The effect of soil fumigants methyl bromide and metham sodium on the microbial degradation of the nematicide Cadusafos

Rapid biodegradation of Cadusafos in a soil from a previously treated greenhouse site from the area of Vasilika in Northern Greece was observed using both incubation and bioassay studies with nematodes. The slow rates of Cadusafos degradation observed in soils from the previously treated site after sterilization with chloroform and also in soils from an adjacent previously untreated site suggested that soil microorganisms adapted to metabolize Cadusafos were responsible for its rapid dissipation. Fumigation of the soil from the previously treated site with methyl bromide (MB) 9 months prior to the experimental addition of Cadusafos (4 microgram g(-1)) only temporarily inhibited degradation of Cadusafos, unlike metham sodium (MS) whose application significantly hampered degradation of the nematicide. However, when the time interval between fumigation of soil from the previously treated greenhouse with MB and MS and the experimental application of Cadusafos was minimized to 20 d, both fumigants halted the rapid degradation of the nematicide. The transient inhibitory effect of MB on the microbial degradation of subsequent Cadusafos applications was attributed to the recovery of the soil microflora within 9 months. In contrast, the persistent inhibitory effect of MS on the microbial degradation of Cadusafos might have been caused by the adverse effects of the fumigant on the microorganisms, which were responsible for the rapid degradation of Cadusafos in the specific soil, although this was not tested in this study. According to these findings, the combined application of MS as a pre-planting treatment with a post-planting application of Cadusafos would maximize the biological efficacy of the non-fumigant nematicide by extending its persistence and preventing the development of enhanced biodegradation for organophosphorus nematicides such Cadusafos.