Size/shape-dependent migration of microplastics in agricultural soil under simulative and natural rainfall

Agricultural soil is a sink of microplastics (MPs) in the environment. MPs in topsoil can be transferred deeply or into surrounding water by rainfall. However, little is known about rainfall-induced migration pattern of different MPs in agricultural soil. In this study, soil leaching experiments of 21 d were performed on Nile red-stained size-different polyethylene terephthalate (PET) particles, and shape-different polyethylene (PE) MPs under simulated or natural rainfall. Results showed that simulated rainfall of 5-25 mm/d caused intensity-dependent migration of MPs in horizontal and vertical directions. Maximum migration depth of MP particles arrived up to 4-7 cm. Rise of soil slopes could significantly increase horizontal mobility of MPs. Comparatively, natural rainfall of similar intensity caused relatively high mobility of MPs. Moreover, under both simulative and natural rainfall, mobility of MPs presented size/shapedifferent characteristics. Comparatively, small-size MPs (especially <1 mm) showed relatively high mobility in horizontal or vertical direction, and had high-frequency presence in runoff water. Of four MPs' shapes, fiber and film had relatively high mobility in comparison to particles. These results indicate that rainfall can cause size/shape-dependent migration ofMPs in agricultural soil. It suggests size/shape-different environment fate of MPs, and provides a reference for MP control.

Automated summary

This study reveals that rainfall can induce size/shape-dependent migration of microplastics (MPs) in agricultural soil. Small-sized MPs, especially those smaller than 1mm, showed higher mobility in both horizontal and vertical directions and were frequently present in runoff water. Among the different shapes of MPs, fiber and film exhibited relatively higher mobility compared to particles.