Accelerated Microbial Degradation of Chlorothalonil in Soils Amended with Farmyard Manure

The degradation rate of the fungicide chlorothalonil, 2, 4, 5, 6-tetrachloroisophthalonitrile, applied at 40 mg kg^<-1> dry weight of soil was compared among four soils subjected to different types of fertilizer application : unfertilized soil (NF-soil), soil amended with chemical fertilizers (CF-soil), soil amended with the chemical fertilizers and 40 t ha^<-1>y^<-1> of farmyard manure (CF+FYM-soil), and soil amended with 400 t ha^<-1>y^<-1> of farmyard manure (FYM-soil). Degradation mainly due to microbial activity was faster in the CF+FYM- and FYM-soils than in the non-FYM soils. The microorganisms required other carbon sources for chlorothalonil degradation. The most probable number of chlorothalonil-degrading microorganisms was not significantly different among the four soil treatments. Accelerated degradation was observed in the autoclaved CF+FYM-soil inoculated with 5% of intact CF-soil but not in the autoclaved CF-soil inoculated with 5% of CF+FYM-soil. Degradation of chlorothalonil increased in the CF- and CF+FYM-soils by adjustment of the soil pH to a neutral value, although the most probable number of degrading microorganisms remained constant. In conclusion, amendment of soil with farmyard manure enhanced microbial degradation of chlorothalonil due to the increase in the degrading capacity by the maintenance of a near neutral pH value in soil and not due to the increase in the number of degrading microorganisms.