Insect-resistant maize: Bt toxin in the soil

"The Bt toxin does not simply disappear after harvesting."

Bt toxin, a substance present in genetically modified plants which acts as a pesticide, could accumulate in the soil. These are the findings of a research project at the Federal Agricultural Research Centre (FAL) in Braunschweig. Here for the first time studies were conducted under field conditions to see what happens to the Bt toxin from the plants in the soil. Christoph Tebbe and his team have developed a new, extremely sensitive measuring procedure for this. No evidence of negative effects on the soil ecosystem has been found so far.

Project Director Dr. Christoph Tebbe and Dr. Susanne Baumgarte have developed a new procedure for measuring Bt content in the soil.

Crop residues. In the spring the rotting remains from the previous year lie on the ground between the young maize plants. They still contain the Bt toxin.

The experiment:

Trial sites: two areas of Bt maize, one in North Rhine-Westphalia and one in Saxony-Anhalt, where all the field trials of the maize project group were conducted.

Layout: Several plots on each site arranged in a specific pattern. Bt maize and the isogenic reference strain with and without insecticide treatment were grown on them.

Bt maize: In this experiment a Bt maize variety (MON 810) which also produces the Bt toxin in the roots was evaluated. The Bt toxin is also used as a pesticide in other transgenic plants such as cotton and soya.

Sampling: Several soil samples were taken from all the Bt maize plots at both sites over a two-year period. The samples from each plot were mixed and the Bt toxin content was studied. This procedure was carried out four times in each growing period at clearly defined stages in the development of the maize. A distinction was also made between "loose soil" and the soil clinging to the roots of the maize (rhizosphere).

When genetically modified plants with Bt pest resistance are grown, the Bt toxin is also present in the soil. The active protein, the gene for which is isolated from soil bacteria and which destroys the intestinal wall of the pests, enters the soil from rotting crop residues and probably also through root excretions. But how quickly does it break down in the soil? Is it possible that the Bt toxin accumulates there if Bt plants are grown on the same field for several years? These are questions which interested Christoph Tebbe and his team at FAL’s Institute of Agroecology.

Although Guenther Stotzky and his colleagues at New York University have already shown that Bt toxin can be relatively stable in the soil under laboratory conditions, these studies were conducted with comparatively unrealistically high concentrations. The FAL group has itself now been able to detect Bt toxin in the soil under realistic growing conditions, thanks to the refinement of a commercially available detection method (ELISA ). The team managed to increase the sensitivity of this method approximately a thousand times. This was enough to quantitatively determine the amounts of Bt toxin from Bt maize grown outdoors.

Researchers at the FAL estimate that approximately 60% of the Bt toxin present in the soil was analytically measurable. The rest is bound to clay minerals or organic substances, as can be surmised from the Stotzky studies. However, even when bound, it could still retain its insecticide affect if it is absorbed by soil organisms (resorption).

Results: More Bt toxin in the second year.

Christoph Tebbe conducted his studies on two Bt maize sites near Bonn and Halle, which had previously been used by several biological safety research projects. In 2002 the Bt concentrations from soil samples taken from several plots were systematically analysed for the first time using the new procedure. The following year the same plots were planted with Bt maize again.

In the second year of cultivation all Bt toxin values were significantly higher than those for 2002 - on both sites, on almost all plots and throughout the growing season.
So far studies have mainly been conducted on soil samples from the root area (rhizosphere ) of maize plants. Here the soil is mixed with fine roots and the Bt content is significantly higher than in the loose soil, as one would expect. The samples taken from the trial plots are still undergoing systematic evaluation.
The rotting crop residues, which had been left in the fields from the previous year, were also examined. Here too the Bt values were "incredibly high" (Tebbe), but had fallen significantly by the summer.

The Bt values in the crop residues from the previous year were approximately a thousand times greater than in the soil. However Susanne Baumgarte, researcher with the FAL, found the measurement results of the soil samples taken from the loose soil in April particularly remarkable. "They are very high. That indicates that, at this point in time, the Bt toxin has not yet fully broken down."

No harmful ecological effects so far.

Even if the Bt values increase from one year to the next, they are evidently too small to harm the soil life." In our estimation, the effect on the diversity of microorganisms is on the low side if anything," says Christoph Tebbe. " We know this from our studies of rhizospheres." Of greater interest is the impact of the high Bt toxin concentrations in the crop residues. "Many minute arthropods and worms which are involved in mineralising these crop residues are dependent upon them. This affects entire terrestrial food chains." Wolfgang Büchs and his team from the Federal Biological Research Centre (BBA) in Braunschweig are studying sciarid fly larvae, for example, which live on rotting plant material.

Accumulation - even over several years?

The critical question, however, is whether Bt toxin gradually accumulates in the soil when Bt maize is grown over a period of several years. It is also conceivable that the Bt toxin gradually breaks down and levels off.

But the areas from which the FAL group took their soil samples will not be planted with Bt maize next year. The study is coming to an end – and the question of whether Bt values would have continued to rise in 2004 remains unanswered. "We can see a trend", says Christoph Tebbe, "but after only two years of study we cannot conclusively say how Bt toxin contents in fields of Bt maize change." It is also not clear at present what effect crop rotation or soil preparation has on the persistence of Bt toxin in the soil. Further studies in the coming years will be able to shed more light on this matter now that the sensitive detection system is available.

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