Heightened threat of aged microplastics in constructed wetlands: impacts on nitrogen cycles and greenhouse gas emissions

Microplastics (MPs) could significantly affect the pollutant treatment performance of constructed wetlands (CWs). However, the impact of aged MPs on nitrogen (N) and greenhouse gas (GHG) emissions within CWs remains limited. This study utilized fibrous MPs, which were widely distributed in wetlands, as representative MPs to explore the effects of different concentrations and aging behaviors of MPs on N-transformation and GHG emissions. The results revealed that high-concentration aged MPs reduced the abundance of ammonia-oxidizing bacteria while promoting the abundance of nitrate-reducing bacteria, leading to a decrease in NH₄⁺-N removal (12.85%) and an increase in NO₃⁻-N removal (39.44%). Furthermore, MPs entered plants through their cell walls, affecting plant growth and inducing oxidative stress, thereby reducing the plants ability to absorb N. Additionally, high-concentration aged MPs enhanced the emission of NO and CH by altering the enzymatic activities and functional genes associated with N and C transformation, while inhibiting CO emission. These findings emphasize that the accumulation and aging of MPs within substrate could significantly influence the operation and pollutants removal performance of CWs.