Enhanced microbial degradation of insecticides in soil

Racke, Kenneth
Major Professor
Joel R. Coats
Committee Member
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This project was concerned with the study of the enhanced microbial degradation of insecticides in soil. Enhanced degradation in the phenomenon whereby a soil-applied insecticide is rapidly degraded by populations of soil microorganisms previously exposed to it or a similar insecticide. A laboratory methodology was employed in these studies;The enhanced degradation of isofenphos was investigated in soil in order to elucidate the chemistry and microbiology of this phenomenon. Isofenphos was most rapidly degraded in Iowa cornfield soils that had a history of isofenphos insecticide use. Between 13 and 42% of an applied dose of 5 ppm remained as isofenphos after 4 weeks in soil with isofenphos use history, whereas between 63 and 75% remained incomparable nonhistory soils. Soils with enhanced isofenphos degradation contained an adapted population of soil microorganisms responsible for its degradation. Degradation products of isofenphos detected in cultures of adapted soil microorganisms included isopropyl salicylate, CO[subscript] 2, and polar products;The specificity of enhanced degradation within the organophosphorus insecticide class was investigated. The degradation rates and product distributions of chloropyrifos, fonofos, ethoprop, terbufos, and phorate were not dramatically altered in soils containing microbial populations adapted to rapidly degrade isofenphos. An Arthrobacter sp. isolated from soils with a history of isofenphos use rapidly metabolized isofenphos in pure culture, but did not metabolize or cometabolize any of the other 5 organophosphorus insecticides. Likewise, only fonofos was rapidly degraded in soil with a long history of fonofos use. None of the organophosphorus insecticides was rapidly degraded in soil containing carbofuran-degrading microbial populations;The specificity of enhanced degradation within the carbamate insecticide class was also investigated. Soils with prior field exposure to carbofuran, cloethocarb, or several carbamates contained adapted microbial populations capable of rapidly degrading carbofuran. Bendiocarb was rapidly degraded in all soils displaying enhanced carbofuran degradation, but carbaryl and cloethocarb were most rapidly degraded only in soil with prior exposure to several carbamates or to cloethocarb. The persistance of aldicarb and its sulfoxide and sulfone metabolites was not altered in soils with enhanced carbofuran degradation.