[Effect of post-emergence application of two herbicides to winter wheat on the soil microflora (author's transl)].

In a field experiment the herbicides U 46 PD-Fluid (dichlorprop salt) and Tribunil (methabenzthiazuron) were applied in April 1976 post-emergence to winter wheat cultivated in 3 different soils. After 4 days, 1, 2 and 3 months samples were taken from 0-5 and 5-10 cm depth of a loamy sand soil. Dehydrogenase activity and respiration were investigated. Microbial populations were counted after 4 days only. In all 3 soils decomposition of straw in 10 cm depth was estimated using nylon gauze bags. The application of U 46 DP-Fluid stimulated bacterial and algal but reduced fungal and actinomycete populations at 0-5 cm depth. About 10% increase in the dehydrogenase activity was recorded during the whole season. Soil respiration investigated immediate after taking samples was stimulated on the 4th day after herbicide application, but reduced after 3 months. Storage of the soil samples at 5 degrees C for 10 months showed similar or opposite results depending on the incubation method used. A somewhat increased decomposition of the straw was also observed in the loamy sand until June. The application of Tribunil at the same depth stimulated populations of actinomycetes and algae but did not affect the other microflora. Reduced dehydrogenase activity was observed during the dry month of June, but the same remained unchanged at the other time of sampling. In freshly drawn soil samples, reduced soil respiration was observed after 1 and 3 months and this remained unaltered at other times. The storage at 5 degrees C of the soil samples revealed inhibition only after 1 month and at the other times a stimulation or no influence could be observed and this was found to be dependent on the incubation method. Straw decomposition was not changed following the application of Tribunil. In 5-10 cm depth, however, the above mentioned trends were only scarcely observed.