Distribution Characteristics of Microplastics and Their Toxic Effects on Earthworms in Long-Term Film-Covered Vegetable Fields in Shenyang, China

The long-term utilization and low recycling rate of agricultural films have resulted in substantial increases in plastic debris and microplastics remaining in the soil, impacting the sustainable utilization of agricultural soil. However, the distribution and ecological toxicity of microplastics in long-term film-covered greenhouses and nongreenhouse vegetable fields on soil animals remain unclear. In this study, six typical greenhouse and nongreenhouse vegetable fields in the Shenyang area, which had been covered with plastic film for more than 20 years, were investigated. The distribution of microplastic abundance, shape, and source across different particle sizes in soil, as well as their oxidative damage toxicity effects on earthworms, were examined. The results demonstrated that the total abundance of microplastics in greenhouse soil was greater than that in nongreenhouse soil. Plastic fragments and microplastics > 2 mm were more prevalent in nongreenhouse soil, whereas microplastics < 2 mm were predominantly found in greenhouse soil, accounting for 89.9–98.6%. Notably, the abundance of microplastics with small particle sizes of 20–40 μm was high in greenhouse soils, and their proportion increased with increasing soil depth, with the cucumber and tomato groups showing increased abundances. Microplastics were identified mainly as thin-film and filamentous forms composed of polyethylene and polypropylene. After 56 d of exposure, a slight increase in malondialdehyde was detected in the earthworms in the soil where the cucumbers and tomatoes were grown. Mantel analysis revealed a significant correlation between the particle size of the microplastics and oxidative stress markers in the earthworms. Although greenhouse soil currently only causes slight oxidative damage to earthworms, over time, the oxidative damage caused by greenhouse systems to earthworms will increase. Therefore, regulatory measures should be implemented to standardize vegetable field management, especially with respect to microplastic pollution.