N²O emission dynamics and its microbial mechanisms within alfalfa grassland in arid areas were previously unclear.Accordingly,gas collection was performed on a 3-year old alfalfa (Medicago sativa L.cv.Longdong) grassland in Qingyang,on the Loess Plateau,using a static,enclosed chamber.N²O emission flux was measured using High Performance Gas Chromatography.Quantities of nitrifying and denitrifying bacteria in soils at a depth of 0～30 cm were determined using the most probable number (MPN) method.The results showed:(1)The N²O emission flux of alfalfa grassland ranged from -0.099～0.085 mg·m^-2·h^-1 through all phonological stages.N²O emission peaked at flowering stage of the first cut and at the re-growth stage before overwintering,whereas the grassland absorbed N²O at re-growth stage of the second cut and during overwintering,which became the N²O repository.(2)The amounts of nitrite bacteria,nitrate bacteria and denitrifying bacteria varied among different phenological stages and soil layers.(3)Significant linear correlation was found between N²O emission flux and soil moisture content and soil pH value and the amounts of nitrate and denitrifying bacteria.(4)Longdong alfalfa grassland in Qingyang was found to be a weaker N²O emission source than other agricultural sources as reported in similar research and the international average of emission level.