Enhanced degradation in soil of the herbicide EPTC and determination of its degradative pathway by an isolated soil microorganism

In many soils, repeated applications of certain pesticides result in their being so rapidly degraded that they lose their efficacy against target pest(s). A series of experiments was conducted to examine the ability of Ohio soils to develop enhanced degradation of the herbicide EPTC (s-ethyl N,N-dipropyl carbamothiaote) and to determine its metabolism by an isolated soil microorganism. Three soils selected to obtain a range in pH, texture, and organic carbon were treated with EPTC for 4 consecutive applications (6 weeks between applications). EPTC concentrations as measured by gas chromatography, decreased 80% or more one week after the second application in all three soils. When EPTC (as the herbicide Eradicane) was applied in the field, EPTC persistence was also affected. After 4 days, 70% of the applied EPTC was degraded in the soil which had received 1, 2, 3, or 4 consecutive years of EPTC applications while only 30% was degraded in the soil without prior EPTC treatment. Inoculation of untreated soil with as low as 1% (w/w) enhanced EPTC soil also induced increased EPTC degradation in an untreated soil.; Metabolism of unlabelled and labelled (at the 1-propyl position) EPTC by an isolated soil microbe was followed by GC/MS and TLC/LSC analysis, respectively. Rapid decrease in 14-C activity in the organic fraction corresponded with rapid {dollar}sp{lcub}14{rcub}{dollar}CO{dollar}sb2{dollar} evolution and transient increase in 14-C activity in the aqueous fraction. Four metabolites were observed in the TLC analysis. Two were identified as EPTC-sulfoxide and N-depropyl EPTC with N-depropyl EPTC being confirmed by GC/MS analysis. It is proposed that the initial reactions catalyzed by soil microbes involve hydroxylation (primary pathway) and sulfoxidation (secondary pathway) resulting in products which are further rapidly mineralized to CO{dollar}sb2{dollar}. The availability of different pathways for EPTC metabolism by soil microbes after repeated applications to the soil results in its very rapid degradation and loss of efficacy.