LAUSR

Urban rivers receive used water derived from anthropogenic activities and are a crucial source of the potent greenhouse gas nitrous oxide (N2O). However, considerable uncertainties still exist regarding the variation and mechanisms of N2O production in response to the discharge of treated sewage from municipal wastewater treatment plants (WWTPs). This study investigated N2O 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 N2O concentrations and inherent N2O consumption activities in the river sediments. In summer and winter, the mean dissolved N2O concentrations were 0.67 µg-N L-1 and 0.82 µg-N L-1, respectively. Although the dissolved N2O was supersaturated (mean 288.7% in summer, mean 240.7% in winter) in the river, the N2O emission factors (EF5r, 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 (NO3-) concentration in the Tama River increased downstream of the WWTPs discharge sites, and it was the main nitrogen constituent. An increasing trend of NO3- concentration was observed from upstream to downstream, along with an increase in the N2O consumption potential of the river sediment. A multiple regression model showed that NO3- is the crucial factor influencing N2O 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 N2O production and consumption suggested that nirK-type denitrifiers likely contributed to N2O production. Structural equation models (SEMs) revealed that treated sewage discharged from WWTPs increased the NO3- loading from upstream to downstream in the river, inducing changes in the microbial communities and enhancing the N2O consumption activities. Collectively, aerobic conditions limited denitrification and in turn facilitated nitrification, leading to low N2O emissions even despite high NO3- loadings in the Tama River. Our findings unravel an overestimation of the N2O emission potential in an urban oxygen-rich river affected by treated sewage discharge.