Effects of environmentally relevant mixtures of microplastics on soil organisms

Soil ecosystems are considered important sinks for microplastics (MPs). However, the effects of environmentally relevant mixtures of MPs on soil organisms have rarely been assessed. This study aimed to evaluate the chronic effects of a mixture of MPs on two model soil organisms, the earthworm Eisenia andrei and the springtail Folsomia candida. The MP mixture was composed of polymers and shapes frequently found in agricultural soils amended with sewage sludge, including high-density polyethylene and polypropylene fragments, and polyester fibers. The organisms were exposed in LUFA 2.2 soil to MP concentrations of 0%-1% dry soil for E. andrei, and 0%-5% for F. candida. This study shows that particle ingestion by E. andrei was proportional to MP exposure levels, and the size distribution taken up was similar to that observed in the exposure medium, suggesting nonselective uptake behavior. In contrast, very low ingestion levels of MPs were found for F. candida, even at the highest test concentration. No significant effects were found on survival, growth, or reproduction of E. andrei. However, significant adverse effects were found on the reproductive output (number of juveniles) and juvenile dry weight for F. candida, with a reduction of approximately 30% in both endpoints at the highest test concentration, and calculated No Observed Effect Concentrations of 0.4% and 1%, respectively. These adverse effects may have been caused by changes in soil properties, mobility reduction, and/or the presence of plastic additives, instead of MP uptake. The comparison of MP exposure concentrations in soils obtained from the literature with the threshold concentrations derived for F. candida indicates insignificant environmental risks at current exposure levels.