| 摘要: | 氮移除過程將活性氮物種轉化成最終產物氮氣（N2）或是氧化亞氮（N2O）並且從生態系統中移除。然而全球氣候變遷對N2O（溫室效應當量為二氧化碳的300倍）或N2卻是有著全然不同的回饋過程。另外，人類製造出的大量活性氮已經嚴重干擾並且加速氮循環過程，導致水域系統的優養化和一系列的環境問題。目前對氮移除過程的瞭解相當廣泛，但是針對移除速率的定量和調控機制的研究卻相對缺乏。最近基因組學的研究也揭露數個先前未知的，新的氮移除過程。針對各種氮移除過程的移除速率和相對貢獻，甚至包含未知過程的探索，提出創新的方法技術因而顯的相當急迫且重要。本論文提出一個新發展出來的穩定同位素15N示蹤方法（IPTanaN2O）可用來定量和區分複雜的氮移除過程，包括脫硝作用、硝化作用、厭氧氨氧化作用、硝化菌脫硝作用以及協同脫硝作用。新的方法也運用在富營養鹽的溼地和沿岸生態系統中。在淡水河溼地和長江口缺氧區這兩個研究中，IPTanaN2O方法被用來同步定量每個氮移除作用的N2和N2O產生速率。在淡水河溼地的案例中，脫硝作用產生的N2O是主要的氮移除過程，而且可能貢獻了大多數的水體往大氣釋放的N2O。然而，在長江口外的沉積物研究中，區分不開的硝化菌脫硝作用和協同脫硝作用佔了超過一半的N2O產率，而且脫硝作用產生的N2是主要的氮移除過程。相較於低氧水體中經由脫硝作用或是消化作用產生的N2O而言，沉積物產生的N2O實際是水體中溶解性N2O的主要來源。據我們所知，這是第一個使用15N示蹤劑方法在海洋沉積物中，證實區分不開的硝化菌脫硝作用和協同脫硝作用的存在，同時也說明了IPTanaN2O的潛力。在全球變遷的研究脈絡下，我們認為使用這個新方法定量不同的轉換以及個別作用的速率，應該進一步運用在對溫度、二氧化碳分壓、溶氧…等更精細的操控實驗中。本論文有四個主要貢獻：第一，我們提出新的工具IPTanaN2O，能夠用來定量數種氮移除過程的速率與相對貢獻量。這將有助於研究者未來針對個別過程的調控機制方面的研究。另外，我們同時也示範如何辨別出硝化菌脫硝作用和協同脫硝作用的存在，這兩種作用或許可以進一步以基因組學的方法區分開來。第三，長江口外側區域在缺氧區逐漸生成的期間，沉積物會出現N2O產率很高而且變動程度很大的情況。我們認為這種不穩定的狀態是由於沉積物中的氮移除過程，隨著水體溶氧逐漸降低而有所轉變所致。未來針對缺氧區氮循環的研究，應該將水體缺氧視為一個連續的發展過程，而非只是一個單點的事件，這樣才能建立一個在水體缺氧的不同時期，各種氮移除作用的回應的完整圖像。最後，長江口外水體缺氧前期，沉積物產生的N2O，證實為水體N2O的主要來源。假若這個觀察結果是一個普遍的現像，以目前沿岸缺氧區的數量和範圍不斷增加的情況下，有必要針對沿岸沉積物的N2O釋放量進行重新評估。;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. |
