Bioassay of Dechlorination of Benthiocarb (Thiobencarb) Herbicide in Flooded Soil Using Germinated Grains of Rice Plants

Abstract

Occurrence of dwarfish symptom in growing rice plants caused by benthiocarb (thiobencarb) application in soil was assayed using germinated grains of rice plant. Two grams of soil containing 0.8% of fresh straw powder was placed in a centrifuge tube and pre-incubated with 3ml of water for one week at 25°C. Two ml of benthiocarb (20ppm on dry soil basis) solution was added to the flooded soil. Remarkable growth inhibition of seedlings by S-benzyl N′N-diethylthiocarbamate, a reductive product of benthiocarb, occurred by placing the grains in soil incubated with Benthiocarb for 3-7 weeks at 25°C. Some factors affecting the dechlorination of Benthiocarb and the degradation of dechlorinated benthiocarb were studied by the method above. Application of straw powder or soluble starch to the soil caused a decrease in the redox potential of the soil. Under the reductive soil conditions, the production of dechlorinated benthiocarb was promoted, and the degradation of the dechlorinated product in the soil was retarded. Optimal temperature of the soil incubation for the dechlorination of benthiocarb was estimated at 25-20°C. Neither the dechlorination of benthiocarb nor the degradation of dechlorinated benthiocarb occurred by sterilizing the soil.