Low nitrous oxide concentration and spatial microbial community transition across an urban river affected by treated sewage.

Urban rivers receive used water derived from anthropogenic activities and are a crucial source of the potent greenhouse gas nitrous oxide (NO). However, considerable uncertainties still exist regarding the variation and mechanisms of NO production in response to the discharge of treated sewage from municipal wastewater treatment plants (WWTPs). This study investigated NO concentrations and microbial processes responsible for nitrogen conversion upstream and downstream of WWTPs along the Tama River flowing through Tokyo, Japan. We evaluated the effect of treated sewage on dissolved NO concentrations and inherent NO consumption activities in the river sediments. In summer and winter, the mean dissolved NO concentrations were 0.67 µg-N L and 0.82 µg-N L, respectively. Although the dissolved NO was supersaturated (mean 288.7% in summer, mean 240.7% in winter) in the river, the NO emission factors (EF, 0.013%-0.025%) were significantly lower than those in other urban rivers and the Intergovernmental Panel on Climate Change default value (0.25%). The nitrate (NO) concentration in the Tama River increased downstream of the WWTPs discharge sites, and it was the main nitrogen constituent. An increasing trend of NO concentration was observed from upstream to downstream, along with an increase in the NO consumption potential of the river sediment. A multiple regression model showed that NO is the crucial factor influencing NO saturation. The diversity in the upstream microbial communities was greater than that in the downstream ones, indicating the involvement of treated sewage discharge in shaping the microbial communities. Functional gene quantification for NO production and consumption suggested that nirK-type denitrifiers likely contributed to NO production. Structural equation models (SEMs) revealed that treated sewage discharged from WWTPs increased the NO loading from upstream to downstream in the river, inducing changes in the microbial communities and enhancing the NO consumption activities. Collectively, aerobic conditions limited denitrification and in turn facilitated nitrification, leading to low NO emissions even despite high NO loadings in the Tama River. Our findings unravel an overestimation of the NO emission potential in an urban oxygen-rich river affected by treated sewage discharge.