| Nitrogen(N)is one of major elements affecting aquatic environments in.In recent years,excessively reactive nitrogen has been transported into urban rivers caused by fossil fuel combustion,domestic emission and industry discharge.These nitrogen in rivers derived mainly from anthropogenic,is in the form of nitrate.The overloaded reactive nitrogen in rivers may induce various environmental problems,such as water quality deterioration,water acidification and decreasing in biodiversity,which posed a threat to urban riverine ecosystem.Therefore,the enriched-nitrogen in urban rivers has been attracted many attentions and been considered as the hot scientific problem in the study of urban environment.However,there is limited information on nitrogen cycling and associated removal mechanism in urban rivers.The understandings of nitrogen removal pathways,fate and eco-environment effect in sediment of urban rivers are very important to deepen urban nitrogen cycling and associated mechanisms.Thus,in this study,we conducted the dissimilatory nitrate reduction processes and revealed the transformation mechanisms and fate of nitrate in urban riverine sediments of Shanghai.Dissimilatory nitrate reduction processes,including denitrification,anaerobic ammonium oxidation(ANAMMOX)and dissimilatory nitrate reduction to ammonium(DNRA).Both denitrification and ANAMMOX by conversion nitrate to gaseous nitrogen,which are considered as dominant microbial pathways in these ecosystems.Dissimilatory nitrate reduction to ammonium(DNRA)is an anaerobic microbial pathway that transforms nitrate into bioavailable ammonium,which still remain in aquatic ecosystems,playing an important role in the transformation mechanisms of nitrate.In this study,we conducted sediment slurry experiments combined isotope tracer technology to investigate the dissimilatory nitrate reduction processes(including denitrification,anammox and DNRA)and reveal their environmental implications in urban riverine sediments of Shanghai.The main findings of this work are as follows:(1)A great spatiotemporal variations of dissimilatory nitrate reduction processes in sediments of urban river networks.The measured rates of denitrification in sediments ranged from 0.19 to 16.9 nmol N g-1 h-1 in winter and 1.98 to 98.7 nmol N g-1 h-1 in summer.The rates of anammox in sediments ranged from 0.0404 to 23.7 nmol N g-1 h-1 in winter and 0.243 to 12.9 nmol N g-1 h-1 in summer;the detected rates of DNRA in sediments ranged from 0 to 7.28 nmol N g-1 h-1 in winter and from 0 to 10.3 nmol N g-1 h-1 in summer.The potential rates of denitrification and DNRA were significantly higher in summer than in winter,while the average rates of anammox were significantly higher in winter than in summer.(2)The dissimilatory nitrate reduction processes of Shanghai urban river networks may be affected by multiple environmental variables with complicated interactions Dissolved oxygen,total organic carbon,nitrate,ammonium,sulfide,Fe(?)and Fe(?)were found to have significant influence on these nitrate reduction processes.Statistical analyses showed that the potential denitrification rates were correlated with dissolved oxygen,sulfide,total organic carbon,the content of nitrate and Fe(?)/Fe(?).In addition,the potential rates of anammox were correlated with ammonium,Fe(?)and dissolved oxygen.However,DNRA was affected by nitrate and C/N.(3)Compared with DNRA,denitrification and anammox are the major process in removing nitrate in Shanghai urban riverine networks.Denitrification contributed 11.5-99.5%to total nitrate reduction,as compared to 0.343-81.6%for anammox and 0-52.3%for DNRA.In this study,the denitrification contributed 50.2%of total nitrate reduction(sum of denitrification,anammox and DNRA)in the sediments of the Shanghai's urban rivers.Anammox accounted for 40.3%of the total reduction of nitrate,implying that anammox plays an important role in nitrogen removal.However,DNRA played a small fraction in the nitrate reduction,accounting for 9.3%of the nitrate loss on average.It is estimated that annual 8.86 × 104t and 4.45 × 104 t of nitrogen loss were linked to respective denitrification and anammox,about 1.33×105 t N yr-1 was linked to both denitrification and anammox processes,which accounted for about 20.1%of total inorganic nitrogen transported annually into the Shanghai urban river networks. |
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