Response Of Nitrogen Removal In The Overlying Water To Sediment Resuspension In The Yangtze Estuary Intertidal Wetlands

With the development of industry and agriculture,a large amount of reactive nitrogen entered estuarine ecosystems through rivers,groundwater and atmospheric deposition in recent years.It was shown that the river nitrogen load will be reduced by nearly 50%in estuaries due to the significant nitrogen removal effect.As a land-sea interaction zone,estuarine wetlands have very complex hydrodynamic conditions,like tide,runoff,wave and storm surge,which lead to frequent sediment resuspension.The resuspension of estuarine wetland sediments has an important influence on reactive nitrogen removal.Denitrification and anaerobic ammonia oxidation?anammox?are main processes for nitrogen removal.As denitrification and anammox processes are considered to occur in anoxic or anaerobic environments,previous studies on nitrogen removal mainly focus on sediments or sediment-water interface,while research on overlying water caused by frequent sediment resuspension is limited.Under the interaction of tide and runoff,different salinity gradient will be formed in the estuary area,which is also the main reason for the complete development of high,middle and low different hydrological gradient tidal flats.Illustrating the influence of sediment resuspension on the denitrification and anammox processes in overlying water of estuary tidal flats is of great practical significance for a thoroughly theoretical understanding of nitrogen removal processes,which also provides scientific evaluation of sink-souce imbalance in estuary areas.The tidal flats of Yangtze estuary was selected as study area,and ¹⁵N isotope tracer technique combined with simulation experiment was conducted in order to discussed the influence mechanism of sediment resuspension under different salinity and hydrological gradient to denitrification and anaerobic ammonium oxidation process.Coupled nitrification-denitrification and coupled nitrification-anammox processes were analyzed under resuspension conditions,the contribution of denitrification and anammox process for nitrogen removal in estuary region were estimated,and the ecological environment effect caused by resuspension was further analyzed.The main results are as follows:?1?The different salinity denitrification rate caused by resuspension of the overlying water system was 52.09³492.08 mg N m^-2 d^-1,and the anammox rate was2.47¹43.54 mg N m^-22 d^-1;the different hydrological gradient denitrification rate of the overlying water system was 27.07³93.29 mg N m^-2 d^-1,and the anammox rate was 18.15¹78.55 mg N m^-22 d^-1.In general,the denitrification and anammox rates increased with the increase of sediment resuspension turbidity,but when the turbidity value increased to a certain value,the process rates gradually leveled off.Additionally,with the increase of sediment resuspension turbidity,the abundance of both denitrifying bacteria nirS gene and anammox bacteria 16S rRNA gene increased,indicating that the increase of the abundance of related functional bacteria in water caused by sediment resuspension is an important reason for the increase of denitrification and anammox rates.Under sediment resuspension,denitrification and anammox rates at the different tidal environments varied,and were mainly controlled by the total organic carbon content in the sediments.?2?Under the conditions of resuspension,the coupled nitrification-denitrification rates of high,medium and low tide flats were between 5.27 and 346.81 mg m^-22 d^-1,which accounted for 72%⁸3%of the denitrification rate,and showed an increasing trend of rates and percentage with the increase of turbidity.The coupled nitrification-anammox rates were between 4.13 and 157.74 mg m^-22 d^-1,which accounted for 71%⁸3%of the anammox rate,and the rates and percentage gradually increased with the increase of turbidity.Both of them showed obvious spatial changes in the study area.Coupled denitrification and anammox are the main nitrogen removal processes under resuspension condition in Yangtze estuary.?3?According to the average denitrification rates and anammox rates in this study,denitrification and anammox were estimated to remove 1.93×10⁶ kg yr^-1 and 9.80×10⁴kg yr^-1 nitrogen in overlying water,respectively.Without resuspension,the nitrogen removed by denitrification and anammox were estimated to be 1.03×10⁵ kg yr^-1 and1.21×10⁴ kg yr^-1,respectively.These results demonstrated that the nitrogen removal effects caused by resuspension has an important impact on nitrogen cycling in estuary ecosystems,where denitrification is the main nitrogen removal process.Influenced by sediment resuspension,the reduction of active nitrogen was enhanced,which can effectively alleviate the water eutrophication caused by excessive nitrogen,and it also made up for some of the nitrogen imbalance.But it was estimated that greenhouse gas N₂O emissions can amount to 2.01×10⁵ kg yr^-1.The promoting effect of sediment resuspension on the release rates of N₂O led to a negative ecological effect affecting the aqutic ecosystems.