Microplastic-derived dissolved organic matter is associated with increased N₂O emissions and altered...

Microplastics (MPs) and aged MPs continuously release dissolved organic matter (DOM), yet the effects of microplastic-derived DOM (MPs-DOM) on carbon-nitrogen coupling and nitrous oxide (N₂O) emissions in constructed wetlands (CWs) remain unclear. Here, polypropylene- and polyethylene-derived MPs-DOM, in pristine (PP-DOM, PE-DOM) and UV-aged forms (LPP-DOM, LPE-DOM), was introduced into laboratory-scale CWs to evaluate pollutant removal, DOM transformation, N₂O fluxes, isotopocule-inferred source structure, and denitrification-related microbes. MPs-DOM maintained high NO₃⁻-N and COD removal and enhanced NH₄⁺-N and TN removal, especially in the LPE-DOM treatment, but also associated with increased N₂O emissions. MPs-DOM was removed; LPP/LPE-DOM responses were driven by composition and DOC. Peak N₂O fluxes in LPP/LPE-DOM treatments were 173%-194% higher than the control, and mean fluxes were approximately doubled. Effluent DOM became more oxidized and unsaturated, and was enriched in lignin- and tannin-like compounds, whereas protein- and lipid-like fractions declined. Isotopocule-based apportionment showed that bacterial-denitrification contribution declined to 38.5%-50.1% under MPs-DOM exposure, whereas isotopocule-attributed fungal-denitrification signatures rose to 46.0%-55.8%, exceeding bacterial denitrification in most MPs-DOM treatments, while chemodenitrification remained minor (2.8%-5.7%). Under MPs-DOM treatments, biofilm microbial abundance and denitrification-gene abundance were elevated, and denitrifier community structure, co-occurrence networks, and niche patterns were reshaped relative to the control. Overall, MPs-DOM enhanced pollutant removal but also increased greenhouse-gas risk in CWs. Under the present exposure design, the stronger responses in aged-MPs-DOM treatments should be interpreted as treatment-associated outcomes linked to DOM composition and higher influent DOC concentration, rather than aging-specific effects.