Eco-labeled composts reduce microplastic contamination and mitigate heavy metal bioavailability in agricultural ecosystems.

This study assessed the abundance, size, weight, shape, and polymer composition of microplastics (MPs) in 11 commercial compost products using ATR-FTIR spectroscopy. MPs were present in all samples, with an average abundance of 137.65 ± 6.01 items/kg, and concentrations up to 631.14 mg/kg. Eco-labeled composts showed significantly lower MPs abundance, size, and concentration than non-eco products. The most contaminated compost was a blonde peat substrate, while an algae-based humus showed the lowest MPs load. MPs larger than 1 mm were predominant, and films were the most common shape, likely resulting from plastic bags and packaging materials. A total of 15 polymer types were identified, with chipboard/agglomerate, modified cellulose, and polyethylene being the most frequent. Polymer diversity was greater in commercial universal composts and positively associated with anthropogenic activities. Physicochemical analysis revealed significant correlations between MPs concentration and compost quality. The presence of MPs was negatively correlated with pH and nitrogen content, but positively with organic matter. Vermicompost exhibited higher nitrogen and pH levels compared to other composts. MPs also influenced the distribution of trace elements: significant negative correlations were found between MPs levels and several elements, including As, Cd, Cr, and Pb. These findings suggest MPs may adsorb heavy metals, reducing their bioavailability. A "diversity index" (DI) of petroleum-based polymers, excluding cellulosic particles, was developed and showed significant correlations with Pb and Cd concentrations, indicating that human-related plastic waste contributes to metal contamination in compost. The results underscore the need for improved waste separation and composting processes to reduce MP and heavy metal contamination. Future regulations mandating separate collection of biowaste and textiles may mitigate these impacts.