SUPPRESSION OF CHLOROTHALONIL DEGRADATION IN SOIL AFTER REPEATED APPLICATION

Mechanisms of degradation suppression of chlorothalonil after repeated application were investigated. Under laboratory conditions, soil was treated sequentially with 40 mg/kg dry weight of chlorothalonil. This resulted in the degradation suppression of chlorothalonil and accumula tion of the metabolite 4-hydroxy-2,5,6-trichloroisophthalonitriIe (TPN -OH) in soil. After pretreatment with,three kinds of chlorothalonil me tabolite, only TPN-OH was found to suppress chlorothalonil degradation in soil. After the pretreatment of TPN-OH, residual chlorothalonil in soil at 1 week after application was 9 to 11% higher than those with pretreatment with other metabolites and control. The TPN-OH pretreatme nt also suppressed soil respiration and TPN-OH was more persistent in soil than chlorothalonil, indicating that toxicity of residual TPN-OK may suppress the degradation of chlorothalonil in soil.