Uncertainty of hydrological processes on greenhouse gas emissions from urban river driven by natural and anthropogenic factors

The drawdown zone is the transitional boundary between the terrestrial and aquatic ecosystems. However, there is a lack of greenhouse gas (GHG) emission data from drawdown zones, which might lead to significant uncertainties in global aquatic GHG emissions. In this study, high-frequency field work was conducted using floating chambers and bubble traps to investigate the temporal and spatial variation of GHG fluxes from drawdown zones in a deltaic urban river affected by tides and to evaluate whether the contribution of drawdown zones is important. The results showed that the nitrous oxide (N₂O) flux from drawdown zones showed two peak emissions under the tidal cycle, which were mainly derived from coupled nitrification–denitrification. The methane (CH₄) flux from drawdown zones showed distinct diurnal changes in all seasons and periodic flooding increased CH₄ bubble events from drawdown zones. In comparison, there was no obvious pattern of carbon dioxide (CO₂) flux, which ranged from − 2966.97 μmol m⁻²h⁻¹ to 4124.5 μmol m⁻²h⁻¹. Water temperature, tidal level, and concentration of dissolved oxygen (%) are the three critical factors that explain GHG emissions. Overall, N₂O, CH₄, and CO₂ from the drawdown zones accounted for 3 %, 13 %, and 17 % of the total annual emissions, respectively. This study indicates that it is necessary to take drawdown areas into consideration to accurately qualify the GHG budgets of deltaic urban rivers.