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Is Bt toxin from genetically modified maize bound in the soil?

(2001 – 2004) University of Göttingen, Institute of Applied Biotechnology University of Trier, Department of Soil Science/Soil Chemistry

Topic

This project is concerned with the effects on the soil of releasing transgenic Bt maize varieties. The purpose of the project is to investigate:

  • whether Bt toxin from Bt maize is transferred to the soil and in what quantities
  • to what extent it persists there and retains its insecticidal effect
  • which soil characteristics affect the binding of Bt toxin.

Summary

The soils in the three locations have a similar mineralogical composition with a high clay mineral content. These are characterized by a large surface area, which increases the binding ability of the Bt toxin. However, the clay minerals are also highly charged, which hinders binding.

As a result of harvest residue being incorporated into the soil, the upper soil layers have higher levels of organic substances than the lower layers, which likewise hinders binding.

Experiment description

The project examined the soil at the release locations set up within the framework of the joint maize project.

Soil characteristics

Soil components were analyzed in order to be able to make general statements about the binding of Bt toxin in the soils at the trial locations:

  • The clay mineral composition of the soils was determined by means of x-ray diffraction analysis.
  • The quantities of organic substances were determined.
  • The surface charge and the surface area of the soil particles were measured.

Binding of the toxin in the soil

A defined quantity of soil was mixed with increasingly concentrated Bt toxin solutions to enable the retention of the Bt toxin in the soil to be examined in more detail. The bound quantity of Bt toxin in the soil samples was calculated from the difference in the Bt toxin concentrations in the solutions at the beginning and end of the trial. Evaluation of the investigations was by means of sorption isotherms (evaluation curves), which can be used to establish the bound quantity of Bt toxin in relation to the Bt toxin concentration. The characteristics of the various soils can then be compared using these sorption isotherms. From this it is possible to deduce to what extent the binding of Bt toxin depends on the chemical parameters of the soil particles.

Results

Soil characteristics

The soils at the three locations are characterized by a similar mineralogical composition – they show high levels of certain clay minerals (smectite, illite, kaolinite and vermiculite).

As the research locations consist of arable land where harvest residue is incorporated into the soil, the upper soil layers (clay fractions of the topsoil, 0-30 centimetres) all have higher levels of organic substances than the subsoil (40-60 centimetres).

Clay minerals are electrically charged due to their crystal structure and, as a result, they are able to bind charged particles. The particles in the clay fraction have a surface area of between 80 and nearly 100 square meters per gram of soil. Most chemical reactions in soils, including the binding of molecules, take place at these interfaces and the same applies to Bt toxin.

The following general statements can be made regarding the chemical parameters of the soil:

  • The higher the level of organic substances, particularly in the topsoil, the lower the binding of Bt toxin. The organic substance closes small gaps between the soil particles so that Bt toxin cannot be bound in the interstitial spaces.
  • The lower the negative charge of the soil particles, the more Bt toxin is bound. The repulsion between the Bt toxin complexes and soil particles decreases with decreasing negative charge of the soil particles so that more Bt toxin can be bound on the surface of the particles.
  • The greater the surface area of the soil particles, the more Bt toxin is bound to the soil particles. When soil particles have a larger surface area, more space is available for binding polymers on the particle surfaces.