| Nitrogen(N),an essential element for all organisms,regulates productivity in the surface waters of many parts of the ocean.As a limiting nutrient in the euphotic zone,nitrogen rapidly interconverts among five major N compartments:particulate organic nitrogen(PN),dissolved organic nitrogen(DON),ammonium(NH4+),nitrite(NO2-),and nitrate(NO3-).The general features of the N cycle in the sunlit region of the ocean are well known,but methodological difficulties have previously confounded simultaneous quantification of transformation rates among the many different forms of N.Studies of the rates of transformation of N in the marine N-cycle have had a major impact on our current understanding of the coupling of autotrophic and heterotrophic processes involving carbon and nitrogen as well as the efficiency of the biological pump.Such information has also facilitated evaluation of ecosystem functions.Thus,by using 15N stable isotope labelling tracer techniques,we made a breakthrough in rate calculation methodology and put forward a new model named it "Isotope Matrix Method" to simultaneously obtain almost all the most fundamental processes in the N cycle.To demonstrate the applicability of the method,we conducted incubation experiments with low-nutrient water from the western North Pacific and with high-nutrient coastal water off the southeastern China coast.Then we focused on the upper sunlit ocean in different trophic nutrient levels,taking the spring western North Pacific and summer Jiulong River Estuary as examples,to measure multiple simultaneously occurring nitrogen flowing rates in the upper aquatic systems and to discuss the controlling factors and the significance of specific process in the biogeochemical cycle.The main findings of the study are as follows:We selected water samples from one or two layers in the sunlit ocean situated in western North Pacific Ocean featured with low nutrient and Wuyuan Bay off southeastern China with high nutrient,and added only trace 15NH4+ into an incubation bottle under simulated in situ conditions for time series incubation.By monitoring the changes in the concentrations and isotopic compositions of the total dissolved nitrogen(TDN),PN,NH4+,NO2-,and NO3-pools allowed us to quantify the 15N and 14N fluxes simultaneously.Constraints expressing the balance of 15N and 14N fluxes between the different N pools were expressed in the form of simultaneous equations,we proposed"Isotope Matrix Method" and the unique solution of which via matrix inversion yielded the relevant N fluxes,including rates of NH4+,NO2-,and NO3-uptake;ammonia oxidation;nitrite oxidation;DON release,and NH4+ uptake by bacteria.The Isotope Matrix Method results were further compared with conventional source-product rate measurements and rates estimated by using ordinary differential equations(ODEs),and we found:(1)accurate measurements of concentrations during a time series is vital for all kinds of transformation rate estimates,including the isotope matrix method and(2)the Isotope Matrix Method can overcome various biases that impact estimates made with traditional methods.As rates based on Isotope Matrix Method were from the same bottle under in situ light condition,it would be more realistic evaluating the relative importance of co-occurring nitrification and new production in the euphotic zone,the relative nitrogenous nutrition preference and the relative magnitude of production or consumption pathways for one specific nitrogen species when compared with traditional incubation methods.When we know about how to obtain an accurate rate by choosing a suitable incubation and rate calculation model,we can step further to explore the specific N processes in the upper ocean.The primary productivity in most of the ocean(?75%)is limited by the availability of inorganic N except those high-nutrient low-chlorophyll areas,thus,the continuous renewable regenerated N sustains the desert upper ocean.Among the regenerated N compounds,NH4+ and urea-N are two major forms.By adding 15N labelled NH4+ and urea-N,we investigated the responses of assimilation and oxidation for regenerated NH4+ and urea-N to light changes at community level under various substrate conditions in spring 2015 across western North Pacific.This is the first field study in open ocean showing that light would shape the specific affinity of assimilators and oxidizers toward NH4+ and urea.Moreover,an enhanced photosensitivity toward low substrate for both phytoplankton and nitrifier was observed shedding lights on the synergistic control of light and substrate on the distribution of nitrogen recycling pathways,aid to nitrogen-based ocean biogeochemical model.The results of light and substrate dynamics study have strengthened our understanding of the regulation and microstructure of euphotic recycled N pathways.Jiulong River Estuary(JLR)is seriously disturbed by human activities features with the unbelievably high areal yield rates of nitrogen fluxes.However,previous studies in JLR associated with N mainly focused on the flux of different nitrogen species,different source and sink contributions,denitrification and N2O flux,the internal N processes studies are insufficient.By adding 15N labeled substrates,including 15NH4+,15NO2-and 15NO3-,to trace 15N flows among different forms of inorganic and organic nitrogen and quantify transformation rates among the N species.We found NH4+ uptake rates(2.1-7.9 ?mol N L-1 d-1)contributed most of the dissolved inorganic N(DIN)uptake with a proportion of large than 97.7%;the dark uptake was universal but the ratio of dark uptake rate to light uptake rates varied for each DIN species;NH4+ oxidation rates(108.9-338.4 nmol N L-1 d-1)were always greater than NO2-oxidation rates(16.0-48.2 nmol N L-1 d-1),and a weak comammox process was detected(1.5—2.7 nmol N L-1 d-1);the rates of source processes for NO2-were larger than those of the sink processes resulting in the accumulation of NO2-in JLR water.Therefore,we got a relatively comprehensive understanding of the N cycle processes in summertime JLR.To sum up,the doctoral research is mainly focused on the upper sunlit ocean.We firstly made a breakthrough in the rate calculation methodology by putting forward"Isotope Matrix Method" to determine multiple simultaneously occurred nitrogen processes rates.Subsequently,this new method is applied in different nutrient level aquatic environments to have a better understanding of the biogeochemical significance of specific nitrogen processes. |
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