Distribution characteristics of microplastics in agricultural soils from the largest vegetable production base in China

The study area is situated in Shouguang City, Shandong Province, as the largest greenhouse vegetable production base in Northern China. Samples of facility agricultural soil, open-field agricultural soil, and agricultural plastic mulch film were collected to investigate the distribution characteristics, influencing factors, and discharging sources of microplastics (MPs). Microplastic abundance of three soil layers at all sampling sites ranged from 310 to 5698 items/kg, with an average value of 1444 ± 986 items/kg. The main size category of MPs was less than 0.5 mm, and the contribution of MPs with sizes <0.5 mm in the 10–25 cm layer of facility agricultural soils was significantly higher (p < 0.05) than that in the 0–5 cm soil layer, which indicated that small MPs tended to migrate to deeper soil layers. The prevailing shapes of MPs were fragment and film, while polypropylene, ethylene-propylene copolymer, and polyethylene dominated among chemical compositions. The fractions of silty and sandy particles were correlated with the abundance of MPs, and the microplastic abundance in sandy loam was significantly higher (p < 0.05) than that in silty loam or loam based on the international classification standard. Thus, the soil texture may affect the distribution of MPs in local agricultural soils. In addition, the planting age of facility agricultural soil was related to microplastic abundance, while there was no significant difference in the microplastic abundances of facility agricultural soils under different irrigation methods. The microplastic abundance in sandy loam surpassed that in silty loam or loam, small size (<0.5 mm) MPs tended to migrate to deeper soil layers, and planting age affected microplastic abundance in facility agricultural soils.