Abiotic dissipation of chlorothalonil in soil accelerated by amendment with high applications of farmyard manure

The dissipation rate of the fungicide chlorothalonil, 2,4,5,6-tetrachloroisophthalonitrile, applied at 4 mg kg−1 dry wt soil was compared among four soils receiving different fertilizing practices. The dissipation of chlorothalonil was accelerated in a soil amended with farmyard manure at 200 t ha−1 twice a year (FYM-soil, pH = 7.2). The acceleration was mainly due to abiotic action. The abiotic dissipation rate apparently followed first-order kinetics. Chlorothalonil was sorbed strongly to FYM-soil. Only a small amount of chlorothalonil was recovered from the humus fractions of FYM-soil extracted with 100 mm Na4P2O7(pH 7.5). Chlorothalonil was dissipated in sterile solutions (pH 7) containing dissolved crude humus extracted by 0.2 n NaOH from FYM-soil. It is suggested that the accelerated abiotic dissipation of chlorothalonil in FYM-soil was due mainly to chemical reactions with soil humus. The dissipation of chlorothalonil from aqueous solution at pH higher than 8 also indicated that microsites in FYM-soil with a high pH might accelerate the dissipation.