Study On Vertical Distribution Characteristics And Flux Of N₂O Dissloved Concentrations In The Large And Large Reservoirs Of Lancang River

The temperature increase in recent decades is mainly caused by greenhouse gas emissions from human activities.The global greenhouse effect potential of nitrous oxide(N2O)is 300 times and 12 times that of carbon dioxide(CO2)and methane(CH4),respectively,and N2O made an important contribution to global warming.It has been reported that after the reservoir is a possible net source of N2O,the assessment of N2O emissions from the reservoir has received increasing attention.In this study,the four deep reservoirs of the Lancang River(Yunnan Section)XW,MW,NZD and JH were taken as the research objects.Based on the N2O concentration of the reservoir atmosphere,water body and sediment interstitial water,the vertical N2O of the reservoir was determined.The distribution characteristics of concentration and the N2O emissions in the reservoir,and comprehensively discussed the environmental factors that affect the water-gas interface and water-sediment N2O emissions,provide a theoretical basis for better control and reduction of water N2O emissions.The research results of this article are as follows:(1)The average concentration of N2O dissolved in the sampling points of the dry season(January 2019)and wet season(August 2019)was 12.82±1.96 and 16.32±4.43nmol/L,respectively.The average N2O saturations of the sampling points in the dry and wet seasons were 262.96±51.19 and 368.94±108.64%,respectively.All the N2Os in the waters were supersaturated,and the waters were all sources of atmospheric N2O.The concentration and saturation of N2O in the dry and wet seasons were generally JH>NZD>XW>MW reservoir.The N2O concentration of interstitial water in the sediments of the wet season reservoirs generally decreased with the increase of sediment depth.The N2O concentration of interstitial water in each reservoir was between 8.36-28.94 nmol/L.The N2O concentration of each reservoir was shown as JH>NZD>XW>MW reservoir.The concentration of N2O in interstitial water was significantly positively correlated with the concentration of NO3-and NO2-in interstitial water,and was significantly negatively correlated with the concentration of NH4+ in interstitial water.(2)The N2O concentration at each point of XW,MW and JH reservoir in the wet season has obvious vertical distribution characteristics,and its N2O concentration as a whole shows a trend of first increasing and then decreasing,increasing from the atmosphere to the gap water interface As the depth of interstitial water increases,the N2O concentration gradually decreases,and the NZD reservoir as a whole shows a trend of decreasing first and then increasing and then decreasing.The nitrification-denitrification coupling reaction mainly occurs in the wet season reservoir water body,and the nitrification-denitrification coupling reaction mainly occurs in the sediment.The water body of each reservoir is the source of atmospheric N2O,and the sediment is the source of water N2O.(3)The average N2O flux at the water-air interface in the dry and wet season reservoirs is 12.08±2.72?mol/(m2·d)and 16.32±4.43?mol/(m2·d),respectively.From XW to MW reservoir,it showed a downward trend first,and from MW to JH reservoir,it showed a trend of rising.In 2019,the total annual N2O release from the water-air interface of the reservoirs in XW,MW,NZD and JH reservoirs is 29.26×105mol N2O.In the dry season,the ?N2O is positively correlated with NO3-in the water body of the Lancangjiang reservoir.Although there is no significant correlation with DO,the trend is opposite,the dry season denitrification reaction is the main production process of N2O in the water body of the Yunnan section of the Lancangjiang River.The ?N2O in the water body of the Lancangjiang reservoir in the wet season is positively correlated with NO3-and positively correlated with NH4+,the coupling reaction of nitrification-denitrification in the wet season is the main production process of N2O in the water body of the Yunnan section of the Lancangjiang River.(4)The measured and diffused N2O fluxes at the wet-sediment interface during the wet season are 0.43±0.01-0.68±0.03?mol/(m2·h)and 0.31±0.08-0.78±0.12?mol/(m2·h).At the water-sediment interface,the N2O flux is positively correlated with NO3-,negatively correlated with water DO,and positively correlated with NH4+,indicating that the coupling of nitrification-denitrification in the system is the main production process of N2O,so controlling the denitrification rate can reduce the reservoir N2O emissions.