Nitrous oxide emissions in the Shanghai river network: Implications for the effects of urban sewage and IPCC methodology

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.