Many farmers and scientists have raised questions recently about the impact of Bt-transgenic crops on soil microbial communities and soil health. Two important papers on this topic are:

"Microbial Utilization of Free and Clay-Bound Insecticidal Toxins from Bt and Their Retention of Insecticidal Activity after Incubation with Microbes," Applied and Env. Microbiology, Sept. 1997, p. 3561-3568. Authors, J. Koskella and G. Stotzky.

"Persistence of the Insecticidal Toxin from Bt subsp. Kurstaki in Soil," Soil Biology and Biochemistry, Vol. 30, No. 4, p. 471-476., 1998, H. Tapp and G. Stotzky

Current understanding and regulatory approvals of Bt-transgenic plants rest largely on ecological effects studies on nontarget organisms carried out in laboratories. Most have used foliar Bt products. For the same reasons that laboratory studies of the impacts of Bt-corn pollen on Monarchs are not necessarily indicative of impacts in the field (see criticism of the Cornell monarch study), laboratory studies exploring direct Bt impacts on non-target organisms are an inadequate basis from which to project -- and dismiss -- longer-run soil ecosystem impacts of Bt-transgenic crops.

There has been very little serious consideration, in the field or laboratory, of the short-term or longer-run impacts of Bt-transgenic crops on soil microbial communities, biodiversity, and function. Such work is critically needed for two reasons. First, Bts engineered into plants are in a more active, or cleaved form whereas the Bt toxins in conventional spray formulations are in a more natural, complex and inactive form. Foliar Bts are not contact poisons; they must be ingested to be activated. When the Bt is exposed to the alkaline conditions in the insect's gut (ph 8 to 10), the molecule is cleaved. The activated toxins then bind to receptors in the insects's stomach lining, where they cause pores to form leading to leakage and ultimately death of the insect.

Second, the volume of Bt entering the soil in a field planted to a Bt-transgenic cultivar vastly exceeds the natural, background levels in the soil, plus any from conventional sprays. It is therefor inappropriate to extrapolate from research on, and experience with foliar Bts when evaluating possible soil ecosystem and soil health impacts of Bt-transgenics, especially over time.

The above cited research also shows that Bt can be quit persistent, contrary to "conventional wisdom." This finding contradicts the general response from industry scientists to questions about soil >persistence/impacts of their Bt-cultivars. (They typically say the Bt in transgenic plants is very short-lived in soil, a matter of days; a recent example was the response given by Eric Sachs, Monsanto, to a question re soil microbial community impacts of Bt corn at the June 18, 1999 EPA-USDA Bt-corn workshop ). For these reasons, work is needed on the environmental fate and temporal dynamics in the soil of the Bt toxins present in corn, cotton, and potato crop residues, as a function of various management/tillage systems. It is known, for example, that chopping corn stalks will accelerate degradation, and hence lessen late season ECB selection pressure. What about planting Bt-corn in a no-till system, with no chopping? Degradation would, in this scenario, be delayed, and might occur in a flush of decomposer activity in the early spring as the soil warms.

Research is needed on the short-term soil microbial community impacts of a big dose of Bt as corn trash and other crop residues break down in the spring and early summer. One might hypothesis that under some circumstances, Bt entering the soil will impact soil microbial communities in ways that lead to complex, multi-tier impacts on microbial and soil insect biocontrol, pathogen pressure, immune response and nutrient cycling. Even if the impacts last only 4 to 8 weeks, that is ample time to leave a lasting mark on the performance of the cropping system, both in one season and over many years as microbial communities evolve to a new steady state.

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