Effects of pristine microplastics and nanoplastics on soil invertebrates: A systematic review and meta-analysis of available data

Highlights: • Pristine microplastics and nanoplastics (MPs/NPs) can be toxic to earthworms and nematodes. • Nematodes show dose dependent response to MPs/NPs. • MP/NP concentrations higher than 1 g kg⁻¹ (in soil) may reduce earthworm growth and survival. • MP/NP concentrations higher than 1 μg L⁻¹ (in water) may reduce nematode reproduction. In laboratory studies, microplastics and/or nanoplastics (MPs/NPs) have been shown to cause a variety of ecotoxicological effects on soil invertebrates. Existing data on the effects of these plastic debris on biological functions and physiological systems, showed a great variability among studies. Thus, how soil invertebrates respond to different types, shapes, sizes and concentrations of pristine MPs/NPs remains to be further characterized. The present work is an up-to-date review on quantitative and qualitative data on the effects of pristine MPs/NPs on soil invertebrates in laboratory conditions. Research priorities are also discussed. Out of a total of 1061 biological endpoints investigated in 56 studies, 49% were significantly affected after exposed to pristine MPs/NPs. The polymers with chloro and phenyl groups had more negative impacts on soil invertebrates than other polymers. Most studies used earthworm and nematode species as model organisms. For nematodes, the impact of MPs/NPs seemed to be concentration-dependent and higher concentrations of pristine MPs/NPs appeared to have more adverse impacts on biological functions and physiological systems, but this trend was not confirmed in earthworms. Meta-analysis revealed that pristine MP/NP concentrations higher than 1 g kg⁻¹ (in soil) may decrease growth and survival of earthworms, while a concentration higher than 1 μg L⁻¹ (in water) may affect nematode reproductive fitness.