Microplastic contamination effects on near-saturated hydraulic conductivity of soils

Accumulation of microplastics (MPs) in agricultural soils is an increasing environmental concern, with potential effects on soil physical and hydraulic properties. This study investigated the impact of MP contamination (concentration 0.5% w/v; incubation period 15 months) on soil hydraulic conductivity in three soil types (Alfisol, Entisol, and Vertisol). We worked at nearly-saturated conditions by applying three different water pressure heads (‒6, ‒3, and ‒1 cm) and explored the influence of MP contamination by testing two polymers (low-density polyethylene, PE, and polypropylene, PP) in two shapes (fiber, film), both individually and in combination (MP-mix). Results indicate that MP fibers, regardless of polymer type, had more pronounced effects on soil physical and hydraulic properties compared to other MP forms, though the extent of these effects varied depending on soil type. Specifically, the presence of MP fibers led to an increase in bulk density in the Vertisol, a slight increase in the Entisol (only with PE fibers), and no change in the Alfisol. Microplastic contamination generally resulted in a decrease in water content, but the intensity of this effect differed among the three soils, being most evident in the Vertisol with PE fibers and in the Alfisol with the MP mix. Additionally, the results revealed a trend of increasing hydraulic conductivity (K) in the Vertisol and Alfisol, whereas the Entisol exhibited a slight reduction, though modest in absolute terms, due to MP contamination. These findings highlight important agro-ecological implications, as MP contamination may increase nutrient leaching while reducing water availability for crops, a critical issue in water-scarce regions like the Mediterranean.