Study On N₂O Distribution And Interface Flux Of Cascade Reservoirs In Lancang River（Yunnan Section）

At present,the construction of hydropower projects has led to the increase of greenhouse gas（especially N₂O）emissions in water bodies,thus aggravating global warming,which has attracted wide attention of scholars at home and abroad.Cascade hydropower developmrnt is an important hydropower project.The study of N₂O under the influence of cascade development measures is of great significance for evaluating the greenhouse gas emission levels of reservoirs.In this study,seven cascade hydropower stations（Miaowei、Gongguoqiao、Xiaowan、Manwan、Dachaoshan、Nuozhadu、Jinghong）in Yunnan section of the Lancang River Basin,one of the thirteen largest hydropower bases in china,was selected as the research object.Through in-situ observation and laboratory simulation experiments,the distribution characteristics and flux process of N₂O at the atmosphere-water-sediment three-phase interface in the region was analyzed.The accumulative effect of cascade hydropower development on the N₂O in the Lancang River Basin was explored,and the“source”and“sink”processes of the N₂O in the reservoir area were revealed.The regional contribution of N₂O provides a scientific basis.The main findings of this paper were as follows:（1）Through in-situ observation and analysis of experimental results,the spatial distribution characteristics of N₂O concentration and saturation in the studied area were revealed.During the summer flood season,the N₂O concentration in the water body of the study area shows a trend of“decreasing and then increasing”along the longitudinal gradient of the river.The N₂O concentration in the Watershed is as follows:Jinghong Reservoir>Nuozhadu Reservoir>Gongguoqiao Reservoir>Miaowei Reservoir>Dachaoshan Reservoir>Xiaowan Reservoir>Manwan Reservoir,reflecting the construction of the reservoir indirectly affects the release process of N₂O in the basin.（2）Through the molecular diffusion model,the N₂O exchange flux at the water-air interface in the reservoir area of the Yunnan section of the Minjiang River was estimated,and the main pathway for the generation of N₂O at the water-air interface was ascertained.In the exploration of the N₂O exchange flux at the Atmosphere-water interface in the reservoir area of the Yunnan section of the Lancang River,it was found that the nitrification of the surface water and the denitrification of the bottom water are the main sources of N₂O production at the interface.The exchange flux ranged from 0.83 to 6.78μmol（m²·d）^-1,indicating that the study area is a medium-scale release“source”of atmospheric N₂O.Affected by natural factors such as human activities and climate differences,the N₂O exchange flux at the water-gas interface in the study area is significantly different in spatial distribution,and the N₂O flux was compared with the water in front of the dam and the dam.The contribution rate of N₂O is higher.Through the analysis of the environmental impact factors of the N₂O flux at the water-air interface,the dissolved N₂O concentration,the N content（TN,NH₄⁺-N,NO₃^--N）and the temperature,TP,p H,DO and water-gas interface N₂O exchange fluxes are all significantly positively correlated,and temperature and N₂O concentration are the most critical environmental driving factors for the interface to affect N₂O exchange flux.（3）Through indoor simulation culture device,it was found that the N₂O at the water-sediment interface in the reservoir area of the Yunnan section of the Minjiang River is mainly produced by denitrification.The N₂O exchange flux ranges from-0.05 to 11.90μmol（m²·d）^-1,reflecting that the water-sediment interface in the study area is the N₂O“source”and“sink”in the water.The results of laboratory experiments showed that the N₂O exchange flux at the water-sediment interface had significant diurnal variation characteristics.Under the action of photosynthesis and respiration,there was a large fluctuation of N₂O flux during the day and a small fluctuation at night.Similar to the environmental impact factor of N₂O flux at the water-air interface,There was a significant correlation between the N₂O exchange flux at the water-sediment interface and the nitrogenous nutrients（TN,NH₄⁺-N,NO₃^--N,NO₂^--N）and organic matter in the reservoir area of the Yunnan section of the Lancang River.And the N:P ratio can also affect the N₂O exchange flux at the water-sediment interface.Therefore,it was inferred that the inhibition of denitrification at the water-sediment interface can reduce the release of N₂O.