Environmental Behavior And Ecological Effects Of Bacillus Thuringiensis (Bt) In Soil

In recent years, microbial pesticides as a manner to keep ecological safety were product and applicate widely. Microbial pesticides are living microorganisms, which are largely used to control pests in the agricultural ecosystem. They have unique growth and decay behaviors in the natural environments and a complex interaction with other non-target organisms and environmental factors. As the representative strains of microbial pesticides, the influence of Bacillus thuringiensis (Bt) on soil ecosystem, society-natural complex ecosystem, has not yet been adequate research. The retained situation and variation tendency of Bt in different soil types can reflect complicated interaction in the soil ecosystem. Therefore, it is important to know what the behavior of Bt in different soil types, and what factors that determine the Bt dynamics in the soil environments. Meanwhile, it is rarely understood the ecological roles of alive Bt cells in the Bt agent. In this study, we have assessed the dynamics and ecological roles of Bt in 6 different soil types (Black soil, Paddy soil, Red soil, Brown soil, Loessal soil and Cinnamon soil) and field soil (Nanjing,china) using a rif Bt mutant Bt-R300.aditional bacterial plate culture and PCR-DGGE techniques. Four treatments have been applied including Bt treatments at the commercial recommended dosage (1×1010 CFU/m2) and high dosage (1×1012CFU/m2), water (negative control) and avermectin treatments.With the aid of rif resistant strain Bt-R300, we studied the dynamics of Bt in different soil types. The number of Bt in the non-sterile soil are lower than that in the sterile soil at the same sampling time. The dynamical patterns of Bt in soil are affected by soil properties and Bt doses, at high dose, Bt reaches the highest population much earlier than that in the other treatments. Moreover, Bt can multiply in the given 6 soils within 60 days after it was applied, but the increase of the its population is not over 50%. According the dynamic trend in the given 6 soils, Bt behaviors in the soil can be classified into 3 groups. (1) Bt population continuously decreased during the test period (less than 10% of the initial population size at 60th day after inoculation in Red soil). (2) Bt population decreased during the first 12 days after inoculation and then maintained stable afterwards(about 30%-50% of the initial abundance in Black soil and Paddy soil). (3) Bt population increased in the first 1-3 days, then decreased, and increased again after 12 days (In Brown soil, Loessal soil and Cinnamon soil).With the traditional bacterial plate culture method and PCR-DGGE technology, the microbial abundance and diversity in soil after the given four treatments applied were analyzed. Result indicated that Bt does not significantly influence the bacterial and actinomycic abundance in the soil, but showed suppression on the fungal abundance at 24th day after treatment. The electrophoresis bands of Bt treated samples and the bands of water control samples were clustered in the same branch after 12 days since treatments were applied, which indicated that Bt agent dose not influence the bacterial community structure thereafter. According to the DGGE profile, Bt may enhance the growth of Acinetobacter sp., Myroides sp., Escherichia sp., Enterobacter sp. and Agrobacterium sp..The soil enzyme activities were analyzed and compared between different treatments. During the whole experiment, there was no significant difference between the urease activity and Sucrase activity in the soil treated by two dosages of Bt and water control. The alkaline phosphatase activity in two dosage of Bt treatments are both significantly higher than in water control at 12th day after treatment and thereafter, which indicated that Bt may increase the alkaline phosphatase activity of soil.In summary, this study examined the environmental behavior of Bt in the six different types of soil and field soil, analyzed the effects of different doses of Bt on field soil enzymes activity, the number of microorganisms and microbial population structure. During the test period, the highest Bt abundance did not exceed 150% of the initial abundance among the given types of soil. Two dosages of Bt treatments did not show adverse effects on enzyme activities, microbial abundance and microbial diversity in soil. According this, we can regard Bt as a safely microbial pesticides in the soil environment.