dc.description.abstract | Nitrogen removal processes transfer reactive nitrogen from ecosystem to end produces of nitrous oxide (N2O) and/or dinitrogen (N2). However, the productions of N2O (300x CO2 in term of GHG potential) and N2 may lead to totally different climate feedback. The general features and understanding of those pathways are known, however, the quantitative information and their controlling factors remains unclear. Moreover, the anthropogenic perturbation on the nitrogen cycle is accelerating by massive artificial nitrogen inputs, leading to eutrophication of aquatic systems and serial environmental problems. Recent genomics studies also revealed many previously-unknown processes. An innovative technique is urgently needed to quantify the relative removal rates by various processes and even explore unknown processes. This thesis presented a newly developed 15N tracer method (IPTanaN2O) to quantify and/or distinguish complicated nitrogen removal pathways, including denitrification, nitrification, anammox, nitrifier denitrification and codenitrification. Applications of IPTanaN2O to the eutrophic wetland and coastal ecosystems were conducted. Both N2 and N2O production rates were determined simultaneous and assigned to each nitrogen removal pathways by IPTanaN2O at the wetland of Danshuei River in Taiwan and the coastal hypoxia area off Changjiang Estuary in China. In the case from Danshuei wetland, N2O production via denitrification contributed major nitrogen removal and might account for most of in situ air-water N2O emission. However, the investigations from the sediments off Changjiang Estuary revealed that entangled nitrifier denitrification and codenitrification explained half of N2O production, and denitrification N2 production was the major nitrogen removal process. Comparing with in situ water columns N2O production via nitrification and denitrification, sedimentary N2O production was the major source of water column dissolved N2O in the pre-hypoxia area off Changjiang Estuary. To our knowledge, this is the first 15N tracer evidence reported entangled nitrifier denitrification and codenitrification activity in marine sediments and illustrated the potential of IPTanaN2O. More manipulation experiments, such temperature, pCO2, DO…etc., can be pursued by using this new technique to examine pathway shifts in a quantitative way under global change context. There are four major contributions in this thesis. First, we presented a new tool, IPTanaN2O. Its capacity of determining rates and relative contributions of N2O in various nitrogen removal pathways will benefit researchers focus on issues of controlling factors specifically for different pathways. We also demonstrate how to identify the activity of entangled nitrifier denitrification and codenitrification, which might be distinguished further with genomics tools. Third, a hotspot of high and dynamic N2O production was revealed in sediments off Changjiang Estuary during pre-hypoxia period, which suggested a changing nitrogen removal processes in sediment accompany with devolving of water column hypoxia. Studies concerned on influences of hypoxia to the nitrogen cycle should treat hypoxia as a continuous process instead of an episode event, in order to have a comprehensive understanding in terms of the responds of each pathways under different hypoxia stages. Finally, sedimentary N2O production were evidenced an important source contributing to water column in a pre-hypoxia area off Changjiang Estuary. If our observation represents a general phenomenon, re-evaluation of N2O emission from costal sediment is required in currently increasing of coastal hypoxia regions. | en_US |