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Summary Band 16

Mechthild Denkler (1994): Microbial toxicity, degradation and translocation of pesticides in a cultivated luvisol: laboratory and field experiments and modeling.

 

The toxicity of pesticides on soil microorganisms was tested in field and laboratory experiments. Furthermore, the total and water soluble contents of the herbicides methabenzthiazuron (MBT) and atrazine were determined over time in different soil depths of a cultivated luvisol. The results were used for the calibration and validation of the simulation model PELMO.

In the field trial only short-term effects of the pesticides on the microbial activity could be observed. Compared to oscillations of the biotic activity caused by e.g. climatic factors these effects can be classified as negligible.

In the laboratory dose response curves were established for the herbicides Tribunil and Goltix, the fungicides Corbel and Desmel, the insecticide Metasystox R and their active ingredients. The results show that the microbial toxicity of the chemicals differed to a high degree (50 % inhibition of the microbial activity at 84,7 mg/kg (Desmel) up to >8000 mg/kg (Tribunil and Corbel)). In the course of two months the negative effects increased or diminished.

After repeated applications during the last 20 years the vertical distributions of MBT and atrazine showed the highest concentrations in the plow layer (0-30 cm) and decreasing contents from 30 to 90 cm depth. High MBT contents in some subsurface samples taken four months after application in autumn indicated a preferential flow of MBT in macropores.

The degradation of MBT and atrazine was high during the first six months after the application in spring and then decreased strongly. The degradation could not be described adequately with a reaction following first-order kinetics. Reactions with orders > 1 (MBT: 1,7; atrazine: 2,6) or two combined exponential functions matched the results much better.

The solubility of MBT and atrazine in the soil samples from the field trial was low compared to the solubility of the herbicides in batch-experiments with comparable total contents. The reason for this discrepancy was the different age of the pesticide residues in the field experiments (1 week to 3 years) and the much shorter time for an interaction between pesticides and soil material in the laboratory experiment. In the course of time the strength of the MBT and atrazine binding increased as was indicated by increasing sorption coefficients (KD, KF values).

Using PELMO an accurate simulation of the total MBT and atrazine contents in the upper soil layer (0-5 and 0-30 cm) was possible, whereas the transport of the pesticides into the subsoil was underestimated. The model was not able to describe herbicide translocation by preferential flow. 

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