Impact and mechanism of microplastics on the denitrification of a bioretention system

Microplastics (MPs) are widely distributed in the natural environment and are transported into bioretention systems through rainwater runoff, where they progressively accumulate. However, their impact on denitrifying bacteria within these systems remains poorly understood. This study aimed to explore the effects of MPs derived from different mask layers on the denitrification efficiency of bioretention systems. The results revealed that the accumulation of MPs led to a decline in the removal efficiency of NH₄⁺ -N, NO₃^--N, and total nitrogen. Furthermore, MPs inhibited the enzymatic activities of dehydrogenase, catalase, and urease. The presence of MPs also reduced the abundance of key denitrifying bacteria, such as Boteobacteria, Bacteroidota, and Denitratisoma. Notably, the experimental results demonstrated that microbial communities exhibit adaptive responses through compositional modulation of extracellular polymeric substances (EPS), specifically modifying its physicochemical properties including viscosity and hydrophobicity, to mitigate the environmental stress induced by microplastic exposure. Among the tested MPs, those from the filter layer of masks exhibited the strongest inhibitory effect, followed by outer-layer MPs and inner-layer MPs. This experimental study provides a theoretical foundation for understanding the denitrification mechanisms in bioretention systems under MPs accumulation and highlights the potential ecological risks posed by MPs in such environments.