台灣北部河岸濕地甲烷通量與環境影響因子研究

Abstract

天然濕地一直被視為是重要的大氣甲烷來源區，而濕地的甲烷排放是產甲烷菌與消耗甲烷菌分別進行產甲烷作用與消耗甲烷作用相互影響的結果，然而目前對於甲烷排放、微生物作用以及環境因子之間的關聯尚未完全釐清。本研究選擇台灣北部的淡水河河岸濕地作為研究地點，依據不同鹽度設立五個樣點，並於夏天和冬天進行甲烷排放測量，以及採集表層土壤與沉積物岩芯做化學分析。夏天的甲烷排放量約為0.01到103.6 mmol m-2 day-1，而在冬天則是約0.001到1.50 mmol m-2 day-1。在淡水與半淡鹹水環境中可觀測到較高甲烷的排放；而在鹹水環境中則是顯示低甲烷排放。沉積物岩芯的化學剖面隨著採樣位置不同而有變化，在上游淡水環境中有著高甲烷與低硫酸鹽的化學剖面；在下游鹹水環境則是顯示低甲烷與高硫酸鹽的化學剖面，顯示產甲烷作用與硫酸還原作用分別控制兩種環境的化學剖面。而在中游半淡鹹水環境中則顯示甲烷與硫酸鹽隨深度相互變化的化學剖面，顯示產甲烷作用、消耗甲烷作用與硫酸還原作用相互影響的結果。甲烷排放速率與土壤溫度呈現顯著正相關，但表層甲烷濃度與甲烷排放速率沒有明顯關係，土壤有機碳含量也與甲烷排放速率沒有顯著相關性。本研究結果顯示，甲烷排放受到土壤溫度、鹽度(硫酸鹽)與微生物交互作用的影響，而實際排放到大氣層中的甲烷量是受到地底下產甲烷與硫代謝作用的垂直分布與組成所影響。

Natural wetlands are considered the most abundant source of atmospheric methane. Methane emission in wetlands is the result of methane production and consumption mediated by methanogens and methanotrophs, respectively. However, the interplay among methane flux, microbial activities and environmental variables is still unclear. In this study, methane fluxes were measured at five riverside wetlands with different salinities and sulfate concentrations along the Tamsui River in northern Taiwan in summer and winter. Surface sediments and sediment cores were collected for complimentary geochemical characterization. The methane fluxes ranged from 0.01 to 103.6 mmol m-2 day-1 in summer and from 0.001 to 1.50 mmol m-2 day-1 in winter. Higher methane fluxes were observed at brackish and freshwater sites, whereas lower methane fluxes were detected at saline sites. The sulfate and methane profiles varied considerably upon salinity well. Sulfate depletion was accompanied with limited variations in methane and low methane abundances at the saline site, whereas the high methane and limited sulfate were observed at the upstream freshwater site. Such geochemical patterns suggest the predominance of either sulfate reduction or methane metabolisms at individual sites. In contrast, various co-variation patterns between sulfate and methane at the midstream sites suggest the interplay among sulfate reduction, methanogenesis and methanotrophy. The measured methane fluxes were also positively correlated with sediment temperatures but weakly correlated with the fluxes calculated from the methane profiles and from surface methane. No or poor correlations were found between the methane flux and organic content. Overall, our results suggest that methane flux is controlled by temperature, salinity (or sulfate) and microbial processes. Despite the seasonal temperature variations, the interaction and vertical organization of sulfur and methane metabolisms are vital in influencing the amount of methane exported to the atmosphere.