Abstract: Sequencing batch reactors (SBR) are used to investigate the greenhouse N₂O production in complete and short-cut denitrifications under different ρ(COD)/ρ(N).Using ethanol as carbon source to control ρ(COD)/ρ(N),which are 1.56,2.83,4.56,6.01,and 10.0 in complete denitrification and 1.51,2.45,3.33,4.13,and 9.7 in short-cut denitrification.Results show that 6.01 and 4.13 are the optimum ρ(COD)/ρ(N) in complete and short-cut denitrifications respectively with little N₂O production.One gram of mixed liquor suspended solids(MLSS) can reduce 0.077 9 nitrate or 0.089 9 nitrite per day.The specific denitrification rates increase with increasing ρ(COD)/ρ(N).However,there is no remarkable impact with adding organic matter when the carbon source is abundant.When the carbon source is insufficient,N₂O production in short-cut denitrification is obviously larger than that in complete denitrification,and the amount topped 0.607 mg/L at ρ(COD)/ρ(N) of 3.33.High N₂O production under low ρ(COD)/ρ(N) operations is mainly due to the inhibition of nitrite to the activity of nitrous oxide reductase.