Processes And Production Rates Of Nitrous Oxide In Aquatic Systems With Different Trophic States

Nitrous oxide(N2O)is a sensitive climatic trace gas with potent greenhouse effect and ozone depletion capability.The microbial mediated nitrogen cycle processes are main sources of N2O,which has been significantly stimulated by anthropogenic reactive nitrogen.The concentration of N2O has been increased since the industrial revolution and reached 320 ?L m-3,which is the highest value of last 800 ka in the Earth's atmosphere and continues to increase with a pace of around 1 ?L m-3 a-1.Consequently,N2O is currently the third largest greenhouse gas and the largest ozone depletion substance.The ocean is a major nature source of atmospheric N2O with its contribution of 1.2×109-11.3×109 g N a-1,which accounts for nearly one third of nature source.Despite efforts have been made for several decades,the large uncertainty in estimating N2o flux from global ocean to the atmosphere remains unresolved.There is to date still limited measurements on oceanic N20 and in addition,quantifying rates of different N2O production processes remain sparse in the field due to the complex reaction web and technique limitation,which severe hindering our ability in estimating N2O flux and predicting how oceanic N2O production response to global changes.Knowing the sources,production and consumption processes and their rates are key issues in current climatic research area.Here we conducted research on surveying production rates of N2O in aquatic systems with distinct nutrient condition,for the first time,we combined the nature abundance of different nitrogen species and the isotope labeling technique to conduct a comprehensive study on sources and production rates of N2O from eutrophic estuary to oligotrophic open ocean.Main findings of our study are concluded as below:By using isotopic compostion of NO3-,NO2-and N2O,we identified sewage is an important source of NO3-and N2O in the upper stream of the Pearl River Estuary(PRE),both the upper and lower stream showed active nitrogen transformations and N2O production,yet with large variation.Active nitrification and coupled NO3-reduction and NO2-oxidation were found in the upper stream,the reduction pathways were major N2O production processes.In the lower stream,NH4+ oxidation rates are higher than NO2-oxidation,accordingly,NH4+ oxidation contributed to a higher fraction of N2O production.The short-term reaction rates from isotope labelling incubation further confirmed simultaneous occurrences of multiple nitrogen transformations in the PRE,in the upper stream,we found higher NO2-oxidation rate than NH4+ oxidation rate,whereas the reverse mode was found in the lower stream,the change of DO and ammonia oxidizer might responsible for such variation.Unexpectedly,we observed active NO3-reduction rates in the lower stream where DO remains nearly saturated,the micro-niche in the particles may largely responsible for it.Overall,nitrification contributed to the largest part of N2O production(56.8±14.3%of total N2O production),however,the contribution of N2O from reduction pathways(including NO3-reduction and NO2-reduction)also contributed to considerable fraction of N2O(43.1 ± 15.0%),which might due to the high turbid environment in the PRE.By comparing with the sea-air flux,the integrated N2O production rates in the water column accounted for 10.6?94.1%(with average value of 34.3±24.6%)of N2O to the total release,suggesting water column was an important source of N2O in eutrophic system.Assimilation and oxidation of ammonium are central parts of the marine nitrogen cycle,controlling ocean productivity and the export of fixed carbon into the ocean's interior.The underlying regulation mechanisms of these two competing processes remain unclear.Here we show ambient nitrate acts as a key variable to bifurcate ammonium flow through assimilation or oxidation,and the depth of the nitracline represents a robust spatial boundary between ammonium oxidizers and assimilators in the stratified oligotrophic ocean.By profiling the ammonium utilization capabilities between nitrifiers and phytoplankton,we find that the phytoplankton assemblage in nitrate depleted regimes holds higher affinity towards ammonium relative to nitrifiers.In nitrate replete conditions,by contrast,phytoplankton reduce their ammonium reliance and thus enhance the success of nitrifiers.This finding helps to explain existing discrepancies in the understanding of light inhibition of surface nitrification in global ocean and provides further insights into the spatial linkage of nitrification and oceanic new production in the ocean.In the SCS and the WNP,we provided robust evidences of in-situ N2O production in euphotic zone.We observed consistent N2O with depleted 515N and ?18O at the vicinity of PNM in the open ocean sites,a mass balance model showed more than 10%of N2O was produced at the depth with lowest isotope value.Further,the production rates which was derived from isotope labelling incubation show in-situ production in the upper 200 m account for around 50%of sea-air flux,suggesting active production and fast release of N2O in the upper ocean.In summary,by using isotopic approaches,we conducted systematic research on N2O production rates in water column with distinct nutrient levels,our results emphasized active production of N2O with different processes.Our study also proved that combination of nature abundance of N isotope and isotope labeling technique is an effective way for water column N2O study,the results were expected to improve our current recognition on N2O production in aquatic system and to provide a novel approach for future research on feedbacks of ocean to global changes.