Accelerated degradation of metam-sodium in soil: Occurrence and possible mechanism

Accelerated degradation (AD) of soil fumigants can result in insufficient pest control. We have documented AD and reduced effectiveness of Methyl-isothiocyanate (MITC), the active ingredient of Metam-sodium in soil, resulting from repeated applications under field conditions. AD of MITC may result from enrichment in the population of microbial degraders in the soils, from increased enzymatic activity of the degraders, from the transfer of extrachromosal elements from the degraders to the other components of the soil microbial community, or from a combination of these factors. Revealing the microbial mechanism underlying this AD may provide tools for managing it. The objective of the present study was to identify and characterize the microorganisms responsible for the occurrence of accelerated degradation of MITC in soil. Under controlled conditions, we were able to induce accelerated degradation of MITC in six different agricultural soils, by inoculating natural (nonhistory) soil with 10% of MS history soil (three repeated MS applica tions). Disinfestation of a history soil by steam or solarization eliminated the AD phenomenon. In contrast, fumigation with methyl bromide, iodomethane, or formalin did not reduce AD in history soil. We developed a soil extraction method in order to increase the density of the degrading organisms. Inoculation of nonhistory soil with such extract from a history soil resulted in AD in the soil, indicating the presence of degraders in the extract. Liquid culture of the soil extract from history soil had a high potency for rapidly degrading MITC. Heating the soil extract from a history soil to 60C for two hours eliminated the accelerated degradation. Initial results indicate that the DNA profiles in history and nonhistory soils after MS application are different. We isolated culturable gram negative bacteria which rapidly degrade MITC in liquid culture as well as in soil.