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