| Nitrogen (N) is a major factor leading to eutrophication in estuarine and coastal regions. Eutrophication caused by overloaded reactive nitrogen input may induce various environmental problems. The nitrogen input is mainly anthropogenic, and most overloaded nitrogen is in the form of nitrate. Reactive nitrogen is imported into estuarine and coastal areas through riverine input, atmosphere deposition and groundwater discharge. Thus, the transformation mechanisms and fate of nitrate in estuarine and coastal regions have attracted much attention. In this study, a fast, simple, precise and economical method is developed for measuring15N in ammonium and other nitrogen compounds. Besides, typical estuarine and coastal regions of China are selected to study the pathway, mechanisms and fate of nitrate reduction processes. The influence of environmental factors including pharmaceutical and personal care products on nitrate transformation processes have also been studied, which will help to understand the mechanisms of nitrate transformation. The results can be used to evaluate and predict the changes in estuarine and coastal ecosystems.Nitrogen pollution in aquatic ecosystems has attracted much attention over the past decades, but the dynamics of this bio-reactive element are difficult to measure in aquatic oxygen-transition environments. Nitrogen-transformation experiments often require measurement of15N-ammonium (15NH4+) ratios in small-volume15N-enriched samples. Published methods to determine N isotope ratios of dissolved ammonium require large samples and/or costly equipment and effort. This study presents a novel ("OX/MIMS") method to determine N isotope ratios for15NH4+in experimental waters previously enriched with15N compounds. Dissolved reduced15N (dominated by15NH44) is oxidized with hypobromite iodine to nitrogen gas (29N2and/or30N2), and analyzed by Membrane Inlet Mass Spectrometry (MIMS) to quantify15NH4+concentrations. The N isotope ratios, obtained by comparing the15NH4+to total ammonium (via autoanalyzer) concentrations, are compared to the ratios of prepared standards. The OX/MIMS method requires only small sample volumes of water or sediment slurries, and is rapid, convenient, accurate, and precise (R2=0.9994, p<0.0001) over a range of salinities and15N:14N ratios. It can provide data needed to quantify rates of ammonium regeneration, potential ammonium uptake, and dissimilatory nitrate reduction to ammonium. Isotope ratio results agreed closely (R=0.998; P=0.001) with those determined independently by Isotope Ratio Mass Spectrometry (IRMS) for DNRA measurements, or by Ammonium Isotope Retention Time Shift (AIRTS) Liquid Chromatography for water-column N-cycling experiments. Application of OX/MIMS should simplify experimental approaches and improve understanding of N-cycling rates and fate in a variety of freshwater and marine environments.Denitrification and anaerobic ammonium oxidization (anammox) are considered the most important processes of removing reactive nitrogen from natural aquatic environments. In situ rates of the two processes across the sediment-water interface of Jinpu Bay were measured and compared using continuous-flow experiments combined with a15NO3-tracing technique to determine their relative importance in this hypereutrophic coastal ecosystem. Rates of denitrification and anammox ranged from1.76-327.97μmol N m-2d-1and0.33-36.32μmol N m2d-1, respectively. Both the denitrification and anammox processes were observed to be associated closely with the bioavailability of organic matter and concentrations of sulfide and iron oxides in sediments. Denitrification was the dominant pathway of eliminating reactive nitrogen and on average accounted for about90%of the total removed nitrogen. Totally, both the denitrification and anammox processes removed about20%of the externally derived inorganic nitrogen within the system. Most of the external nitrogen was still retained in the ecosystem, which may cause the severe eutrophication and algae blooms occurring at the study area. Denitrification is the dominant pathway of nitrate removal in many coastal ecosystems, and it plays an important role in counteracting the effects of aquatic eutrophication induced by nitrogen overload. With an increasing production and usage, antibiotics become pseudo-persistent in aquatic ecosystems, thus leading to inhibition on natural processes including denitrification. In this study, slurry experiments were conducted in order to determine how antibiotics influence on denitrification rates and N2O release rates. Quantification of denitrification gene and antibiotic resistance gene were also conducted to find out the mechanisms of the influence. Antibiotics have an obvious inhibition on denitrification rates, while the N2O release rates correlate positively with the concentration of antibiotics. The inhibition on denitrification rates could be reflected by the changes of denitrification gene abundance, which correlate positively with the denitrification rates. The presence of antibiotics stimulates the increase of antibiotic resistance gene, and it makes a limit for the cut down of denitrification rates. The presence of antibiotics may lead to the decrease of denitrification rates and increase of N2O release rates, which could cause serious global environmental problems, such as eutrophication, greenhouse effect and depletion of ozone sphere.Environmental and ecological effects of nitrate reduction processes have been assessed in this study. Denitrification and ANAMMOX are the most important nitrate reduction processes in estuarine and coastal regions, they both cut down the flux of reactive nitrogen input and lightened the eutrophication in aquatic ecosystems, which is a positive environmental and ecological effect. However, in hypereutrophic aquatic ecosystems, the reduction of nitrate reached its limitation, and a large amount of reactive nitrogen was retained in coastal areas. Antibiotics have an inhibition on denitrification, which lead to nitrate retention in aquatic ecosystems and other environmental problems. Antibiotics have a promotion on the release rates of N2O, which is a negative environmental and ecological effect, as increasing N2O contents may lead to global environmental problems. |
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