Effects of Different-Sized Polyethylene Microplastics on Nitrogen Transformation in River Sediment-Water Systems

As an emerging pollutant, the impact of microplastics’ physical properties on key biogeochemical cycles remains unclear. This study investigated the effects and mechanisms of four particle sizes (25, 150, 200, 500 μm) of polyethylene microplastics (PE-MPs) on nitrogen transformation in river systems through a 120-day indoor micro-cultivation experiment. Results indicate that microplastic addition significantly enhanced nitrogen partitioning from the water phase to sediments, leading to reduced total nitrogen and nitrate concentrations in overlying water. Small-sized (25 μm) microplastics strongly promoted ammonium nitrogen accumulation in sediments during long-term exposure, while large-sized (500 μm) microplastics induced dramatic fluctuations in sediment total nitrogen content, revealing complex adsorption-desorption dynamics. Correlation analysis indicates that sediment ammonium accumulation is significantly positively correlated with iron content. This study reveals that microplastics primarily disrupt nitrogen transformation processes by altering sediment microenvironments and providing microbial attachment sites, ultimately inhibiting nitrification and leading to the accumulation of nitrogen in the sediment as reduced ammonium salts. The study emphasizes that microplastic size is a key physical parameter governing its interference with the nitrogen cycle, providing crucial evidence for scientifically assessing its ecological risks.