could release chemicals that indirectly affect soil chemistry.
Conclusion: The effects of MPs on soil chemical properties resulted in significant increases in pH, EC, calcium and magnesium, soil organic matter, phosphorus, and total nitrogen. Soil organic matter, phosphorus, total nitrogen, and salinity increased by 3.4, 2.2, 7.2, and 2 times, respectively. The presence of MPs increased microbial respiration and microbial biomass in the surface soil, but at lower depths, decreased due to excessive salinity. Overall, this study demonstrates that MPs can have substantial effects on soil chemical and biological properties.