The emission of greenhouse gasses from agriculture has been a global concern. We investigated the emission of nitrous oxide (N₂O) from an Andisol grass field, where dairy cow slurry was applied, in northeastern Japan. Fifteen locations were selected for gas flux measurement along a 150 m long transect of the sloped grass field. Dairy cow slurry was splashed over the field on the day of year (DOY) 300 in 2004. Nitrous oxide flux from the soil surface was routinely measured with the chamber method. Chambers used were 0.285 m in diameter and 0.24 m high, and about 0.05 m of the bottom of the chamber was inserted into the soil surface. Average soil water content between the soil surface and 0.2 m deep near each sampling site was also measured with time domain reflectometry. Soil samples were analyzed for both NH₄- and NO₃-N concentration and the natural abundance of nitrogen isotope (d¹⁵N). A large spike of N₂O flux was observed on DOY 309, nine days after application. The N₂O flux decreased just after the spike and stayed steady for at least 20 days with small fluctuations mostly attributed to changes in soil water content. The concentration of NH₄, the major constituent of the slurry applied, in soil deceased with time while that of NO₃ did not increase accordingly. Temporal increases in d¹⁵N values for the residual NH₄ indicated that NH₄ mainly decreased not due to denitrification but to volatilization. A small amount of NH₄, however, was nitrified during the first ten days after the application. The large spike of N₂O flux might be attributed to both processes of nitrification and subsequent denitrification. Denitrification was evidenced by significant increases in d¹⁵N values of the residual of NO₃. Simultaneous nitrification and denitrification might occur in the moist Andisol grass field.