【Objective】 In order to explore effects of oxygen level (0%, 1%, 3%, 10% and 21%) and carbon input on major sources of N₂O emission in greenhouse vegetable soil, an on-line robotized monitoring and incubation system was used to automatically monitor real-time dynamics of O₂, N₂O and N₂ in greenhouse vegetable soil after a crop of tomato was harvested. In addition, dicyandiamide (DCD), a kind of nitrification inhibitor, was added to investigate contribution of nitrification to N₂O emission from the soil relative to carbon source and oxygen level.【Method】 A certain amount of soil sample collected from a vegetable field under greenhouse after a crop of tomato was harvested, was washed with deionized water and divided into two groups, labeled as T1 and T2, separately. T2 was spiked with glutamate, whereas T1 was not. NH₄NO₃ was added as nitrogen source for both groups. Then the pretreated soil samples were put into 120ml vials, 10.0 g in dry soil weight in each. Deionized water or solution containing NH₄NO₃, C₅H₈NO₄Na·H₂O or DCD was sprayed onto the soil surface to adjust moisture content of the soil up to 250 g·kg^-1 in line with the requirement of the treatment, T1 or T2. Then the soils in the sealed vials were placed in a thermostatic (20°C) waterbath trough for incubation. Concentrations of O₂, N₂O, N₂ and CO₂ in the headspace of a vial were monitored online at intervals of 6 h, and pure O₂ was supplemented with a hermetic syringe in the light of the monitoring results of the gases in the headspace to maintain an approximately constant oxygen concentration (1%, 3%, 10% or 21% (v/v)) in the sealed vials. 【Result】Results show that N₂O emission declined exponentially with rising soil oxygen partial pressure (OPP) (R²=0.82, P＜0.001). It peaked when OPP was 0% or 1%, and fell below 30% of the peak when OPP got equal to or higher than 3%. Addition of available carbon into the vial reduced N₂O and N₂ production in the soil under aerobic conditions, while significantly increasing the contribution of the process of heterotrophic denitrification in the soil to N₂O emission under near-anaerobic conditions (P＜0.01), which suggests that this kind of soil is highly capable of triggering heterotrophic denitrification, and anaerobicity and near-anaerobicity is more favorable to N₂O emission. Compared with Group T1, Group T2 was 64.4% and 88.8% lower in N₂O emission, 23.4% and 18.6% lower in N₂ emission, and 14.5% and 62.3% lower in N₂O/(N₂O+N₂) index (I_N2O), respectively, when OPP was 1% and 3% and no carbon supplemented. However, when carbon was supplemented, the two groups did not vary much in N₂O and N₂ emissions and I_N2O, which suggests that strong nitrification occurs in the soil with no carbon supplemented and N₂O comes mainly from heterotrophic denitrification process (HD) under aerobic conditions in the soil with sufficint carbon supply. DCD would lower the accumulation of NO₂^-, a substrate of nitrification-coupled denitrification (NCD), which is considered to be an important contributor to N₂O emission under aerobic conditions; nevertheless, the process of NCD is still a heterotrophic one in nature. That is to say, DCD would reduce N₂O emission from nitrification processes, and from heterotrophic denitrification processes, too. With OPP rising from 1% to 3%, N₂O emission from nitrifier-induced denitrification (ND) increased from 17.2% to 42.6%, however, the contribution of ND to N₂O emission in the vegetable soil was still quite low due to the drastic reduction in total N₂O emission. Moreover, the commonly used dual isotopic labeling method would lead to overestimation of the contribution of ND to N₂O emission because this method may rule partial NCD or HD processes into ND. As the soil in this study was quite high in pH and remained almost unchanged in nitrate content after the treatment with DCD, simultaneous heterotrophic nitrification-denitrification processes might be very weak. 【Conclusion】Soil N₂O emission mainly occurs under anaerobic and near-anaerobic conditions (OPP=1%). Heterotrophic denitrifiers make the biggest direct contribution to soil N₂O emissions, especially when carbon sources are abundant. NO₂^- is an important substrate of the process of nitrification-coupled denitrification.