Effect Of Cascade Hydropower Development On Sediment Nutrient Diffusion Flux And Denitrification Rate In Lancang River

The construction of cascade hydropower stations in Lancang River Basin had changed the hydrological and material transport process of the river.And the nitrogen difference and the influencing factors of sediment interstitial water in different reaches need to be clarified urgently.In this paper,the data of nutrients and environmental factors in interstitial water were collected from the natural reach and cascade reservoir of Lancang River（Yunnan Province）.Therefore the differences of nitrogen,the influencing factors,microbial community structure and denitrification rate were analyzed.The main conclusions obtained are as follows:（1）The distribution trend of NH₄-N and NO₃-N in interstitial water of surface sediments was opposite and the spatial difference was significant.The NO₃-N content in natural reach was higher than that in reservoir reach,while the NH₄-N content was opposite.In the vertical direction of each reservoir area,the concentration of NH₄-N in sediment interstitial water increased with the increase of depth,the interstitial water was higher than the overlying water,and the concentration distribution of NO₃-N was opposite.（2）The DO content of the overlying water on the surface sediments of the reservoir decreased along the course and the conductivity was higher in the natural reach than in the reservoir in winter.The surface sediment temperature of the natural reach is lower than that of the reservoir,and the temperature in the reservoir decreased first and then increased with the increase of depth.ORP was a transition state from weak oxidation state to weak reductive state in the natural river reach,and it is weak reductive state to reductive state in the reservoir.The vertical reduction environment increases with the increase of depth,and the sediment is in neutral-weak alkaline environment.ORP was in the transition state from weak oxidation to weak reduction in natural reach,from weak reduction state to reduced state in the reservoir,the vertical reduction environment increased with the increase of depth,and the sediments were in the neutral^weak alkaline environment.（3）NH₄-N in surface sediment interstitial water of cascade reservoir reach had a significant negative correlation with pH value,ORP and Temp,while TN had a significant negative correlation with ORP and pH.Vertically,NH₄-N diffused upward from interstitial water to overlying water,and sediment was the"source".NO₃-N diffused from overlying water to interstitial water,and the sediment was a"sink".The pH value of natural river reach was the main influencing factor of ammonia nitrogen and nitrate nitrogen diffusion fluxes,while ORP and SPCond were the main influencing factors of cascade reservoir reach.（4）The microbial ACE and Shannon indexes in the reservoir were both higher than those in the natural reach,and the diversity of microflora in summer was higher than that in winter.The dominant bacteria in natural river reach were Proteobacteria and Bacteroidetes,and the dominant bacteria in reservoir reach were Proteobacteria.Acinetobacter was the dominant bacterium in natural river reach,Acinetobacter and Exiguobacterium were the dominant bacteria in reservoir reach.The microbial community structure in the natural reach was very similar,and SpCond and Temp were the most important influencing factors for the composition of microbial community structure in sediments（p<0.01）.（5）The denitrification rate of sediments in the natural reach was higher than that in reservoir reach,and the denitrification rate in the Xiaowan was much higher than that in the other reservoir sites.The denitrification rate in each reservoir was lower in the surface sediments and first increased and then decreased in the vertical direction with the increase of depth.（6）The abundance of denitrifying functional genes in the sediments was amoA>napA>nirS>nirK>nosZ,and the abundance of amoA,napA and other genes was significantly different（p<0.05）.There was a significant negative correlation between amoA gene abundance and ORP,a significant positive correlation between napA gene abundance and Temp,a significant positive correlation between napA gene abundance and NO₃-N and Temp,and a very significant positive correlation between nirK gene abundance and NO₃-N.