Europium-labeled polystyrene tracing reveals earthworm-mediated inhibition of root uptake and reduced phytotoxicity in lettuce

Microplastic (MP) contamination in agricultural soils poses a significant threat to crop productivity and food safety. Earthworms, functioning as key ecosystem engineers, may mitigate such stress. Using Europium (Eu)-labeled polystyrene (PS) microplastics, we quantified MP accumulation in lettuce (Lactuca sativa L., 1753) and evaluated the role of earthworms in mitigating PS-induced stress. We found that PS microplastics mainly accumulated in the roots and were subsequently translocated to the shoots, resulting in reduced biomass, impaired photosynthetic performance, and lower soluble sugar and protein contents. However, earthworm activity substantially mitigated these adverse effects, decreasing MP accumulation in roots and leaves by 15% and 5%, respectively, increasing chlorophyll content (SPAD value) by 43%, and restoring photosynthetic performance (103% increase in net photosynthetic rate). Consequently, root and leaf dry weights increased by 17% and 43% (P < 0.05), respectively. Mechanistically, earthworms increased soil pH, reduced acidification, restored microbial diversity, and enhanced nutrient cycling. Partial least squares modeling further revealed that earthworms promoted lettuce growth primarily by improving soil physicochemical properties, reducing oxidative stress, and limiting MP uptake. Collectively, these findings identify earthworms as important and effective bioregulators that buffer MP stress through bioturbation, microbial regulation, and soil amelioration, providing a sustainable strategy for mitigating MP-induced stress and regulating MP bioavailability in agricultural soils.