Methane and nitrous oxide concentration and emission flux of Yangtze Delta plain river net

Methane (CH₄) and nitrous oxide (N₂O) saturation concentration and gas-water interface emission flux in surface water of the Yangtze Delta plain river net were investigated in summer at representative sites including the upper reaches of the Huangpu River and the rivers in the Chongming Island. The results show that the CH₄ concentration in river water ranged from 0.30±0.03 to 6.66±0.14 mol•L ^-1, and N₂O concentration ranged from 13.8±2.33 to 435±116 nmol L^-1. River surface water had a very high saturation level of CH₄ (from 468±49.0% to 11560±235%) and that of N₂O (from 175±29.5% to 4914±1304%). Dissolved oxygen (DO) was the primary factor controlling the CH₄ concentration in water. N₂O concentration had significant negative correlation with salinity and a significant positive correlation with nitrate (NO ₃ ^- ), nitrite (NO ₂ ^- ), chemical oxygen demand (CODcr) concentration and pH of river water. CH ₄ and N₂O of river water were brought about mainly by methanogenesis and denitrification in river bottom sediment that diffused through sediment-water interface into the water body and then into atmosphere through the gas-water interface. The emission flux of CH₄ and N ₂O at river gas-water interface reached 778±59.8 and 236±63.6 mol•m^-2•h^-1, respectively in summer. The river net was a potential source of atmospheric CH₄ and N₂O because of eutrophication of the water body.