Earthworm-Driven Vertical Redistribution of Mixed Microplastics in Vermicompost-Amended Soil

Microplastics in soil are an emerging environmental concern, yet their vertical transport under biological activity remains poorly understood. This study investigates the movement of mixed microplastics in vermicompost-amended soil using the earthworm Eisenia fetida as a biological driver. The experiment was conducted in ten tapered earthen pots (10.21 L), each containing 10 kg of soil with 15% vermicompost. A mixture of microplastics (18 g) was introduced into the top 0–5 cm layer of treatment pots, while control pots remained unamended. After 60 days, soils were destructively sampled at 2 cm intervals to assess particle distribution, earthworm survival, and temperature. Results showed non-uniform vertical transport, strongly dependent on particle size and type. Larger polypropylene particles remained within the upper 0–6 cm, whereas nylon fibers and PET fragments migrated to intermediate depths. The smallest particles (polystyrene and PTFE) reached the deepest layers, with particles < 500 µm observed in earthworm casts, indicating active ingestion–egestion transport. Biological responses revealed reduced survival and weight loss in treatment groups compared to controls. AFM observations further indicated that surface aging may enhance particle–soil interactions. Overall, the findings demonstrate that earthworm activity significantly influences microplastic redistribution, with implications for subsurface contamination and soil ecosystem health.