The Impact of Microplastic on Selected Soil Physical and Chemical Properties

Document Type : Research Paper

Authors

1 Department of Physical Geography, Faculty of Earth Sciences, Shahid Beheshti University, 1983969411 Tehran, Iran

2 Department of Physical Geography, Faculty of Earth Sciences, Shahid Beheshti University.

3 Department of Chemical Engineering, University of Science and Technology of Mazandaran, School of Geography, Earth and Environmental Sciences, University of Birmingham,

Abstract

The use of plastics has become highly prevalent in modern human life, making their elimination nearly impossible. Plastics break down into smaller microplastics particles due to environmental factors, eventually entering the soil. Microplastics alter the physical, chemical and biological properties of soil. In this study, the effects of polyethylene terephthalate (PET) microplastics on certain physicochemical properties of soil were investigated in rangeland and agricultural lands within the Nahrmian watershed. Experimental plots were constructed, and PET microplastics smaller than 500 micrometers were added to the soil. Three stages of irrigation and tillage were conducted in agricultural plots, while three rainfall events and three tillage stages were performed in rangeland plots. Parameters such as clay, silt, and sand percentages, pH, sodium (Na), potassium (K), electrical conductivity (EC), and organic carbon (OC) were measured before and after the addition of microplastic particles and throughout the irrigation, rainfall, and surface tillage processes. In the rangeland plots, after the introduction of microplastics into the soil, all chemical properties examined except for pH showed non-significant increases. Clay content increased by 2%, and silt content increased by 0.19%, while sand content decreased by 6.04%. In agricultural plots, pH (t-value = 4.3, p = 0.002) and K (t-value = 5.9, p = 0.001) increased significantly, whereas Na and OC showed non-significant increases, and EC decreased. The percentage of soil clay (t-value = 3.99, p = 0.003) decreased significantly, while silt (t-value = 2.33, p = 0.042) and sand (t-value = 4.93, p = 0.001) percentages increased significantly. Due to the flood irrigation method used in agricultural plots, most changes in soil texture were attributed to surface erosion, and the addition of PET microplastics did not result in significant alterations in soil texture. This was further supported by the lack of significant changes in soil texture in rangeland plots. Therefore, the effects of microplastics on soil texture require more extensive research. However, the introduction of microplastics into soil affects its chemical properties over time, with the extent of these effects varying based on land use type and specific soil conditions.

Keywords

Main Subjects

Introduction:

Plastics are present in the environment in three primary size categories: macro, meso, and micro. Microplastics (MPs) primarily enter the environment through two pathways: as primary MPs, which are intentionally manufactured for use in products such as cosmetics, and as secondary MPs, which result from the degradation of larger plastic items due to environmental factors like ultraviolet (UV) radiation, water, and wind erosion. Beyond aquatic environments such as oceans and lakes, soil serves as a significant repository for MPs. The composition and properties of microplastics can vary widely, influencing their interaction with the soil environment. The accumulation of MPs in soil has substantial implications for its physical and chemical characteristics, impacting soil quality and functioning either directly or indirectly. The investigation of the effect of MPs on the physicochemical characteristics of soil is primarily conducted in laboratory and pilot-scale environments, with few studies performed at the field or landscape scale. Additionally, there is limited research on this issue across different land uses. Polyethylene terephthalate (PET) is one of the materials used in the production of agricultural irrigation pipes and other products such as Disposable plastic containers. As a result, this MPs has entered agricultural soils and bareland. In this research, we examine the effects of PET on several key soil characteristics. This study investigates the impact of PET MPs on some soil properties, including Percentage of clay, silt and sand, OC, Na, K, EC, and pH, within two rangeland uses and agricultural land.

Data and research method:

Two types of land, rangelands and agricultural, were selected for the study, and a pair of test plots were established in each land type. A total of 3000 grams of MPs was evenly distributed across the surface of one of two available plots in both the rangelands and agricultural areas. The MPs were thoroughly mixed with the soil to a depth of 0–10 cm using a manual chaser. One plot in each land type (rangelands and agricultural) was designated as a control plot, with no MPs. After the establishment of the rangeland’s plots, 12 samples of 200 grams were collected from the surface of the plots, at a depth of 0–10 cm. The agricultural plots were irrigated biweekly for a period of 8 weeks. Upon completion of the irrigation period, 12 samples, each weighing 200 grams, were taken from the surface of the plots at a depth of 0–10 cm. In the laboratory, the concentrations of K, Na, EC, OC, and soil pH were analyzed. Additionally, soil texture was determined using the hydrometric method, and the percentages of clay, silt, and sand in the soil were calculated. A t-test was used to check the correlation and relationships between the investigated parameters.

Results:

In the rangeland’s plots, after the test period concluded, the average levels of Na, K, OC, and EC of the soil increased, while the soil acidity slightly decreased. These changes were not statistically significant. Additionally, the clay content increased slightly, whereas the amount of silt did not change but the amount of sand decreased slightly. In the agricultural plots, the levels of Na and OC in the soil increased, while the EC decreased. However, these changes were not statistically significant. On the other hand, both pH and K showed a significant increase. The percentage of clay decreased significantly, while the percentage of sand and silt increased significantly. This is despite the fact that no significant changes were observed in the physical and chemical properties of the soil in the control plots on range land pliots, despite the processes of plowing and irrigation. Therefore, the type of land use, soil type, and the duration of the presence of microplastics in the soil are effective in the extent of the effect of microplastics on the physicochemical properties of the soil.

Conclusion:

Based on the findings from previous and current research on the effects of various microplastics on soil physicochemical properties, it is recommended that further studies be conducted to examine the impact of commonly used microplastics, such as light and heavy polyethylene and biodegradable microplastics such as polylactic acid, on essential soil elements like phosphorus and nitrogen. These investigations should be carried out in multiple agricultural fields subjected to different irrigation methods, to better understand the comprehensive influence of microplastics on soil quality under varied management practices.

Author Contributions

Conceptualization, K.N. and R.D.; methodology, K.N. and A.S.; software, A.S.; validation, K.N., A.S. and R.D.; formal analysis, A.S.; investigation, K.N., A.S. and R.D.; resources, K.N.; data curation, A.S.; writing—original draft preparation, A.S.; writing—review and editing, K.N. and A.S.; visualization, K.N., A.S. and R.D.; supervision, K.N.; project administration, K.N.; funding acquisition, K.N. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement

Data is available on reasonable request from the authors.

Acknowledgments

We acknowledge the support of Grant number 600.871 funded by the research council of Shahid Beheshti University, Tehran, Iran.

Ethical considerations

The authors avoided data fabrication, falsification, plagiarism, and misconduct.

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Iranian Journal of Soil and Water Research
Volume 56, Issue 4
July 2025
Pages 919-934

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