Temperature and moisture effects on fumigant degradation in soil

Recent discovery of the contribution of methyl bromide fumigation to stratospheric ozone depletion has revealed our limited understanding of the environmental processes of fumigants. For instance, little is known about fumigant degradation in soil under high temperature or low moisture conditions that prevail near the soil surface during fumigation. In this study we determined the interaction of soil temperature and moisture with degradation of 1,3-dichloropropene (1,3-D) and methyl isothiocyanate (MITC) for extended soil temperature and moisture ranges. Fumigant degradation increased 5 to 12 times when temperature increased from 20 to 50 degrees C. It was further shown that chemical transformation of fumigants always increased with increasing temperature, but temperature effects on microbial degradation were fumigant dependent. The relative contribution of microbial degradation to the overall fumigant degradation was highest for the soil with highest organic matter content, and was greater for MITC than for 1,3-D isomers, When the temperature was >30 degrees C, microbial degradation of 1,3-D was substantially suppressed, while that of MITC was greatly stimulated. As soil moisture content increased, 1,3-D degradation accelerated, but that of MITC decreased. The specific responses of fumigant degradation to temperature and moisture variations should be considered when describing their transport in the environment, and also may be used for designing fumigation practices that allow reduced atmospheric emissions.