The soil organisms responsible for the enhanced biodegradation of metham sodium

Abstract.

On a farm near Perth, Western Australia, where metham sodium (sodium N-methyldithiocarbamate) soil fumigant has been extensively used for the past decade, a dramatic case of enhanced biodegradation has been identified. On this soil, the applied dose of metham sodium produced less than half the maximum concentration of the toxin methyl isothiocyanate (MITC) reached in similar, previously untreated soil. Also, the MITC formed was present in the soil for less than 5% of the time of that in the untreated soil. On sterilisation of the affected soil in an autoclave, followed by treatment with metham sodium, the MITC concentration/time relationship was similar to that of the previously untreated soil, confirming that the enhanced degradation was the result of a biological process. Dry heat treatment of the affected soil (100°C, 1 h) did not destroy the microorganisms responsible for enhanced biodegradation, as the soil recovered its degrading ability upon rehydration. This indicated that microorganisms with resistant stages were involved, at least in part, in the biodegradation of MITC. Agar containing MITC as a carbon source was used to isolate bacteria from fresh soil exhibiting enhanced biodegradation. All 18 isolates selected were Gram positive. Morphologically, 11 isolates resembled Rhodococcus spp., 4 isolates resembled Bacillus spp. and 3 were unidentified. Inoculation with these isolates grouped together into soil that had been sterilised by autoclaving resulted in biodegradation of MITC in that soil.