The objective of this study was to evaluate N₂O emissions during the first three months after different nitrogen (N) source treatments involving green manure and chemical fertilizer in a sugarcane field in Okinawa, Japan. Four treatment plots were established: 100 and 300 kg N ha⁻¹ chemical fertilizer (100N and 300N plots, respectively); green manure (Crotalaria juncea, 150 kg N ha⁻¹, C:N=20; GM plot); and the control (0N plot). We regularly measured N₂O flux and soil inorganic N (NH₄⁺-N and NO₃^--N), and also recorded environmental data. There were no significant differences in the fluctuation pattern and peaks of N₂O emissions between the 100N and GM plots, and total N₂O emissions were comparable, though the applied N amount was higher in the GM plot. Little rainfall in the early period should limit microbial activity in the 100N plot, though soil NH₄⁺-N and NO₃^--N contents were higher in the 100N plot than in the GM plot. Total N₂O emissions in the 300N plot were 1.5-times higher than in the 100N and GM plots. Collectively, the results suggest that the amount of applied N more strongly affected N₂O emission than the difference in N substrate quality.