Micro- and Nanoplastic Effects on Nematodes: A Systematic Review of Toxicity Mechanisms, Exposure Characteristics, and Soil Ecological Implications

Micro- and nanoplastics (MNPs) are now widely recognized as pervasive contaminants in terrestrial ecosystems. However, their effects on soil nematodes remain largely confined to dispersed experimental studies, resulting in a fragmented evidence base that necessitates integrative synthesis. In this review, we consolidate current knowledge on the biological and ecological consequences of MNP exposure in nematodes, with particular emphasis on agricultural and greenhouse soil environments where plastic inputs are especially intensive. To achieve this, we followed PRISMA guidelines and screened 251 records retrieved from the Web of Science and Scopus databases. After duplicate removal and eligibility assessment, 166 studies were retained for qualitative evaluation, of which 154 provided extractable experimental datasets suitable for comparative analysis. Species representation revealed a pronounced taxonomic skew, with Caenorhabditis elegans serving as the dominant experimental model in more than 150 studies. In contrast, other free-living and entomopathogenic nematodes were only sporadically investigated. Exposure characterization further demonstrated a strong reliance on polystyrene (PS)-based materials, with both micro- and nanopolystyrene frequently employed in experimental designs, followed by polyethylene (PE) as the second most studied polymer. Particle size distributions showed a marked bias toward nanoscale fractions—most commonly around ~100 nm—whereas larger microplastics more representative of soil contamination profiles were comparatively underexplored. Reported exposure concentrations spanned a wide gradient, ranging from environmentally relevant µg L⁻¹ levels to elevated mechanistic doses designed to elicit toxic responses. Across studies, recurring toxicological outcomes included oxidative stress induction, intestinal injury, neurotoxicity, reproductive impairment, and transgenerational effects mediated through epigenetic pathways. Toxicity was often intensified by environmental aging processes, polymer-specific chemistry, and interactions with co-occurring contaminants. Taken together, the available evidence indicates that although mechanistic understanding of MNP toxicity in nematodes has advanced substantially, significant gaps persist in ecological realism, polymer diversity, and multi-species soil assessments. Addressing these limitations will be critical for refining terrestrial risk frameworks and clarifying the long term implications of plastic contamination for soil ecosystem functioning.