Enhanced degradation of pesticide wastes in soil: implications for bioremediation of agrochemical dealer sites

Abstract

The fate of C-atrazine and C-metolachlor, applied as a mixture (50 fxglg each), in soils taken from two pesticide-contaminated sites in Iowa (Alpha and Bravo) was determined in laboratory studies. In the Bravo site study, degradation of metolachlor was greater in the Kochia scoparia rhizosphere soil with a mean of 53% of the applied C remaining in the rhizosphere soil compared to 71% in the nonvegetated soils after 60 days of incubation. A significantly greater percentage of applied C-atrazine was mineralized in soil taken from the rhizosphere of Kochia than from nonvegetated soils. Atrazine was less persistent in the rhizosphere soil than in nonvegetated soils. Soil-bound residues were present in greater quantity in nonvegetated soils compared with the rhizosphere soils. In the Alpha site study, significant differences were seen between the rhizosphere and nonvegetated soils in the amount of COj evolution, atrazine remaining and soil-bound residues. The half-life for atrazine was significantly less in the 'Pesticide Toxicology Laboratory, Iowa State University, Ames, lA 50011 ^Department of Plant Pathology, Texas A & M University, College Station, TX 77843 17 rhizosphere soil than in nonvegetated soil (p ^ 0.05), with a half-life of 50 d compared to 193 d in the nonvegetated soils. In plant/soil studies, chemical analyses revealed significant differences between the Xoc/jw-vegetated and Brassica-wegetzted soils (192 d post-herbicide treatment). Combustion of plant material revealed that significantly more C was taken up by Kochia plants (9.9% of the applied 'C) than by Brassica plants (0.8%) (p =0.0001). Significantly less atrazine was extractable from soils vegetated with Kochia (4.3% of the applied C) than soils vegetated with Brassica (9.8%) (p < 0.05). At 240 d post-herbicide application, significantly less atrazine was extractable from soils vegetated with Kochia (5.3% of the applied ^^C) than from nonvegetated soils (8.3%) (p < 0.01). Combustion of the plants revealed that 6.5% of the applied 'C-atrazine was taken up into the plant tissue. Low populations of atrazine degraders were seen in soils from Alpha site, ranging from 7 to 326 organisms g ' of soil. Atrazine degraders at the Alpha site were significantly more numerous in Kochia rhizosphere soils than in soil from a nonvegetated area. Greater numbers of atrazine degraders were noted at Bravo site, with significantly more in the nonvegetated soil (17,412 organisms g^) compared with the Kochia rhizosphere soil (1,107 organisms g'). The potential for vegetation and degrader microorganisms to aid in bioremediating pesticide wastes in soil is promising.