Effect of Soil Organic Matter on Adsorption and Insecticidal Activity of Toxins from Bacillus thuringiensis

With the large-scale cultivation of transgenic crops expressing Bacillus thuringiensis (Bt) insecticidal toxin in the world, the problem of environmental safety caused by these Bt crops has received extensive attention. The effects of soil organic matter (SOM) on the adsorption and insecticidal activity of Bt toxin in variable- and constant-charge soils (red and brown soils, respectively) were studied. Organic carbon in the soils was removed using hydrogen peroxide (H2O2). After H2O2 treatment, the SOM in the red and brown soils decreased by 71.26% and 82.82%, respectively. Mineral composition of the H2O2-treated soils showed no significant changes, but soil texture showed a slight change. After SOM removal, the cation exchange capacity (CEC) and pH decreased, while the specific surface area (SSA), point of zero charge (PZC), and zeta potential increased. The adsorption isotherm experiment showed that the Bt toxin adsorption on the natural and H2O2-treated soils fitted both the Langmuir model (R2≥ 0.985 7) and the Freundlich model (R2≥ 0.984 1), and the amount of toxin adsorbed on the H2O2-treated soils was higher than that on the natural soils. There was a high correlation between the maximum adsorption of Bt toxin and the PZC of soils (R2 = 0.935 7); thus, Bt toxin adsorption was not only influenced by SOM content, but also by soil texture, as well as the SSA, CEC, PZC, and zeta potential. The LC50 (lethal concentration required to kill 50% of the larvae) values for Bt toxin in the H2O2-treated soils were slightly lower than those in the natural soils, suggesting that the environmental risk from Bt toxin may increase if SOM decreases. As the measurement of insecticidal activity using insects is expensive and time consuming, a rapid and convenient in vitro method of enzyme-linked immunosorbent assays is recommended for evaluating Bt toxin degradation in soils in future studies.