Adsorption of the insecticidal toxin from Bacillus thuringiensis subspecies tenebrionis to clay fractions of tropical soils

Genetically modified plants, which produce Bacillus thuringiensis toxins, release the toxins into the soil through root exudates and upon decomposition of residues. There is need to assess the fate of these toxins in the soil. Our aim was to analyze the adsorption of the toxin from Bacillus thuringiensis subsp. tenebrionis (Btt) to surface active soil constituents. We separated clay-sized fractions from a vertisol, an alfisol and an oxisol after complete dispersion of the soils by mechanical means and purified the toxin from a Btt commercial preparation. Adsorption isotherms were established by the difference method in deionized water or in phosphate buffer (pH 6.8). Adsorption of Btt toxin was in the order: vertisol > alfisol > oxisol, and it was mostly irreversible, except for the alfisol. Adsorption could be explained by physicochemical conditions (pH and ionic strength) and the nature of clay constituents, i.e., the dominant clay mineralogy: montmorillonite (vertisol) and kaolinite (alfisol, oxisol). The results show that Bt toxins released into the soil may be retained by soil colloids. The impact of such adsorption on biodegradation and pesticidal activity should be studied.