Abstract
To elucidate the competitive relation between Bacillus thuringiensis and inherent Bacillus species and other microorganisms in soil, dynamics of this bacterium was studied using soil samples with different properties obtained from three mulberry fields (A; alluvium layer loam, B; diluvium volcanic ash soil clayloam, C; alluvium layer sandy soil). Two strains of B.t. (AF101 and var. kurstaki) were mixed with untreated soil, and the number of B.t. decreased with the time in 3 samples. In the sterilized soil samples from mulberry field A, cell numbers of AF 101 increased about 100 folds and those of var. kurstaki about 10 folds 5 days after the onset of experiment and the numbers maintained nearly the same level for 80 days in AF101 and for 60 days in var. kurstaki. However, such increases were not observed in soils from mulberry fields B and C. AF101 was mixed with the sterilized soil sampled from 9 points of the mulberry field A. In 3 out of 9 samples, AF101 increased 10∼100 folds, and no increase was observed in 6 samples. In the 3 samples, B.t. displayed the pattern of germination, growth, and sporuration. Sporulated B.t. did not show any regermination and no secondary growth during the experimental period. These results indicate that the fate of B.t. in the mulberry field soil depends on the competition with the inherent soil microorganisms.
