Nitrate contamination has become one of the most important issues for surface water and groundwater. N₂O, with an increasing contribution to global warming, has been more and more attention by the IPCC recently. As well known, denitrification plays a major role in nitrogen cycle of aquatic ecosystems and operates at rates far below its potential under proper conditions. Sediments are the single largest pool of nitrogen in wetland ecosystems. During this process, facultative anaerobic bacteria transform nitrite into nitrogen gas which dissolves in the groundwater and diffuses into the atmosphere finally when it shows up with seepage or spring in the wetland. To seek a mechanistic understanding of N removal in natural wetland ecosystem, a case study was carried in terms of denitrification rate at the Ochi catchment, Chiba, Japan. In this study, samples of intact soil cores in 0–20cm were taken along the groundwater flow path, which including 2 samples in the unsaturated zone and 2 in saturated wetland ecosystem. Denitrification capacity of soil was quantified using acetylene (C₂H₂) inhibition/gas chromatography ECD method with time intervals of 0, 2, 6, 12, 24 h. Total—N and Total—C contents and amount of denitrifying bacteria were also analyzed. It is found that denitrification ability is low for all 2 samples in the unsaturated zone and high in saturated zone. Results show that N₂O emission flux after C₂H₂—inhibition ranges from 0 to 1.17 gN m^-2h^-1, with an increase value prior 6 hours and slow down after that.