Investigating the effect of plant presence and leachate irrigation on the distribution of microplastics in different soil depths and the rhizosphere

• Landfills produce significant microplastic pollution. • Plant roots enhance microplastic movement into deeper soil layers. • Leachate irrigation increases microplastic spread in soils. • Smaller microplastics (<0.1 mm) are most abundant and harmful. • Polyethylene and polypropylene are the dominant microplastic polymers. Microplastics are recognized as an emerging environmental challenge, posing a serious threat to human life and other organisms. Landfills act as hotspots for the accumulation and generation of microplastics, as discarded plastic waste gradually fragments under environmental and chemical influences. This study specifically examined soils within the rhizosphere of Ceratocarpus arenarius L. compared to adjacent non-vegetated soils, as well as the effect of landfill leachate on microplastic transport at the Halgheh-Darreh landfill in Karaj. The results showed that soils with both plants and leachate contained the highest number of microplastics (1720 particle/kg), followed by soils without plants but irrigated with leachate (1413 particle/kg). Soils with plants but no leachate contained 953 particle/kg, while the lowest abundance occurred in soils without plants and without leachate (562 particle/kg). These findings demonstrate that both plant and leachate strongly influence the retention and vertical redistribution of microplastics in soil. The percentage of microplastic particles in the size ranges of 1–5 mm, 0.2–1 mm, 0.1–0.2 mm, and <0.1 mm were 16%, 26%, 29%, and 29%, respectively. Black and transparent particles were the most frequently detected colors (23% and 20%), which likely reflects the prevalence of these plastic types in the landfill waste stream. Fibers (34%) and fragments (26%) dominated the shape distribution, and polyethylene and polypropylene were the most common polymers (32% and 28%). Overall, this study provides direct evidence that plant rhizospheres can facilitate the downward migration of microplastic particles, while leachate serves as both a source and a transport medium, jointly determining the fate of microplastics in semi-arid landfill soils.