Comparative Study Of Denitrification Process And Correlative Microorganism In River Sediments Under Urban Pollution In Tianjin

City and its nearby rivers are main water bodies in water ecosystem and the pollution from city has direct impact to these rivers.The pollution levels of these urban rivers are closely related to the security and stability of urban ecological environment.As widely acknowdeged,urbanization level is rising rapidly in China,therefore,urban water pollution is becoming increasingly obvious and contaminated river water issue is particularly prominent.For this reason,river pollution has become a research focus.Sediment in urban river receives different levels of nitrogen pollutants from city,and developes into a potential but unobservable source of river pollution.It has been widely recognized that denitrification process is one of approaches to remove nitrogen from riparian environment permanently.Tianjin has several typical water body types,and the rivers under different pollution levels have significant different characteristics.Due to the differentiated pollution conditions,river's abilities of denitrification are distinct.Therefore,it is significant to study the denitrification process in river sediment under the impacts of different pollution conditions and investigate space and temporal differences as well as the major influencing factors.Particularly,this study conducted comparative study between urban polluted river?drainage?and other types of rivers?nature rivers and transitional rivers?from both macroscopic and microscopic perspectives and placed emphasis on the impact of urban water pollution.Meanwhile,in order to further explore the nitrification and denitrification processes in river sediments in different river ecosystems,this study also designed experiment to analyse microbial community characteristics of related denitrifiers under different contamination in typical rivers in Tianjin city and compared the richness,diversity and community structure of denitrifying bacteria in sediment between natural river and drainage.This study aims at rivers in different pollution features,as main research objectives,combining methods of field sampling,in situ measurement and laboratory analysis.Conclusions in different seasons and sites list as follows:?1?The average NO₃^--N,NH₄⁺-N in river water showed a certain spatial distribution feature.NO₃^--N ranged from ND to 9.26mg·L^-1(3.70±2.29mg·L^-1 in average),and NH₄⁺-N maximum content was 58.44mg·L^-1(6.23±8.98mg·L^-1 in average).The average NO₃^--N and NH₄⁺-N concentration in summer and fall were lower than spring and winter.The water content changed from 27.54%to 212.58%.Bulk density ranged from 0.81 to 2.47g·cm^-3.PH was between 5.56 and 9.56,showed weak alkalinity in average.The SOC varied from 1.67 to208.16mg·g^-1(average 20.78±7.95g·kg^-1),and SOC was the lowest in spring,while the highest in winter.On vertical distribution,SOC was gradually reduced as depth increased.Extractable NO₃^--N and NH₄⁺-N in sediment were ranged from 0.02 to 48.26mg·kg^-1(average 7.08mg·kg^-1)and 155.59 to 2170.91mg·kg^-1 respectively.This study found that the higher the pollution level,the higher the concentration in spacial variation.On vertical distribution,extractable NO₃^--N and NH₄⁺-N were gradually reduced as depth increased.?2?The denitrification rate of river sediment in Tianjin city varied from 0.51 to8.99mg-N·m^-2·h^-1.Results showed that summer had the highest rate and fall showed the lowest.The average denitrification rates in spring,summer,fall and winter were 1.91±1.53,3.07±1.25,1.61±0.53 and 1.87±0.79mg-N·m^-2·h^-11 respectively.From different study sites,sediment denitrification rates showed regional difference in different sites.NYN had the highest rate in2.59±1.63mg-N·m^-2·h^-1;BTD and DGD were the second,2.09±0.98mg-N·m^-2·h^-1 in average;natural rivers YDN and DLN had the lowest rate of 1.98±1.29mg-N·m^-2·h^-1 in average.The sites located near severe and complex pollution source showed higher denitrification rates,and the sites in ideal surrounding?featured in wide river and high water velocity?had lower denitrification rates.The denitrification rate in the 0² cm depth of sediment ranged from 0.51to 8.99 mg-N·m^-2·h^-1.This study found that the deeper the sediment profile,the lower the rate.The highest rate was in the top 0⁰.5cm?42.01%in total?.The denitrification rate was positively correlated with pH,SOC,water content,bulk density,extractable nitrate nitrogen in sediment and nitrate nitrogen in upper water?p<0.05?based on correlation analysis.While,this study found the rate had no correlation with Eh and ammonium nitrogen in upper water.?3?Yearly total denitrification abilities in average in Tianjin river sediment under different pollution level were different.BTD drainage was 6.01±1.95mg-N·m^-2·h^-1;DGD drainage was6.52±2.53mg-N·m^-2·h^-1;YDN nature river was 5.72±2.75mg-N·m^-2·h^-1;DLN nature river was6.15±3.87mg-N·m^-2·h^-1;NYN transitional river was 7.77±4.14mg-N·m^-2·h^-1.Summer had the highest ability in 46.11±0.71mg-N·m^-2·h^-1?36.52%?;spring was 28.71±2.30mg-N·m^-2·h^-1?22.79%?;winterwas27.29±1.21mg-N·m^-2·h^-1?21.61%?andfallwas24.16±0.50mg-N·m^-2·h^-1?19.13%?.Drainage rivers had the highest denitrification potential(275.3ng-N·g^-1·h^-1)than other river sites.In seasonal variation,denitrification potential in winter was higher than the rate in summer.In vertical variation in sediment,top 0-5cm of sample showed larger denitrification potential than 5-10cm.?4?N₂O natural rate ranged from 0.39 to 12.99 mg-N·m^-2·h^-1 in sediment,which summer had the highest rate,and spring showed the lowest.The average N₂O natural rate was4.50±1.14mg-N·m^-2·h^-1 in summer;1.36±0.25 mg-N·m^-2·h^-1 in fall;1.30±0.19mg-N·m_-2·h^-1 in winter and 0.98±0.12mg-N·m^-2·h^-1 in spring.In different typical polluted rivers,N₂O natural rate in nature rivers DLN and YDN group showed the highest,which average rate were2.33±1.96mg-N·m^-2·h^-1 and 2.47±2.10mg-N·m^-2·h^-1 respectively.Draniage rivers BTD and DGD were 1.89±1.38mg-N·m^-2·h^-1 and 1.91±1.31mg-N·m^-2·h^-1 respectively.Transitional river NYN was 1.98±1.24mg-N·m^-2·h^-1.In vertical variation,the N₂O natural rate was the highest in upper sediment surface layer and decreased with increase of depth.N₂O natural rate showed correlation with pH,SOC,water content and extractable NO₃^--N in sediment?p<0.05?positively,however it was negatively correlated with Eh?p<0.05?and had no significant correlation with NO₃^-,NH4⁺in upper water and bulk density.?5?Microbial samples of Tianjin river sediment had high Coverage Index,which Coverage Index was greater than 0.95.It means that the lab samples can represent the denitrifying microorganisms in real sites.The polluted drainage river sites had higher abundance of nirS gene in denitrifying bacteria than natural rivers.The drainages showed higher genetic richness than natural rivers.In vertical variation,the estimated richness in underlying sediment layer was slightly higher than the sediment surface above.The Shannon Index in drainage was higher than the natural river site,and the denitrifying microbial diversity in drainage sediment was higher than the natural river.In different layers in depth,Shannon index was higher in top surface sediment than the bottom sediment in all sampled rivers,which means the surface of sediment had higher diversity of denitrifying bacteria group than the bottom sediment.Overall,denitrifying microorganisms'richness and diversity were higher in drainage sediment than the natural river sample.According to classification of related gene sequence,denitrifying bacteria group in natural river sediment?YDN?mainly belonged to Proteobacteria,including Alphaproteobacteria and Betaproteobacteria.Dominant bacteria group in bottom sediment was Betaproteobacteria,including Azoarcus,Dechloromonas and Sulfuritalea species.In drainage BTD,denitrifying bacteria group in river sediment mainly belonged to Betaproteobacteria,mainly including Azoarcus and Sulfuritalea species.