Natural Aging Intensifies Microplastic Phytotoxicity in Brassica chinensis

Microplastics (MPs) are increasingly recognized as stressors in agricultural soils, yet the consequences of aging remain poorly understood. Here, we evaluated the effects of pristine and aged polyethylene (PE) and polystyrene (PS) MPs on Brassica chinensis and soil function in a 45-day pot experiment. MPs were artificially aged by photo-oxidation and incubation before application at 0.1 and 1% (w/w). Compared with pristine MPs, aged MPs with oxidized surfaces induced stronger toxicity, reducing plant height (-24.1%) and biomass (-34.3%), lowering chlorophyll content, and elevating oxidative stress markers (malondialdehyde (MDA), superoxide dismutase (SOD), peroxidase (POD), and glutathione peroxidase (GSH-Px)). Metabolomic analysis indicated disruptions in carbohydrate, amino acid, and energy metabolism. In parallel, soil phosphatase and amylase activities as well as total nitrogen were significantly suppressed, with aged PE also impairing Zn uptake. These findings demonstrate that aging amplifies MPs' phytotoxicity and soil dysfunction, underscoring their heightened ecological risk in agroecosystems.