Earthworms multifacetedly drive size- and type-dependent microplastic transport in soils

Area of Science:

• Environmental Science
• Ecotoxicology
• Soil Ecology

Background:

• Soil ecosystems act as significant reservoirs for microplastics (MPs).
• Understanding how soil fauna influence the subsurface transport of MPs with varying characteristics is crucial but limited.
• Earthworms are key soil engineers with potential roles in MP redistribution.

Purpose of the Study:

• To investigate the role of earthworms in the subsurface transport of microplastics.
• To determine how earthworm density and MP properties affect vertical MP movement.
• To elucidate the mechanisms of MP mobilization and retention by earthworms.

Main Methods:

• Soil incubation experiments were conducted to assess earthworm-mediated MP transport.
• MP properties such as size, shape, and polymer type were varied.
• Earthworm density was manipulated to determine optimal transport conditions.
• Mechanisms including bioturbation, ingestion, and mucus adhesion were analyzed.

Main Results:

• Optimal vertical MP transport efficiency was observed at 28.49 ± 5.70 earthworms/m².
• Smaller PET particles and submillimeter MPs (20-400 μm) were transported to deeper soil layers more effectively.
• Earthworm ingestion and mucus adhesion showed polymer-specific dynamics, influencing MP retention.
• Flexible fibers and film-shaped MPs exhibited different transport and retention behaviors compared to rigid particles and spheres.

Conclusions:

• Earthworms are multifunctional engineers driving MP redistribution through density-dependent bioturbation and selective interactions.
• Biologically moderated soil MP transport necessitates risk frameworks integrating faunal activity and depth-dependent contamination.
• Effective soil conservation strategies require understanding earthworm influence on MP fate and transport.