Sewer-relevant aging of microplastics drives plastic-class-dependent N₂O emissions from denitrifying sludge

Microplastics (MPs) accumulate in sewer systems and wastewater treatment plants (WWTPs), where they undergo aging processes distinct from photo-oxidation in surface waters. How such sewer-relevant aging impacts denitrification-associated nitrous oxide (N₂O) emissions remains unknown. Here, we simulated sewer-relevant aging (sulfidation and mechanical abrasion) for non-biodegradable, compostable and biodegradable MPs (NMPs, CMPs, BMPs) and evaluated their effects on N₂O emissions relative to pristine MPs, while linking MP-derived dissolved organic carbon (DOC) characteristics to microbial physiological and community responses. Aging increased DOC release from CMPs and BMPs by 30.0% and 115.8%, respectively, and generated plastic-dependent molecular signatures enriched in aromatic and sulfur-containing compounds. Rather than bioavailable carbon sources, aged MP-derived DOC could barely be consumed as the electron donor yet acted as a potent physiological disruptor. Correspondingly, aged CMPs and BMPs increased N₂O emissions to 2.5- and 8.0-fold those of their pristine counterparts, with a stimulation order of BMPs > CMPs > NMPs under carbon-sufficient conditions (C/N = 4:1). Stronger N₂O stimulation was associated with more severe membrane damage and greater impairment of electron supply. Metagenomic analysis further revealed enrichment of Paracoccus under stronger N₂O stimulation, together with shifts in denitrification-related genomic potential. As a genus capable of degrading diverse plastic additives while harboring a complete denitrification pathway, Paracoccus could favor N₂O accumulation under electron-limited conditions, where N₂O reduction is disadvantaged relative to upstream denitrification steps. These findings showed that sewer-relevant aging reshaped MP effects on denitrification-associated N₂O emissions in a plastic-dependent manner and challenged the presumed environmental benignity of compostable and biodegradable plastics, underscoring the need to incorporate MPs aging into risk assessment for WWTPs.