Compost microbiomes in the plastisphere: microplastic fate and trade-offs in organic waste valorisation

Composting is widely promoted for organic waste valorisation, but a growing body of evidence shows that compost products often contain microplastics (MPs), raising concerns over secondary pollution. Within composting systems, plastisphere microbiomes colonize plastic particles, on which biofilms mediate the formation of distinct microenvironments and can modulate surface oxidation, partial depolymerisation, and fragmentation. Although biodegradable polymers may mineralise under thermophilic conditions, conventional plastics persist and may shift toward smaller fragments, potentially including nanoplastics, highlighting a sustainability paradox: composting mitigates waste and supplies nutrients, but simultaneously can contribute to MP dissemination. Once applied to soils, compost-derived MPs can disrupt soil aggregation, microbial balance, and plant–microbe interactions. Plants exposed to MPs exhibit oxidative stress responses—including stress-related responses and impaired plant performance under certain exposure scenarios—along with reduced photosynthetic efficiency and biomass. Soil fauna ingest MPs, leading to intestinal damage, oxidative imbalance, and trophic transfer to higher organisms. Human exposure via compost-amended agroecosystems is plausible, but exposure attribution and toxicological thresholds remain uncertain. Despite advances in microbial and enzymatic degradation, their efficiency under realistic composting conditions remains poorly constrained because plastisphere activity is highly context-dependent and difficult to capture in simplified laboratory settings. This mini-review synthesises current evidence on compost plastisphere ecology and polymer-dependent transformation constraints, and discusses methodological and field-scale limitations that shape interpretation, with emphasis on size-resolved risk framing, mixture contexts, and cross-kingdom interactions.