MICROBIAL-DEGRADATION OF PARAQUAT SORBED TO PLANT RESIDUES

It was demonstrated that paraquat (1,1'-dimethyl-4,4'-bipyridynium dichloride) sorbed to plant residues was degraded by natural microbial populations associated with plants and/or soil under laboratory conditions. The microorganisms associated with plants showed a higher degradation rate than those from soil. The degradation rate was higher in rice straw (C/N ratio = 53) than in dropwort (C/N = 20) and Chinese milk vetch (C/N = 17). Urea, ammonium chloride, and sodium nitrate suppressed the paraquat degradation rate by microorganisms associated with rice straw, suggesting a relationship to nitrogen metabolism. The degradation rate was much higher under aerobic conditions than under anaerobic conditions. When rice straw containing sorbed paraquat was placed on the surface of soil or mixed with soil, the herbicide was degraded under the former condition but not under the latter condition. Monopyridone (1',2'-dihydro-1,1'-dimethyl-2'-oxo-4,4'-bipyridynium ion) and (CO2)-C-14 were detected as metabolites in rice straw spiked with [methyl-C-14(2)]-paraquat.