Abstract: In this study, the antagonistic bacteria against plant diseases were isolated and screened from soybean paste. The antagonistic substances were extracted from fermentation broth. After that, we examined the inhibition mechanism and basic properties of antagonistic substances. Bacteria were isolated by dilution-plate method. Antagonistic bacteria were screened through plate-confrontation method. The strains were identified by morphological observation, physiological and biochemical experiments, and phylogenetic tree based on the strain’s 16S rDNAsequences. The antagonistic substance of fermentation broth was extracted by ammonium sulfate fractional precipitation. The protein content of the crude extracts was measured by Bradford method. The inhibition effects of antagonistic substances were observed by scanning electron microscope. The antimicrobial spectrum of antagonistic substances and stability of antimicrobial activity of antagonistic substances were determined through agar bore diffusion method. Within the 18 bacterial strains  isolated from soybean paste, the best antagonistic bacterium strain was screened and named as 4-z-3, which was identified as Bacillus siamensis. Its inhibitory rate to pathogen was 62.78%, and the inhibitory rate of fermentation broth of 4-z-3 to pathogen was 53%. The crude extracts of 4-z-3 had the highest inhibition rate at 60.8% when the saturation of ammonium sulfate was 20%-30%. At the optimum saturation of ammonium sulfate, the protein content of the crude extracts was 3.45%. The antagonistic substances had good antagonistic effect against various pathogens. The hyphae of the pathogenic fungi treated with crude extracts of 4-z-3 were destroyed, which caused great absence of cell shape and integrity. In terms of stability, crude extract was insensitive to heat, with the inhibition activity significantly decreased only at 90 and 100 ℃. The crude extract was insensitive to acids and alkalis, with 81.6% inhibition activities of crude extracts being remained at pH 4. The crude extract was insensitive to protease, and theinhibition activity of crude extracts only decreased by 19.6% when the inhibition rate was the lowest. The crude extract was not sensitive to ultraviolet light, with 92.5% inhibition activities of crude extracts being remained after irradiated by ultraviolet light for 12 h. Our results demonstrated that strain 4-z-3 would be a promising agricultural biocontrol bacterium. Antagonistic substances of 4-z-3 can effectively control various plant diseases. Moreover, the antibacterial activity of antagonistic substances is stable.

Key words: seasonal dynamics, soil physicochemical properties., soil microbial biomass carbon and nitrogen, soil layer, forest conversion