Nitrogen Fixation Rate And Its Influencing Factors In Surface Water Of Shanghai Rivers

Biological nitrogen fixation plays a very important role in nitrogen cycling and maintaining ecological balance.The nitrogen fixation and nitrogen fixation ability of river water in Shanghai are unknown and will affect the accurate understanding of nitrogen load and nitrogen source in Shanghai.Therefore,this study takes eight representative rivers in different areas of Shanghai as the research object,discussing the temporal and spatial distribution of nitrogen fixation rate and the effect of P nutrient on chlorophyll a and nitrogen fixation rate,analyzing the effects of various environmental factors on nitrogen fixation rate and the main influencing factors and estimating the annual nitrogen fixation amount and its environmental significance of river water in Shanghai,which can provide the basis for the future construction of nitrogen balance and nutrient reduction target,and can also provide the theory for the prevention and resolution of eutrophication and effective basis.This study lasted for one year and the main conclusions are as follows:?1?This study was carried out in four seasons of spring?April 2015?,summer?July 2015?,autumn?October 2015?and winter?January 2016?,respectively.During the period,the water temperature changed among 6.8?34.6 ?,DO changed in the range of 1.30mg/L?10.31mg/L,ranges of pH,Eh and salinity were:5.59-8.23,-83.4?-2.1 mv and 0.1?0.4ppt.There were significant differences in water temperature?p<0.01?,DO?p<0.05?and pH?p<0.05?in different seasons.In the spatial distribution,the DO concentration and salinity of the sampling points in different rivers were significantly different?p<0.05?.?2?There were significant temporal and spatial characteristics of water quality in eight water samples.TP,PP and NO₃^--N were significantly different in time variation?p<0.05?.The contents of TP?PP and NO₃^--N were were distributed in 0.04?0.43mg/L?0.008-0.24mg/L and 0.20-3.41mg/L,respectively.In seasonal variation,TP and PP showed high in spring and summer,showed low in autumn and winter;NO₃^--N showed high in spring and winter,showed low in summer and autumn.The contents of TDP and NH₄⁺-N were 0.02?0.34mg/L and 0.37?5.56mg/L,respectively.The average content of NH₄⁺-N in winter was higher than that in spring and autumn,and the content of NH₄⁺-N was the lowest in summer.From the spatial distribution,the water quality factors of the river sampling points in Shanghai had certain spatial distribution characteristics.?TP:0.2mg/L?;the NH₄⁺-N contents of JD and PT all exceeded the surface water quality standard?NH₄⁺-N?,and the water content of the samples was higher than that of the surface water:2mg/L);the NO₃^--N content of the eight sampling points did not exceed the limit.QP and CM TP,NH₄⁺-N and NO₃^--N content was low,water quality environment was good.?3?The algae of the surface water in the rivers of Shanghai are mainly composed of six major categories,namely,cyanobacteria,dinoflagellates,algae phytophytes,diatomophytes,cryptophytes and chlorophyta.?p<0.01?:spring>summer>autumn>winter,the total density of algae in the surface water of the river showed obvious time distribution.In the spatial distribution,the total density of the four seasons is the highest,and the total density of the algae of JD,PT,JS and DM were close,while the QP and CM were lower.Cyanophyta in the spring and summer accounted for a higher proportion of algae density,were lower in autumn and winter,with Anabaena and Microcystis as dominant species.In the eight river sampling sites of this study,only anabaena was seen all year round and was dominant species in summer.The density of nitrogen-fixing algae in the river sampling site has obvious seasonal variation characteristics?p<0.01?,showing the obvious regularity of summer>autumn>spring>winter.As for the spatial distribution,the total density of nitrogen,algae of PT,FX and PD were the highest,SJ,JD,JS solid nitrogen algae density were in middle,two sampling points QP,CM identified the least density of nitrogen and algae.?4?The total nitrogen fixation rate of surface water in Shanghai was among 33.99±14.39ngN·L^-1·h^-1 and 109.53±14.31ngN L^-1·h^-1,and had significant regularity in time distribution and spatial distribution.Seasonal changes performed highest in summer,followed by spring and autumn,and lowest winter law.The average nitrogen fixation rate in summer and autumn was 75.21±12.95 ngN L^-1·h^-1 and 76.52±12.91 ngN·L^-1·h^-1,respectively,and the average nitrogen fixation rate was 90.51 ± 12.95 ngN·L^-1·h^-1.The average nitrogen fixation rate was the lowest in summer,which was 57.98±15.73 ngN·L^-1·h^-1.Spatially,the average nitrogen fixation rate of the surface water of eight sampling sites was sorted by:PT>>PD>FX>SJ>JD>JS>QP>CM.The higher the nitrogen fixation rate of the surface water of the river appeared in the three sampling points of PT,PD and FX,and the mean values of SJ,JD and JS were similar,and the lower values appeared at QP and CM sampling points.?5?Environmental factors such as light,water temperature,temperature,nitrogen fixation density,chlorophyll a,salinity,Eh,pH and water environment closely related to nitrogen fixation rate?p<0.01?.The nitrogen fixation rate and chlorophyll a had a significant positive correlation with the nitrogen fixation rate?p<0.01?.The results showed that the photosynthetic rate,water temperature and nitrogen fixation rate showed a positive linear correlation?p<0.01?.DO?p<0.01?,Eh?p<0.05?had a linear negative correlation with nitrogen fixation rate,and had a linear positive correlation with salinity?p<0.05?.There was a significant linear correlation between TP?p<0.01?,TDP?p<0.01?,PP?p<0.05?and nitrogen fixation rate,indicating that P had a significant effect on nitrogen fixation rate.?p<0.01?.The effects of NH4 +-N and NO₃^--N on the rate of nitrogen fixation were very close.The results showed that NH₄⁺-N and NO₃^--N had a significant negative correlation with nitrogen fixation rate?p<0.01?.The nitrogen fixation rate is affected by various environmental factors.The water temperature and the density of nitrogen and algae are the main environmental factors affecting the rate of nitrogen fixation.This is also an important reason for the difference of nitrogen and nitrogen rate.?6?The chlorophyll a contents of eight rivers in four seasons in Shanghai were measured.The chlorophyll a contents of the four sampling points in four seasons was analyzed by Pearson correlation with TP,TDP and PP.It was found that there was a significant correlation between chlorophyll a and phosphorus?p<0.01?,indicating that P had a significant effect on chlorophyll a.The effect of the addition of P on the chlorophyll a in the surface water of Shanghai rivers was-49.53%?65.75%,-78.23%?175.15%in summer,-43.37%?195.08%in autumn and-85.89%in winter,?34.80%,showing that the effect of the addition of P on chlorophyll a was in the order of autumn,summer,spring and winter.The effect of applying P to chlorophyll a in Shanghai river surface water showed that the effect of P on chlorophyll a was affected by the population of algae in river water body.Because of the different dominant species of phytoplankton,the absorption degree of P was different,the primary productivity level and The initial P concentration of river water was different,and the effect of adding P to chlorophyll a is not the same.A certain concentration of P addition promoted the growth of chlorophyll a,but the effect of each sampling season and four different applied P concentrations was not consistent.?7?The effect of applying P on the nitrogen fixation rate of surface water in Shanghai were-52.60%?105.86%in spring,-87.52%?32.69%in summer,-36.68%?56.80%in autumn and-30.83%?63.01%in winter.The order of the effect of adding P on the nitrogen fixation rate of surface water in Shanghai was:spring,winter,autumn and summer.The effect of addition of P on the nitrogen fixation rate of surface water in Shanghai was promoted under certain concentration conditions,but the effect of different sampling points,different seasons and different P concentrations on nitrogen fixation rate was not consistent.The addition of high concentration of P to the nitrogen fixation rate had an inhibitory effect,and the higher the original P concentration in the surface of the river,the more obvious the effect of the addition of P on the nitrogen fixation rate.?8?The annual nitrogen fixation amount of surface water in Shanghai was an important N source.The nitrogen fixation rate of different counties and counties is different,mainly due to the difference of nitrogen fixation rate and the difference of water area in the surface water of the river.The annual nitrogen fixation rate was the largest in Pudong District,and the annual nitrogen fixation in Putuo District was the smallest.In different seasons,the amount of nitrogen fixation in winter accounts for 30.43%of annual nitrogen fixation,25%of annual nitrogen fixation in spring and autumn,and only 19.57%in winter.Therefore,the N input problem in summer was more concerned.Algae a large number of breeding,resulting in deterioration of water quality,water eutrophication of an important reason.The annual nitrogen fixation rate of surface water in Shanghai is about 2.10Gg,which was 3.07%of the nitrogen input in Shanghai,showing that the nitrogen fixation of river surface water is of great significance to nitrogen input and nitrogen load.