Effects of soil water content and temperature on Bt protein degradation in leaves at thinning stage and buds of Bt cotton

In China, transgenic Bt (Bacillus thuringiensis) cotton has been the most widely cultivated transgenic crop since 1997, accounting for 71.5% of total planted area of cotton in 2011. Bt protein of transgenic Bt crops can enter into the soil system via the root system, aboveground system, post-harvest residues ploughed into soils and dissemination of pollen grains. Once in the soil, Bt protein is rapidly absorbed and bounded on active surface particles of soils. This is the probable reason behind the persistence of Bt protein in soils. Thus the degradation dynamics and amount of Bt protein residues in soils have been the core issues in assessing the ecological risks of transgenic Bt cotton in soils. In this study, ELISA (enzyme-linked immunosorbent assays) method was used to detect the degradation of Bt protein in Bt cotton leaves at thinning stage and buds under different soil environmental factors. Two soil factors were included in the study - soil water content and soil temperature. Then three levels of soil water content were designed - 50%, 70% and 100% field capacity of soil moisture. Also three levels of soil temperatures were designed - 15 ℃, 25 ℃ and 35 ℃. Then the entire study was conducted in a two-factor completely randomized design method. Based on the contents of Bt protein in leaves and buds, the calculated Bt protein contents in soil in the initial period of degradation under 50%, 70% and 100% field capacity of soil moisture were respectively 613.19 ng·g^-1, 586.8 ng·g^-1 and 555.84 ng·g^-1 for the leaves treatment and 252.57 ng·g^-1, 241.70 ng·g^-1 and 228.95 ng·g^-1 for the buds treatment. Exponential model for degradation dynamics of Bt protein was estimated along with DT50 and DT90 values. The results showed that on 48 d, 56.18% 93.26% of the initial amounts of Bt protein in leaves and in buds were degraded in soil under different soil water contents and soil temperatures. It was demonstrated that Bt protein in all the treatments was degraded more rapid at the early stage and then slowly stabilized at the later stage. Bt protein degraded most rapidly in the early period at 35 ℃ and 70% field capacity of soil moisture. DT₅₀ was 12.29 d in the leaves and 10.07 d in the buds while DT₉₀ was 41.06 d in the leaves and 33.96 d in the buds. Soil water contents and temperatures had significant effects on Bt protein degradation in the soil. Also there was a significant interaction effect of soil water content and temperature at early stage (before 32 d) on Bt protein degradation. Temperature was the main factor influencing Bt protein degradation. Under the same water content, the rate of degradation of Bt protein increased with increasing temperature. Under the same temperature and 100% field capacity of soil moisture, the degradation rate of Bt protein was slowest. Also under higher temperatures (25 ℃ and 35 ℃), Bt protein degradation was higher at 70% field capacity of soil moisture than at 50% field capacity of soil moisture at the early stage (before 32 d or 48 d). It was concluded that high temperature and suitable soil water content enhanced Bt protein degradation at the early stage.