ACCELERATED MICROBIAL-DEGRADATION OF CHLOROTHALONIL IN SOILS AMENDED WITH FARMYARD MANURE

The degradation rate of the fungicide chlorothalonil, 2,4,5,6-tetrachl oroisophthalonitrile, applied at 40 mg kg(-1) dry weight of soil was c ompared among four soils subjected to different types of fertilizer ap plication: unfertilized soil (NF-soil), soil amended with chemical fer tilizers (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 source s for chlorothalonil degradation. The most probable number of chloroth alonildegrading microorganisms was not significantly different among t he four soil treatments, Accelerated degradation was observed in the a utoclaved CF+ FYM-soil inoculated with 5% of intact CF-soil but not in the autoclaved CF-soil inoculated with 5% of CF+FY1M-soil. Degradatio n of chlorothalonil increased in the CF- and CF+FYM-soils by adjustmen t of the soil pH to a neutral value, although the most probable number of degrading microorganisms remained constant. In conclusion, amendme nt of soil with farmyard manure enhanced microbial degradation of chlo rothalonil due to the increase in the degrading capacity by the mainte nance of a near neutral pH value in soil and not due to the increase i n the number of degrading microorganisms.