Soil Biological Nitrogen Fixation Along With Hydrological Gradient In Chongming Dongtan Wetland

Nitrogen?N?is one of major elements affecting aquatic environments in.The industrial sewage and domestic wastewater discharged into the river been discharged into the sea.These human activities make the pollutants in the environment of the reactive nitrogen content increased by nearly 120%,and an excess of nitrogen inputed in estuary coastal ecosystems caused a series of ecological problems,for example,the incidence of algae blooms increased,biodiversity loss in coastal areas and population structure changed.In recent years,the research on the circulation of nitrogen in the estuary of the estuary has become one of the hot scientific problems in the world.Therefore,continue to conduct the research on nitrogen cycle estuary inshore area,and analyzes the key factors that influence the results of the study not only deepen the nitrogen cycle of theoretical knowledge,but also for estuarine tidal flat wetland nitrogen pollution control to provide the scientific basis.Nitrogen fixation is the process of reducing the molecular nitrogen in the atmosphere to ammonia.At present,the research on microbial nitrogen results show that a large number of cyanobacteria in tropical and subtropical waters nitrogen fixing bacteria?trichodesmium,intracellular vegetative algal?of Marine microbial nitrogen fixation,and in ocean temperature is relatively low?<20??waters inhibit the growth of a large number of cyanobacteria,and form the high solubility of the inorganic nitrogen concentration,thus inhibiting rate of nitrogen fixation.However,recent studies have shown that Marine nitrogen fixation,may be mediated by single-celled cyanobacteria and strange bacteria,even in relatively cold?<10??and the Atlantic coast and the Baltic haihe river mouth also discovered the nitrogen-fixing organisms.Studies on the nitrogen fixation and its influencing factors in the east of honhondo island in Japan show that the temperature,nifH abundance and species directly affect the microbial nitrogen fixation rate.The results showed that the nitrogen fixation rate under the influence of nitrogen fixation was influenced by iron in the environment.Studies on soil microbial nitrogen fixation in wetland ecosystem show that temperature is the most significant factor affecting the distribution of nitrogen fixation microorganisms.In addition,the dissolved inorganic nitrogen has a inhibitory effect on the nitrogen fixation of the estuary,but the low level of nitrogen and phosphorus ratio and trace elements of iron can promote the nitrogen fixation.Based on the Yangtze river estuary of Chongming Dongtan wetland as the study area,We were used mud culture experiment combined with isotope tracer technology and molecular biotechnology,in Chongming Dongtan tidal flat sediments under different hydrological gradient,rate of nitrogen fixation and nitrogen fixation function gene research,and the analysis of physical and chemical factors in sediment environment and the influence of microorganism on nitrogen fixation process.This study includes two parts:field sampling combined with indoor mud culture experiment and indoor control experiment,the main research results obtained are as follows:?1?The soil BNF generally decreases with the increase of water flooding frequency,and the fluctuation amplitude is more obvious in summer.There is a significant difference?P<0.05?.The soil BNF shows that summer is greater than winter,and the topsoil of vegetation cover is higher than that of bare beach.Summer plant and bare beach soil BNF were 0.61-17.90?mol N kg^-1 h^-1 and 1.41-12.10?mol N kg^-1 h^-1 respectively.Winter were 0.64-3.54?mol N kg^-1 h^-1 and 0.47-2.60?mol N kg^-1 h^-11 respectively.?2?The distribution characteristics of nitrogen-fixing nifH abundance along the hydrological gradient were similar to that of BNF,The nifH abundance of vegetation and bare beach soil were 1.28×10⁷-4.61×10⁷ copies g^-1 and 2.56×10⁷-12.3×10⁸ copies g^-11 respectively.The nitrogen fixation function gene nifH abundance was significantly positively correlated with the nitrogen fixation rate distribution,it is indicated that the amount of nifH abundance in soil determines the distribution of BNF to large extent,and the change of BNF could reflect the change of nifH abundance.In addition,both BNF and nifH abundance of plant surface soil were higher than that of bare beach,indicating that plant root system could effectively promote the circulation of soil microorganisms.?3?The soil physical and chemical properties had a certain effect on BNF and nifH content of soil nitrogen fixing gene.NO₃^-and plant in winter and GuangTan summer are in very significant negative correlation,the surface BNF plant surface BNF has significant positive correlation with winter,sulphate and TOC and summer were very significant or significant positive correlation,plant BNF Fe³⁺and summer GuangTan were very significant positive correlation,BNF Fe²⁺plant BNF has significant positive correlation with winter.?4?Salinity is the key factor affecting soil nitrogen fixation in the soil environmental factors of changjiang estuary.The nitrogen fixation rate of surface soil decreased with the increase of salinity.With the growth of culture time,the surface soil nitrogen fixation rate showed a trend of increasing and decreasing.In the vertical direction,the nitrogen fixation rate in the topsoil?0-5 cm?with the maximum effect of overlying water increased with the increase of NO₃^-concentration in the above water,which showed a decreasing trend as a whole.In the horizontal direction,the nitrogen fixation rate of the surface soil showed a certain degree of increase and then decreased with the increase of culture time.At the time scale,the change of soil nitrogen fixation rate of different iron oxide(Fe³⁺)concentration was consistent with that of soil salinity gradient and soil NO₃^-change.In the vertical direction,the nitrogen fixation rate of different iron oxide concentration of soil also showed the same change pattern.