Abstract: In a laboratory incubation under constant temperature and aerated conditions, the effects of 4 fumigants - chloropicrin (Pic), 1,3-dichloropropene (1,3-D), methyl disulfide (DMDS) and metham-sodium (MS) - on soil dissolved organic nitrogen (DON) transformation and microbial biomass under vegetable greenhouse conditions in Beijing area were investigated. The results revealed that DON concentration increased significantly after fumigant treatment. In 0 day after fumigation (DAF), cumulative DON under Pic, MS, DMDS and 1,3-D treatments were 47.55 mg·kg^-1, 42.15 mg·kg ^-1, 40.34 mg·kg^-1 and 32.02 mg·kg^-1, respectively. This represented a corresponding increase of 58.67%, 40.65%, 34.61% and 6.87% compared with 29.97 mg·kg^-1 of untreated soils. In the 14 84 DAF, the content of DON in Pic, DMDS and MS groups increased. A significant difference was noted in DON content in different groups. Fumigation also strongly increased the proportion of dissolved amino acid (DAA) in DON, which was important in the transformation process of soil N. DAA content increased greatly after fumigation, compared with CK, and peaked in 7 DAF. The highest DAA content (12.87 mg·kg^-1) was found under Pic treatment, while the lowest (5.74 mg·kg^-1) under CK. After fumigation, the content of soil microorganisms decreased sharply. Pic exhibited the strongest killing effect. Soil microbial biomass carbon and soil microbial biomass nitrogen contents decreased by 69.39% and 70.95%, respectively, under Pic treatment, followed by DMDS and MS treatments. Then 1,3-D had the least effect on microorganisms. While nitrification was inhibited for at least 2 weeks after treatment with Pic, 1,3-D and MS; it was inhibited for at least 1 week after DMDC treatment. Nitrification and mineralization gradually recovered under long-term incubation.