Microplastics affect organic nitrogen in sediment: The response of organic nitrogen mineralization to microbes and benthic animals

Organic nitrogen is a crucial component of the nitrogen element, and its mineralization by sediment organisms may trigger river eutrophication. Microplastics (MPs), as emerging contaminants, influence nitrogen cycling in sediments. However, the response of organic nitrogen mineralization to sediment organisms (microbes and benthic animals) under MPs pollution remains unclear. This study used microcosm experiments and structural equation modeling to explore the effects of benthic animals and sediment microbes on organic nitrogen mineralization in the presence of polystyrene, polypropylene, and polylactic acid. Compared to the sediment without MPs, the addition of MPs caused a 17 % decrease in ammonium concentration. However, benthic animals enhanced the organic nitrogen mineralization, resulting in a 63 % increase in ammonium concentration. MPs pollution also significantly increase the nodes and edges in the microbial network and decrease the abundance of the organic nitrogen mineralization gene (UreC). In contrast, benthic animals reduce the microbial network complexity, shape keystone microbes (Lysobacter, Flavobacterium, Terrimonas, and Pontibacter), and enhanced UreC abundance. Structural equation modeling was conducted to confirm that benthic animals regulate keystone microbes, thereby promoting organic nitrogen mineralization. These findings indicate that the bioturbation by benthic animals may give arise to a sustained increase in ammonium concentrations in rivers polluted by MPs, therefore, exacerbate river eutrophication.