Degradation of methyl iodide in soil: effects of environmental factors.

Methyl iodide (MeI) is a promising alternative to the phased-out fumigant methyl bromide (MeBr); however, there are concerns about its environmental fate following soil fumigation. Laboratory experiments were conducted to investigate the effect of various environmental factors on the rate of MeI degradation in soil. The chemical was added to soil at 48.6 mg kg(-1) and incubated under different conditions. The MeI degradation rate in soil was determined by extracting and measuring residual concentrations over a 15 d incubation period. In soil, MeI degradation followed availability-adjusted first-order kinetics. At 20 degrees C MeI had a calculated half-life of 32 d in a sandy loam containing 4.3 g kg(-1) of organic carbon. It degraded more rapidly as temperature increased, exhibiting a half-life of 23 d at 30 degrees C. Amendment with 10% cattle manure shortened the half-life to 4 d at 20 degrees C. In both unamended and manure-amended soils, the half-life of MeI greatly increased as the organic matter (OM) was removed and it only slightly increased in soils that were sterilized, indicating predominance of chemical reactions in MeI degradation. Soil texture, mineralogy, and moderate moisture content had little influence on MeI degradation. The degradation slowed as the chemical application rate increased. The results suggest that environmental factors, especially soil temperature and organic amendments, should be considered in combination with the minimum effective MeI application rate for achieving satisfactory pest-control efficacy, reducing atmospheric volatilization, and minimizing groundwater contamination.