Adsorption of the pesticidal toxin from Bacillus thuringiensis subsp. tenebrionis on tropical soils and their particle-size fractions

Genetically modified crops, which produce Bacillus thuringiensis toxins, release the toxins into soils through root exudates and upon decomposition of residues. Several studies showed that Bacillus thuringiensis toxins adsorbed on pure clay minerals and model clay-OM or oxides associations as well as on soils. We determined the adsorption of the toxin from Bacillus thuringiensis subsp. tenebrionis (Btt), active against Coleoptera, on tropical soils and their constituents. Particle-size fractions were separated from a vertisol, an alfisol and an oxisol and the role of constituents of the clay-size fractions was analysed using clay-size fractions from which specific components had been extracted. The adsorbed amounts and affinities of the soil were in the order, oxisol < alfisol < vertisol. Clay-size fractions adsorbed more, with higher affinities and less reversibility, than the silt or particulate organic matter fractions or bulk soils. In the clay-size fractions, organic matter increased adsorption whereas Fe oxides decreased it, and the mineralogy had a prominent role. Smectitic clay fractions adsorbed much more of the toxin from Btt than kaolinitic ones, in relation with the surface area and charge of these minerals. The features of adsorption of the toxin from Btt on tropical soils and their constituents show that the toxin is likely to be retained by soil after cropping with transgenic plants.