Size and concentration-dependent effects of polyethylene microplastics on soil chemistry in a microcosm study

The continuous use of plastics is expected to increase microplastic (MP) contamination in soils, raising concerns about impacts on soil ecosystems and crop productivity. This work investigated the effects of different sizes and concentrations of polyethylene microplastics (PE-MPs) on soil properties in a controlled microcosm experiment. Microplastics of three sizes (300-600, 600-2000, and 2000-5000 µm) were tested at three concentrations (0.02 %, 0.1 %, and 1 % by weight). Significant changes were observed in soil cation exchange capacity (CEC) and dissolved organic matter (DOM), accompanied by the leaching of phthalate acid esters (PAEs) from PE-MPs, with concentrations reaching up to 0.2 mg kg⁻¹ . In contrast, soil pH, electrical conductivity (EC), water-extractable organic carbon (WEOC), and enzyme activity were not significantly affected. In particular, the smallest PE-MPs caused a 12.9 % reduction in soil CEC and a negative priming effect was observed in soil DOM. Although no clear dose-response relationship was observed, the findings suggest that MP-induced changes in soil chemistry are driven by both size-dependent surface interactions and complex soil matrix dynamics. These results demonstrate that PE-MPs can disrupt essential soil functions related to nutrient retention, organic matter dynamics, and pollutant transport, indicating broader impacts on soil health. As smaller MPs continue to accumulate, further research is needed to assess their long-term effects under varied environmental conditions and to inform effective mitigation strategies in agroecosystems. SYNOPSIS: Polyethylene microplastics altered key soil chemical properties, with smaller sizes and higher concentrations leading to greater changes in cation exchange capacity, dissolved organic matter, and diethyl phthalate release, raising concerns for long-term soil health.