| The greenhouse vegetable area has reached to 38.5 M ha in China. It has been reported that N2O emissions from greenhouse vegetable fields were higher than those from other land use soils mainly attributed to high N fertilization rates, however, the underlying mechanisms were not clearly understood. In this study, N2O fluxes were measured using static closed chambers, and gross N transformation rates and the source of soil N2O production were quantified using 15N tracing model and N2O source partitioning methods, respectively, to clarify the mechanisms of high N2O emissions in the greenhouse vegetable fields.We observed the N2O fluxes of the greenhouse-vegetable fields and soybean fields which are under local farmers’management of fertilizers with the method of static box-GS. The results showed that the characters of N2O season emission from greenhouse vegetable fields and soybean fields are similar, high in summer ang low in winter. The N2O emission of year of greenhouse-vegetable fields (organic+ inorganic treatment:928.5±305 mg m-2 yr-1, single inorganic treatment:864.7±224 mg m-2 yr-1) are higher than soybean soil (226.6±41 mg m-2 yr-1) (P<0.05). In the scale of one hundred years, the order of aGWP of greenhouse vegetable fields and soybean soils is greenhouse-vegetable fields (organic+ inorganic treatment:8235 g CO2 m-2, single inorganic treatment:6821 g CO2 m-2)>soybean fields(2216 g CO2 m-2). In addition, we did not find relationships between the greenhouse flux and soil temperature and moisture.N2O emission from soil is controlled by the rate of nitrogen transformation and the propotion in the process of N2O emission, in order to clarify the mechanism of high N2O emission ingreenhouse vegetable soil. Gross N transformation rates in different fertilization treatment greenhouse vegetable soils and soybean soil were determined using 15N stable isotope labelled and numerical model method. The results showed that gross mineralization rate in greenhouse-vegetable soils (organic+ inorganic treatment:4.37±1.5 mg N kg-1 d-1, single inorganic treatment:3.99±0.15 mg N kg-1 d-1), were significantly greater than soybean soil (1.13±0.24 mg N kg-1 d-1) (P<0.05). There were obvious relationships between mineralization and SOC and TN. The autotrophic nitrification rate in the organic+ inorganic treatment (4.92±0.11 mg N kg-1 d-1) was significantly higher than soybean soils (1.51±0.33 mg N kg-1 d-1) (P<0.05). Gross heterotrophic nitrification of organic nitrogen rate followed the order of greenhouse-vegetable soils (organic+inorganic fertilizer treatment:19.5±7.9 mg N kg-1 d-1, single inorganic fertilizer treatment:4.78±0.74 mg N kg-1 d-1)> soybean soils (0.009±0.002 mg N kg-1 d-1) (P<0.05). Heterotrophic nitrification was the main source of NO3- in greenhouse-vegetable soils, which was significant correlated with SOC and TN content.In order to clarify further the mechanism of high N2O emission in greenhouse vegetable soil from the perspective of nitrogen transformation process, the sources of N2O (i.e. the contributions of autotrophic nitrification, heterotrophic nitrification and denitrification process to N2O production) in different fertilization treatments in greenhouse vegetable soils and soybean soil were determined using 15N stable isotope lablled in pairs indoor incubation and source analysis method. The results showed that the contributions of denitrification, heterotrophic nitrification and autotrophic nitrification to N2O ranged in 8.8%～40.3%,46.2%～84.0% and 7.2%～13.5%, respevtively, for different treatments. Compared with soybean soil (46.2%), the main source of soil N2O was the heterotrophic nitrification (66.2%-84.0%) in greenhouse vegetable soils. The ratio of N2O emission from heterotrophic nitrification in the greenhouse vegetable soils (organic+inorganic fertilizer treatment was 2.4%o, and single inorganic fertilizer treatment was 4.7%o) were lower than that of soybean soil (11.6‰). The heterotrophic nitrification is the dominant process of N2O emission in greenhouse-vegetable fields. Thus, the large heterotrophic nitrification rate due to differences of kinds and amounts of fertilizers in greenhouse-vegetable soils was the key factor resulting in N2O emissions from greenhouse-vegetables soils greater than soybean soil.
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