Earthworms improve the rhizosphere micro-environment to mitigate the toxicity of microplastics to tomato (Solanum lycopersicum)

Microplastics (MPs) are widespread in agricultural soil, potentially threatening soil environmental quality and plant growth. However, toxicological research on MPs has mainly been limited to individual components (such as plants, microbes, and animals), without considering their interactions. Here, we examined earthworm-mediated effects on tomato growth and the rhizosphere micro-environment under MPs contamination. Earthworms (Eisenia fetida) mitigated the growth-inhibiting effect of MPs on tomato plant. Particularly, when exposed to environmentally relevant concentrations (ERC, 0.02% w/w) of MPs, the addition of earthworms significantly (p < 0.05) increased shoot and root dry weight by 12-13% and 13-14%, respectively. MPs significantly reduced (p < 0.05) soil ammonium (NH4+-N) (0.55-0.69mg/kg), nitrate nitrogen (NO3−-N) (7.02-8.65mg/kg) contents, and N cycle related enzyme activities (33.47-42.39μg/h/g) by 37.7-50.9%, 22.6-37.2%, and 34.2-48.0%, respectively, while earthworms significantly enhanced (p < 0.05) inorganic N mineralization and bioavailability. Furthermore, earthworms increased bacterial network complexity, thereby enhancing the robustness of the bacterial system to resist soil MPs stress. Meanwhile, partial least squares modelling showed that earthworms significantly influenced (p < 0.01) soil nutrients, which in turn significantly affected (p < 0.01) plant growth. Therefore, the comprehensive consideration of soil ecological composition is important for assessing MPs ecological risk. Environmental implication The accumulation of MPs can adversely affect soil physio-chemical properties, and disrupt microbial communities, indirectly affecting crop growth. Earthworms are recognized as 'ecosystem engineers', playing an essential functional role in soil. Nevertheless, the precise mechanisms by which earthworms regulate the impact of microplastics on plants remain poorly understood. The present study investigated the integrated effects of earthworms on MP-mediated rhizosphere micro-environments and crops. The findings of this study will contribute to a more comprehensive understanding of the role of earthworm activity in mitigating soil contamination with microplastics and assess the significance of earthworms for the sustainable development of agro-ecosystems.