Impacts of Low and High-Density Polyethylene Microplastics and Their Biodegradation on Soil Organic Carbon Pools

Document Type : Research Paper

Authors

1 Graduated from the Department of Soil Science and Engineering, College of Agriculture and Natural Resources, University of Tehran.

2 Professor of Soil Science Department, College of Agriculture and Natural Resources, University of Tehran, Karaj, Iran.

Abstract

This study aimed to investigate the effects of polyethylene microplastics and the application of an efficient bio-recombinant agent composed of bacterial and fungal strains to mitigate the adverse impacts of microplastic particles on various properties of a soil–plant system. To achieve this objective, a factorial experiment was conducted in a completely randomized design, incorporating two types of polyethylene microplastics, low-density polyethylene (LDPE) and high-density polyethylene (HDPE), at three concentration levels (0%, 1%, and 2%), with and without inoculation of the bio-recombinant agent. The experiment was carried out in a sandy loam soil with three replications per treatment, following a 135-day incubation period and subsequent cultivation of sunflower plants over a 60-day growth cycle. The results revealed that in treatments without the bio-recombinant agent, the presence of LDPE and HDPE microplastics in the soil, particularly at higher concentrations, led to a significant increase in total soil organic carbon. Specifically, LDPE treatments showed increases of 0.35% in P1 and 0.55% in P2 compared to the control, while HDPE treatments exhibited increases of 0.23% in P1 and 0.51% in P2. Moreover, the type of microplastic influenced the extent of organic carbon accumulation; LDPE in the P1 treatment resulted in a 13% higher increase in total organic carbon compared to HDPE. Additionally, the findings demonstrated that the presence of both types of microplastics, when combined with bio-recombinant inoculation, significantly enhanced total organic carbon and water-soluble organic carbon, particularly the fraction soluble in hot water, compared to the control and other non-inoculated treatments. Notably, the impact of higher concentrations of LDPE was more pronounced than HDPE, with increases of 32.1%, 18.1%, and 35.1%, respectively.

Keywords

Main Subjects

Introduction

Microplastics (MPs), particularly polyethylene (PE), have become significant pollutants in agricultural soils, affecting soil physical, chemical, and biological properties. PE-MPs, including low density (LDPE) and high density (HDPE), alter carbon cycling and microbial interactions. Despite increasing awareness, few studies investigate their differential effects and biodegradation potential in soil–plant systems. This study aimed to (1) assess the impact of LDPE- and HDPE-MPs on soil organic carbon (SOC) and dissolved organic carbon (DOC) dynamics, and (2) evaluate the role of a novel microbial consortium in mitigating these effects. The post positivist explanatory approach enables insight into carbon redistribution and microbial activity modulation under MPs stress.

Data and research method

Approximately 500 kg of the bulk soil was collected and promptly transported to the laboratory from a farmland adjacent to Karaj City, Alborz Province, Iran. In this study, we examined two different types of PE: LDPE derived from plastic bags, and HDPE derived from plastics used in greenhouses, which were obtained from a plastic factory. After being cut into small pieces and frozen in liquid nitrogen, the raw plastic sheets were cut again using a commercial machinery cutter equipped with a 5-mm sieve, as well as were UV-aged by using two 55 W UV lamps. A factorial randomized design was employed using sandy loam soil from Karaj, Iran. Treatments included LDPE and HDPE at 0%, 1%, and 2% levels, with and without a novel microbial inoculation (+MC) and (−MC). respectively. Thirty-six soil columns were prepared (3 concentrations × 2 polymers × 2 microbial levels × 3 replicates). The microbial consortium comprised Pseudomonas putida, Lysinibacillus xylanilyticus, Ralstonia pickettii, and Aspergillus niger, validated for plastic biodegradation, which were stored in the genome bank of the Soil Microbiology Lab, Department of Soil Sciences, University of Tehran, Iran. Eventually, all samples were incubated for 135 days; thereafter, sunflower plants were cultivated for 60 days. SOC and DOC (cold/hot water extractable) were measured via standard protocols. Statistical analysis employed SAS and Duncan’s test (p< 0.05).

Results

SOC increased with rising MPs concentrations, more prominently under LDPE and microbial treatment. LDPE at 1% +MC showed the highest SOC accumulation. DOC levels rose significantly with LDPE in both cold and hot extracts, especially in hot water. Microbial inoculation amplified DOC, highlighting enhanced plastic degradation and carbon solubilization. Statistical tests confirmed significant interactions among polymer type, concentration, and inoculation (p< 0.05).

Conclusions

LDPE-MPs more strongly influence SOC and DOC enrichment than HDPE, particularly when coupled with microbial inoculation. The microbial consortium accelerated plastic degradation, improving carbon redistribution and soil health. These findings inform strategies for MPs remediation and sustainable carbon management in agroecosystems. Further field-scale studies are recommended to validate laboratory insights.

Author Contributions

Milad Mirzaei Aminiyan: Conceptualization, Writing – original draft, Visualization, Software, Methodology, Investigation, Formal analysis, Data curation. Mahdi Shorafa: Writing – review & editing, Validation, Supervision, Resources, Project administration, Funding acquisition.

Data Availability Statement

Data is available on reasonable request from the authors.

Acknowledgment

All authors express special thanks to anonymous reviewers for their detailed comments that have immensely benefited the ideas expressed in this manuscript. This paper is sponsored by the University of Tehran, Islamic Republic of Iran and funded by the Iran National Science Foundation, Islamic Republic of Iran (INSF; No. 4004288).

Ethical considerations

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

Conflict of interest

The authors declare no conflict of interest.

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Iranian Journal of Soil and Water Research
Volume 56, Issue 12
March 2026
Pages 3315-3329

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