Microplastics altered soil microbiome and nitrogen cycling: The role of phthalate plasticizer

Highlights: • Changes in microbial community and essential nitrogen cycling processes were examined. • PVC microplastics enriched pathogenic fungal taxa in the soil studied. • PVC microplastics (0.5% w/w) resulted in increased NH₄⁺ and decreased NO₃^-. • Plasticizer released from microplastics was the main driver of changes in soil microbiota and function. Microplastics are emerging contaminants that are increasingly detected in soil environment, but their impact on soil microbiota and related biogeochemical processes remains poorly understood. In particular, the mechanisms involved (e.g., the role of chemical additives) are still elusive. In this study, we found that plasticizer-containing polyvinyl chloride (PVC) microplastics at 0.5% (w/w) significantly increased soil NH₄⁺-N content and decreased NO₃^--N content by up to 91%, and shaped soil microbiota into a microbial system with more nitrogen-fixing microorganisms (as indicated by nifDHK gene abundance), urea decomposers (ureABC genes and urease activity) and nitrate reducers (nasA, NR, NIT-6 and napAB genes), and less nitrifiers (amoC gene and potential nitrification rate). Exposure to plasticizer alone had a similar effect on soil nitrogen parameters but microplastics of pure PVC polymer (either granule or film) had little effect over 60 days, indicating that phthalate plasticizer released from microplastics was the main driver of effects observed. Furthermore, a direct link between phthalate plasticizer, microbial taxonomic changes and altered nitrogen metabolism was established by the isolation of phthalate-degrading bacteria involved in nitrogen cycling. This study highlights the importance of chemical additives in determining the interplay of microplastics with microbes and nutrient cycling, which needs to be considered in future studies.