<p>This study investigated the effect and microbial mechanism of microplastics to the N₂O emission in underground and aboveground wastewater treatment plants. The microplastics in the influent of Uwwtp and Awwtp were 4953 ± 558 and 2253 ± 563 particles/L. The microplastics rejection rate were 86.36 ± 0.02% (Awwtp) and 90.56 ± 0.02% (Uwwtp), respectively. The N₂O concentration were 106.40 ± 134.17&#xa0;nmol/L in Uwwtp and 53.58 ± 32.68&#xa0;nmol/L in Awwtp. The N₂O/NO₃^– ratio was 0.55% in the aerobic tank of Uwwtp. The N₂O/NH₄⁺ ratio was 0.39% in aerobic tank and 0.52% in secondary sedimentation tank of Awwtp. The microplastics were significantly correlated with NH₄⁺ (<i>p</i> &lt; 0.05) in Awwtp and significantly correlated with NH₄⁺ (<i>p</i> &lt; 0.01) and NO₂^–, NO₃^– (<i>p</i> &lt; 0.01) in Uwwtps. Mental test results indicated that microplastics significantly correlated with the nitrifier and denitrifier in the systems. This suggest that microplastics could affect the nitrification and denitrification process in the two plants and thus affect the N₂O emission. Microplastics in wwtps, i.e., the plastisphere, may be a novel microbial colonization site that could be vital to N₂O emissions.</p>