New insights into the impact of microplastics on nitrogen transformation in urban rivers based on field experiments

Nitrogen (N) is the primary driving factor of river eutrophication and a crucial nutrients in riverine systems. Microplastics (MPs) in urban rivers may affect N transformation in sediments. Although previous incubation experiments revealed the effect of MPs on N transformation, the relationship between MPs and N transformation in field conditions remain unclear. This study measured nitrogen concentrations, nitrogen transformation rates, the abundances and types of MPs, and keystone microbes in urban river sediments to reveal their interaction. The result indicated that MPs ranged from 200 to 1250 items/kg (dry weight) in urban river sediment, with polyethylene terephthalate (24.4 %), polyethylene (22.4 %), polystyrene (15.7 %), and polypropylene (10.2 %) being predominant. River sediment nitrification rates ranged from 0.05 to 4.19 μmol N kg⁻¹ h⁻¹ , and denitrification and anaerobic ammonium oxidation rates were 3.30–206.76 and 0.07–3.99 nmol N g⁻¹ h⁻¹ , respectively. MPs pollution may promote nitrification, while inhibiting denitrification and anaerobic ammonium oxidation in sediment. Additionally, mathematical model confirmed that MPs pollutions directly changed the keystone microbes (Hyphomicrobium, Actinocorallia, Rhodoferax, Taylorella, and Magnetospirillum) and N transformation rates, thereby affecting N transformation in sediment. Compared to field experiments, incubation experiments might overestimate the impact of MPs on nitrification processes and underestimate their impact on denitrification processes. This study provided field evidence of the impacts of MPs on N transformation in urban rivers, informing the control of MPs and N pollutions in urban rivers.