Ecotoxic Effects of Microplastics from Compostable and Conventional Plastic Bags on the Growth and Reproduction of Eisenia andrei in a Sewage Sludge-Based Substrate

Compostable plastics have been proposed as a more sustainable alternative to conventional polyethylene for bags. However, very little is known about the effects of these plastics on soil biota. This study evaluated the ecotoxicological effects of microplastics (MPs) derived from a compostable bag, a commercial polyethylene bag (PE1), and virgin polyethylene resin (PE2) on the earthworm Eisenia andrei. To this end, the OECD standardized reproduction test was adapted by replacing artificial soil with sewage sludge as the exposure substrate. This approach incorporates the physicochemical complexity and pre-existing contaminants characteristic of a real environmental organic matrix, providing a more ecologically representative exposure scenario. A total of 480 individual earthworms were evaluated after exposure to three different concentrations (250, 750, and 1250 mg·kg−1) of each MP type. No significant differences in survival or adult biomass were observed among treatments. By contrast, reproductive parameters were particularly sensitive to exposure. Cocoon production was significantly altered (p = 0.001), with a 39.9% increase in the compostable-material treatment and reductions of up to 54.2% in the polyethylene treatments in comparison with the control. Likewise, the number of juveniles decreased significantly only in the groups exposed to polyethylene, with an average reduction of up to 34 individuals (44%, p < 0.001), and PE2 had the greatest inhibitory effect. The findings call into question the supposed biological inertness of virgin polymers and highlight the need to develop ecotoxicological assessment strategies based on environmentally realistic matrices that allow for an understanding of the polymer’s intrinsic effect and its interaction with pre-existing contaminants.