Dissimilatory Nitrate Reduction Processes And Associated Contribution To Nitrogen Removal In Sediments Of The Yangtze Estuary

Nitrogen is a key element limiting primary production in estuarine and coastal ecosystems. Over the past few decades, reactive nitrogen production has increased by 120%, and global nitrogen overload has become an emerging environmental issue in this century. Increasing input of nitrogen (mainly in the form of nitrate) is an important driver of eco-environmental problems in estuarine and coastal ecosystems, by altering food-web structure, decreasing biodiversity, and causing widespread anoxia and increasing frequency of algal blooms. An improved understanding of nitrogen transformation is thus required to assess nitrogen fate and control nitrogen-pollution in these aquatic environments.Dissimilatory nitrate reduction processes, including denitrification, anaerobic ammonium oxidation (ANAMMOX) and dissimilatory nitrate reduction to ammonium (DNRA), are important pathways of nitrate transformation in aquatic environments. However, these processes play diverse roles in controlling the fate of nitrate. Both denitrification and ANAMMOX remove nitrate from aquatic ecosystems by conversion to gaseous nitrogen (mainly dinitrogen gas). Denitrification is the dissimilatory reduction of nitrate to dinitrogen gas, whereas in ANAMMOX, ammonium is oxidized to dinitrogen gas, with nitrate or nitrite serving as the electron acceptor under anaerobic conditions. In contrast to those processes, DNRA does not remove nitrate from aquatic environments in the form of dinitrogen gas, but instead reduces it to ammonium. Therefore, DNRA still retains the transformed nitrogen in aquatic ecosystems, as a biologically-available form.In the present study, the YangtzeEstuary as a typical study area was selected to study the dissimilatory nitrate reduction processes. Slurry experiments were conducted to quantify the rates of the denitrification, ANAMMOX and DNRA processes with nitrogen isotope-tracing techniques. We also compared the relative contributions of denitrification, ANAMMOX and DNRA to the total nitrate removal at the study area. Furthermore, environmental factors were determined and compared to elucidate their influences on the processes of denitrification, ANAMMOX and DNRA. The main findings of this work are as follows:(1) Measured rates of denitrification, ANAMMOX and DNRA ranged from 0.06 to 4.51 μmol N kg-1 h-1,0.01 to 0.52 μmol N kg-1 h-1, and 0.03 to 0.89 μmol N kg-1 h-1, respectively, in the underlying sediments of the Yangtze Estuary. The potential rates of denitrification, ANAMMOX and DNRA ranged from 3.63-10.68 μmol N kg-1 h-1,0.03-0.71 μmol N kg-1 h-1 and 0.87-5.57 μmol N kg-1 h-1 in the vegetation sediments of the Yangtze Estuary. The rates of denitrification, ANAMMOX and DNRA in the bare sediments, with the values of 1.8-6.22 μmol N kg-1 h-1,0.03-0.23 μmol N kg-1 h-1 and 0.14-2.98 μmol N kg-1 h-1.(2) Statistical analyses showed that the potential denitrification rates correlated significantly with temperature, sulfide, organic carbon and the concentration of Fe(Ⅱ), in the Yangtze Estuary. It was observed that the ANAMMOX rates were significantly and positively related to temperature and sulfide. By comparing the correlations of the DNRA with environmental factors, dominant factors affecting the DNRA process were sulfide, organic carbon and the concentration of Fe(Ⅱ).(3) Denitrification consumed about 22% of the total terrigenous inorganic nitrogen transported annually into the Yangtze Estuary and, as expected, was the dominant pathway for fixed nitrate removal. ANAMMOX accounted for approximately 3% of the total terrigenous inorganic nitrogen and thus also removed significant terrigenous inorganic nitrogen from the Yangtze Estuary. In total, both denitrification and ANAMMOX processes removed about 25% of the total inorganic nitrogen from agriculture activity, fish farming, and industrial wastewater transported into the Yangtze Estuary...