Sedimentary Nitrogen Removal Pathways In Coastal Oceans Of China:the Environmental Factors And Climatic Effects

Nitrogen is one of the essential elements for all living things in the earth system and is the key factor in regulating primary productivity.Nitrogen cycle is an important component of biogeochemical cycle on the globe.Since the industrial revolution,the production and use of reactive nitrogen(Nr)increased exponentially and exceeded the amount of natural nitrogen fixation.The use of artificial Nr causes serious disturbance to the natural nitrogen cycle and induces inevitable environmental problems.In the context of global change,nitrogen pollution and climate change have become the major environmental problem after biodiversity loss.The Land-Ocean Transition Zones(LOTZs)connect land and ocean,include estuaries and marginal sea areas which are most affected by human activities and global changes,and where nitrogen cycle is most active and complex.The LOTZ receives large amount of anthropogenic Nr and acts as hotspots for Nr removal,sediments in the LOTZ are the major places for Nr removal.Bioavailable nitrogen is mainly removed via microbially-mediated denitrification and anaerobic ammonium oxidation(anammox)in aquatic ecosystems,which are widespread in sediments globally.Sedimentary Nr removal plays an important role in alleviating eutrophication in coastal waters.In addition,nitrous oxide(N2O),a kind of greenhouse gas,is an intermediate product during denitrification,and sedimentary Nr removal is an important source of N2O emission.The LOTZ is the key place to manage and control N2O emission,and subsequently,mitigate the greenhouse effect Therefore,study of sedimentary Nr removal in LOTZ is of great significance since it is closely related to the health of coastal ecosystems and even the climatic problems at local,region and global scale.To understand the mechanisms of sedimentary Nr removal processes and environmental controls and associated climatic effects in the context of global change,Jiulong River Estuary(JRE),Yangtze River Plume(YRP),Sansha Bay and Bohai Sea were taken as research areas in this study.Basing on the isotopic pairing technique,both slurry incubation and intact core incubation were conducted in the study area.The major Nr removal pathways and the potential rates of each process were identified and quantified via slurry incubation.While the in situ rates of denitrification,anammox and associated N2O production in sediments of JRE and YRP were quantified via intact core incubation.We evaluated the effective environmental factors for Nr removal processes,and then articulated the role of sediments in the coastal nitrogen cycle and in climate under the context of anthropogenic influence.Additionally,the responses of sedimentary Nr removal and associated N2O production to the changing of different environmental factors,such as temperature,15NO3-and glucose concentration,were explored by the laboratory control experiments.Our results provide a novel insight into the nitrogen cycle in estuaries and coastal oceans.The main innovative conclusions included:(1)The results from slurry incubation indicated that surface sediments in LOTZ were potential places for Nr removal.Denitrification and anammox were the two major Nr removal pathways,within the two processes,the potential rates of denitrification was 1-2 orders of magnitude higher than the anammox potential,and thus dominated the Nr removal pathways.Among the detected environmental factors,temperature,sedimentary organic carbon and nitrogen contents co-regulated the latitudinal distribution of denitrification potential in the surface sediments,while temperature and salinity were the main environmental controls for the spatial variation of potential anammox rates.Surface sediments acted as a potential N2O source and the potential rates were regulated by temperature,organic matter quality and porosity of sediments.The vertical profile of nitrogen removal and N2O production potential rates in JRE and YRP sediments showed that deep sediments can also be a place for Nr removal under the abundant substrates condition.This phenomenon implied that the capacity of benthic nitrogen removal and N2O emission can be stimulated by the disturbance of sediments that caused by extreme events,including heavy rain,typhoons and floods,and thus making profound impacts on the environment and climate change.(2)In situ rates of denitrification and anammox detected from intact core incubation suggested that sediments in the JRE and YRP acted as an important role in removing Nr.Sedimentary nitrogen removal was predominated by denitrification(84-100%)relative to a minimal contribution from anammox(<16%).Both water column and sedimentary nitrification-produced nitrate sustained denitrification and anammox in the study regions.The relative contribution of coupled nitrification-denitrification to total nitrogen removal in JRE and YRP was estimated to be 48-71%and?59.0%,respectively,with other contribution from direct denitrification(supported by water-column-delivered nitrate).Obviously,nitrification in the aerobic sediments was the major source of substrates for Nr removal in the two study regions,while physical diffusion of nitrate from the water column was another significant but less important substrate source.The higher contribution from coupled nitrification and denitrification indicated that organic nitrogen degradation was the main form of nitrogen removal in estuarine and coastal sediments.Only 2-4%and 7-12%of total inputted dissolved inorganic nitrogen was removal via sedimentary denitrification and anammox in JRE and YRP,respectively,suggesting that the major parts of riverine Nr are still retained in the ecosystems,and may further cause environmental problems,such as water eutrophication and hypoxia.Sedimentary Nr removal pathways were regulated by single or multiple environmental factors,including nitrate concentration,temperature,salinity in the bottom water,and porosity,the quality and quantity of organic matter in the surface sediments.In addition,up to?7%of removed Nr was transformed to N2O,representing that the sediments in JRE and YRP might play an important role in N2O emission.The C/N ratio of organic matter in surface sediments showed a good correlation with N2O production rates.Based on our calculation,the sedimentary N2O emission would occupy 12-39%and?53.9%of the daily N2O emission in JRE and YRP,respectively.(3)Both denitrification and anammox showed strong temperature dependence,while denitrifying bacteria and anammox bacteria responded differently to the changing temperatures.The optimal temperature for denitrification was significantly higher than that for anammox in both summer and autumn.Likewise,the apparent activation energy for denitrification was systematically higher than corresponding anammox in the two seasons,yet,both processes showed no obvious seasonal variation in apparent activation energy,indicating a similar microbial community composition for the two processes in different seasons.Both experimental results and compiled global datasets suggested that the denitrifying communities are more temperature-tolerant,while the anammox bacteria were relatively cryophilic.Environmental variables(e.g.in situ temperature,microbial assemblage,organic carbon availability and substrate concentration)may have synergistic effects on the temperature performance of nitrogen removal pathways,future climate warming may stimulate both sedimentary denitrification and anammox,however,anammox is likely more vulnerable to warming.Moreover,the N2O production was also stimulated as temperature increases and the N2O/N2 exhibited higher values at both low temperature(1-15?)and high temperature(30-35 C).We thus inferred that there is an optimum temperature for the working of N2O reductase,the N2O reductase is more easily suppressed by temperature than other enzymes involved in N2O production when the environmental temperature deviates from the optimum.(4)15NO3-addition showed no effect on the potential rates of denitrification and anammox in sediments from JRE,suggesting that the Nr removal processes in the study area may have low half saturation constant for NO3-.However,glucose addition promoted the potential rates of denitrification,while showed no effect on anammox,while both the potential rates of N2O production and the N2O/N2 value during denitrification were slightly stimulated by the addition of nitrate and glucose.