[Research Progress on Production and Emission of CH₄ and N₂O from Urban Rivers]

Methane （CH₄） and nitrous oxide （N₂O） are concerning greenhouse gases. Urban rivers have been important emission sources of CH₄ and N₂O in recent years. It is meaningful for city greenhouse gas reduction to provide a systematic analysis of spatiotemporal characteristics, mechanisms, and influencing factors of the production and emission of CH₄ and N₂O from urban rivers. This study combed measured data of urban river CH₄ and N₂O dissolution concentrations and emission fluxes from related literature published in the past 20 years and also concluded the spatiotemporal characteristics of urban river CH₄ and N₂O emissions. This study estimated that CH₄ and N₂O emissions （expressed by CO₂-eq） from urban rivers in Beijing were 234.63 and 59.53 Gg CO₂-eq in 2018, whereas CH₄ and N₂O emissions （expressed by CO₂-eq） from urban rivers in Shanghai were 159.86 and 260.24 Gg CO₂-eq in 2018, respectively. These results demonstrated that urban rivers have become important CH₄ and N₂O emission sources. This study summarized the production/consumption processes and import/export pathways of CH₄ and N₂O in urban rivers. What is more, this study discussed the main influencing factors of urban river CH₄ and N₂O production and emissions from the perspectives of river environmental conditions and urbanization effects. At last, the present work prospected the future research trends of urban river CH₄ and N₂O emissions and provides urban rivers with scientific support for greenhouse gas reduction.