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Global nitrogen (N) enrichment has resulted in increased nitrous oxide ( N2O) emission that greatly contributes to climate change and stratospheric ozone destruction, but little is known about the N2O emissions from urban river networks receiving anthropogenic N inputs. We examined N2O saturation and emission in the Shanghai city river network, covering 6300 km2, over 27 months. The overall mean saturation and emission from 87 locations was 770% and 1.91 mg N2O-N m−2 d−1, respectively. Nitrous oxide ( N2O) saturation did not exhibit a clear seasonality, but the temporal pattern was co-regulated by both water temperature and N loadings. Rivers draining through urban and suburban areas receiving more sewage N inputs had higher N2O saturation and emission than those in rural areas. Regression analysis indicated that water ammonium ( NH4+) and dissolved oxygen ( DO) level had great control on N2O production and were better predictors of N2O emission in urban watershed. About 0.29 Gg N2O-N yr−1 N2O was emitted from the Shanghai river network annually, which was about 131% of IPCC's prediction using default emission values. Given the rapid progress of global urbanization, more study efforts, particularly on nitrification and its N2O yielding, are needed to better quantify the role of urban rivers in global riverine N2O emission. [ABSTRACT FROM AUTHOR]

[Display omitted] • Without considering rainfall events, GHG emissions from urban rivers will be underestimated by 31%. • The CO 2 –equivalent emissions of GHG from urban rivers during a typhoon are 13.55 times higher than during fair weather. • Pumping drainage, sediment disturbance, and high gas transfer velocity are the primary factors behind GHG release during typhoons. Urban rivers have been recognized as significant hotspots for greenhouse gas (GHG) emissions. However, the impact of storms or typhoons on GHG emissions from urban rivers is not well understood. Suzhou Creek located in the highly developed urban area of Shanghai was selected to investigate of changes in CO 2 , CH 4 , and N 2 O concentrations and diffusive fluxes during different rainfall events from May to October 2021. The findings showed that urban rivers were sources of GHG emissions during various events. The average diffusive fluxes of CO 2 , CH 4 , and N 2 O during typhoon events were 1 to 2 orders of magnitude higher than those during non–typhoon period, revealing that typhoon greatly promote GHG evasion. GHG CO 2 –equivalents were ranked as follows: during typhoon (3.93 g m–2h−1) > post–typhoon (0.89 g m–2h−1) > high precipitation (0.33 g m–2h−1) > non–precipitation (0.29 g m–2h−1) > small precipitation (0.25 g m–2h−1). Rainfall dilution, pump station drainage, sediment disturbance, and a high gas transfer coefficient induced by typhoons were the main factors affecting GHG emissions. Nutrients, chlorophyll-a, and dissolved oxygen in river water were important control parameters for GHG production. Overall, typhoon would release large amounts of GHG from urban rivers. Given the projected increase in the frequency and complexity of extreme weather events, this study highlights the importance of better understanding the dynamic response of urban river GHG to rainfall and typhoons. [ABSTRACT FROM AUTHOR]

The focus of urban water environment renovation has shifted to high nitrate (NO 3 −) load. Nitrate input and nitrogen conversion are responsible for the continuous increase in nitrate levels in urban rivers. This study utilized nitrate stable isotopes (δ15N–NO 3 − and δ18O–NO 3 −) to investigate NO 3 − sources and transformation processes in Suzhou Creek, located in Shanghai. The results demonstrated that NO 3 − was the most common form of dissolved inorganic nitrogen (DIN), accounting for 66 ± 14% of total DIN with a mean value of 1.86 ± 0.85 mg L−1. The δ15N–NO 3 − and δ18O–NO 3 − values ranged from 5.72 to 12.42‰ (mean value: 8.38 ± 1.54‰) and −5.01 to 10.39‰ (mean value: 0.58 ± 1.76‰), respectively. Based on isotopic evidence, the river received a significant amount of nitrate through direct exogenous input and sewage ammonium nitrification, while nitrate removal (denitrification) was insignificant, resulting in nitrate accumulation. Analysis using the MixSIAR model revealed that treated wastewater (68.3 ± 9.7%), soil nitrogen (15.7 ± 4.8%) and nitrogen fertilizer (15.5 ± 4.9%) were the main sources of NO 3 − in rivers. Despite the fact that Shanghai's urban domestic sewage recovery rate has reached 92%, reducing nitrate concentrations in treated wastewater is crucial for addressing nitrogen pollution in urban rivers. Additional efforts are needed to upgrade urban sewage treatment during low flow periods and/or in the main stream, and to control non-point sources of nitrate, such as soil nitrogen and nitrogen fertilizer, during high flow periods and/or tributaries. This research provides insights into NO 3 − sources and transformations, and serves as a scientific basis for controlling NO 3 − in urban rivers. • Urban rivers have received significant amounts of nitrate through direct exogenous input and ammonium nitrification from sewage. • A weak permanent nitrate removal mechanism results in the accumulation of nitrate. • The main source of nitrate pollution in urban rivers is mainly due to the substantial input of treated wastewater. [ABSTRACT FROM AUTHOR]