Vertical migration of microplastics along soil profile under different crop root systems.

Microplastics are highly accumulated in soils and supposed to migrate vertically due to water infiltration, fauna activities, and root growth. In this study, the vertical migration of microplastics along soil profile under three crop roots (corn, soybean, and ryegrass) was analyzed by a laboratory-scale incubation experiment. When microplastics were initially distributed in the surface layer, crop roots showed little effects on the vertical migration of microplastics. But in terms of homogenous microplastic distribution along soil profile, corn roots could contribute to the upward movement of microplastics in the middle layers (7-12 cm). It could be related to more pores and fissures created by primary and secondary corn roots and buoyancy effects once the pores and fissures were filled with water. Additionally, a significant positive correlation between microplastic numbers and tertiary roots of ryegrass has been observed and indicated the microplastic retention ability of fine crop roots. According to the results, in contrast to the downward microplastic migration caused by water infiltration and soil fauna activities, crop roots tended to move microplastics upwards or maintain them in soil layers.