Effect Of Salinity Intrusion On Nitrogen Migration And Transformation In Sediment-water System Of Minjiang River

Estuaries are important passageways which connect land,freshwater and ocean.Nutrients are transported to the ocean through estuaries.Excessive nitrogen discharged to the estuary will cause aquatic eutrophication and then further damage the aquatic ecological balance.In tidal river,the migration and transformation mechanism of nitrogen are more complicated because of complicated hydrodynamic and water environment.Hence,nitrogen cycling mechanism researched in this paper can provide important theoretical benchmark and technical support for environmental protection and management of estuaries.In this study,Minjiang estuary was selected as the observation prototype to research the spatiotemporal distribution of basic physiochemical characters,nitrogenous compounds and their relationship in the tidal cycle.The results showed that salinity intrusion can directly affect the nitrogenous compounds.During the flood tide,NH₄⁺-N and TN both increased with the increase of salinity.The vertical distribution of NH₄⁺-N and TN showed the same trend with the salinity stratification.The average concentration in the bottom layer was generally higher than that in the surface layer.However,NO₃^--N showed the opposite trend that the average concentration in the bottom layer was lower than that in the surface.Salinity had little influence on the NO₃^--N changes.To sum up,TN in Minjiang river estuary was significantly affected by NH₄⁺-N,and they showed the similar change.Using the variables separation method,the effects of salinity,initial nitrogen concentration and dynamic disturbance on adsorption and desorption process of NH₄⁺-N and NO₃^--N in sediments were studied.The results suggested salinity inhibited the NH₄⁺-N adsorption of the sediments.The sediment desorption capacity for NH₄⁺-N increased with the salinity.It was about 13 times higher under 30 salinity compared with 0 salinity.Low salinity was more conducive to sediment desorption for NO₃^--N.According to the response surface results,salinity and rotation speed had significant effects on the NH₄⁺-N desorption of the sediments,while initial nitrogen influenced the NO₃^--N desorption more.The in-site water and sediment of Minjiang river were used in the annular flume to simulate the change of nitrogen-containing compounds caused by actual river salinity.Nitrogen functional genes were tested to research the nitrogen migration and transformation mechanism influenced by salinity.The results suggested that when the salinity increased,the NH₄⁺-N concentration was increased.However,the NO₃^--N concentration was decreased.The absolute abundance of nfrA gene in the experimental group increased obviously when the salinity increased,which led to the accumulation of NH₄⁺-N in water.When the salinity increased,the absolute abundance of nxrA gene in the suspended solids decreased.However,the absolute abundance and relative abundance of narG and napA were significantly increased.Which induced that the high salinity was not conducive to the accumulation of NO₃^--N.