Coupling And Uncoupling Of Nitrogen Removal Processes With Nitrification And Their Influencing Mechanism In The Yangtze Estuary

In the last few decades,human activities have contributed to excessive input of reactive nitrogen into environments.Increased nitrogen loading triggers severe environmental and ecological issues.As the critical transition areas of both land and sea,nitrogen pollution is particularly serious in estuarine and coastal areas.Nitrogen removal processes(including denitrification and anammox)play a vital role in counteracting the adverse effects caused by nitrogen overloading.Therefore,the research about the nitrogen removal processes in estuarine and coastal environment is a hot issue and scientific frontier.Depending on the sources of the reaction substrates,nitrogen removal processes can be categorized as uncoupled nitrogen removal processes(oxidized nitrogen deriving directly from overlying water)and coupled nitrification nitrogen removal processes(oxidized nitrogen deriving from nitrification).These two nitrogen removal processes could effectively remove nitrate and ammonia,respectively.Although numurous studies have beed done on estuarine and coastal nitrogen removal processes,there is a lack of systematic understanding of uncoupled and coupled nitrification nitrogen removal processes and their environmental regulating mechanisms remain unclear.Against this background,the Yangtze Estuary was selected as the research area,and continuous-flow experiments combined with ¹⁵N isotope technique were conducted to investigate nitrogen removal processes,to distinguish the uncoupled and coupled nitrification nitrogen removal processes and to reveal their environmental regulating factors.Then,combining with the data about nitrogen fluxes into the estuarine and coastal environments,we analyzed the scavenging capacity of different forms of reactive nitrogen and clarified the importance of nitrogen removal processes in the Yangtze Estuary.This research could not only deepen the theoretical understanding of estuarine nitrogen cycling,but also provide reference for protection of estuarine and coastal eco-environments.The main results are as follows:(1)In intertidal wetlands of the Yangtze Estuary,the in situ rates of denitrification and anammox ranged from below the detection limit(ND)to 152.39?mol N m^-1 h^-1 and from ND to 43.06?mol N m^-1 h^-1,respectively.In situ denitrification rates were significantly positively correlated with the concentrations of nitrate,nitrite,organic carbon and sulfide,while the in situ anammox rates were significantly correlated with the nitrite concentrations.The rates of uncoupled and coupled nitrification nitrogen removal processes ranged from ND to 195.45?mol N m^-1 h^-1(mean value:31.12?mol N m^-1 h^-1)and from ND to 21.36?mol N m^-1 h^-1(mean value:4.48?mol N m^-1 h^-1),respectively.The controlling factors for uncoupled and coupled nitrification nitrogen removal processes were different.The former was mainly affected by the concentrations of nitrate,nitrite,organic carbon and sulfide,while the latter had significant positive correlations with the concentrations of dissolved oxygen and ferric iron.Uncoupled nitrogen removal processes were the main nitrogen removal pathways in the tidal flat wetlands of the Yangtze Estuary,accounting for 89%of the total nitrogen removal,while coupled nitrification nitrogen removal processes only accounted for 11%of the total nitrogen removal.(2)Along the salinity gradient of the Yangtze Estuary,the in situ rates of denitrification and anammox ranged from 0.44–7.03?mol N m^-1 h^-1 and 0.09–1.21?mol N m^-1 h^-1,respectively.There was no significant correlation between the in situ nitrogen removal rates and changes of salinity.The in situ denitrification rate was mainly affected by the sulfide concentration,active organic carbon content and particle size composition,while the in situ anammox rate was closely related to temperature.Along the salinity gradient of the Yangtze Estuary,the rates of uncoupled and coupled nitrification nitrogen removal processes ranged from 0.21–5.09?mol N m^-1 h^-1(mean value:1.64?mol N m^-1 h^-1)and from ND–4.99?mol N m^-1 h^-1(mean value:1.76?mol N m^-1 h^-1),respectively.With the increase of salinity,the rates of the uncoupled nitrogen removal processes tended to decrease,while the rates of the coupled nitrification nitrogen removal processes tended to increase.In addition,NO₃^-,ferrous iron and organic carbon were favorable to uncoupled nitrogen removal processes,while NO₃^-had adverse impact on coupled nitrification nitrogen removal processes to some extent.Along the salinity gradient,the relative importance of uncoupled and coupled nitrification nitrogen removal processes existed significant spatial variability.With salinity increase and NO₃^-concentration decrease,the relative importance of coupled nitrification nitrogen removal processes increased,while the relative importance of uncoupled nitrogen removal processes decreased.(3)In the seasonal hypoxic areas of the Yangtze Estuary,the in situ rates of denitrification and anammox ranged from 0.01–20.30?mol N m^-2 h^-1 and from ND–1.04?mol N m^-2 h^-1,respectively.Summer hypoxia is one of the most important environmental characteristics in study area,which was in favor of in situ denitrification rates,but adverse to in situ anammox rates.In the seasonal hypoxic areas of the Yangtze Estuary,the rates of uncoupled and coupled nitrification nitrogen removal processes ranged from ND–5.32?mol N m^-2 h^-1(mean value:0.47?mol N m^-2 h^-1)and from ND–15.06?mol N m^-2 h^-1(mean value:2.39?mol N m^-2 h^-1),respectively.Coupled nitrification nitrogen removal processes were the dominant nitrogen removal pathway,with an average contribution of 84%to the total nitrogen removal.In the present study,coupled nitrification nitrogen removal processes had the highest rates under summer hypoxic condition,which indicated that compared with aerobic nitrification,anaerobic nitrogen removal processes were the controlling steps of coupled nitrification nitrogen removal processes in the seasonal hypoxic areas of the Yangtze Estuary.In general,dissolved oxygen levels,temperature and ammonia concentrations were the important environmental factors affecting the coupling of nitrogen removal processes with nitrification.(4)In general,the annual removal fluxes of dissolved inorganic nitrogen were 8.02×10⁴t in the Yangtze Estuary,of which the annual removal fluxes of NH₄⁺and NO_x^-were3.18×10⁴ t and 4.84×10⁴ t,respectively.Combined with the fluxes of dissolved inorganic nitrogen into the sea,the in situ nitrogen removal processes in the Yangtze River estuary could remove 8%of the total dissolved inorganic nitrogen discharge into the estuarine and coastal environments every year.In detail,the removal efficiency of NH₄⁺and NO_x^-was 78%and 5%,respectively.Overall,denitrification was generally the main nitrogen removal process,and coupled nitrification nitrogen removal processes were generally the main nitrogen removal processes in the study area,compared with uncoupled nitrogen removal processes.The relative importance of different nitrogen removal processes was closely related to salinity and nitrate concentrations.