Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/20/2008
Publication Date: 1/5/2009
Citation: Guo, M., and S. Gao. 2009. Degradation of methyl iodide in soil: effects of environmental factors. J. Environ. Qual. 38:513-519. Interpretive Summary: Methyl iodide is a highly effective soil fumigant and important alternative to the phased-out methyl bromide. However, methyl iodide is carcinogenic and exhibits moderate to high acute toxicity by inhalation and ingestion. There is a need for better understanding of its environmental fate after application to soils. The effect of various environmental factors including temperature, soil moisture, soil water content, organic amendment, microorganisms, and soil texture on the degradation rate of MeI in soil were investigated. Increasing soil temperature and organic amendment amount enhanced methyl iodide degradation greatly while changes of other factors had minor effect. This study provided knowledge on methyl iodide behavior in soils that is useful for developing soil management to increase fumigation efficacy and reduce potential negative impacts.
Technical Abstract: Methyl iodide (MeI) is a promising alternative to the phased-out fumigant methyl bromide; 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 degradation rate of MeI in soil. The chemical was spiked into soils at 48.6 mg kg-1 and incubated under different conditions. The degradation rate of MeI in soils was determined by extracting and measuring residual concentrations in soil over a 15 d timecourse. In soil, MeI degradation followed availability-adjusted first-order kinetics. At 20ºC MeI had a calculated half life of 32 d in a sandy loam containing 4.3 mg kg-1 of organic carbon. It degraded more rapidly as temperature increased, exhibiting a half life of 23 d at 30ºC. Amendment with 10% cattle manure shortened the half life to 4 d at 20ºC. In both unamended and manure-amended soils, the half life of MeI greatly increased as the OM was removed and it only slightly increased as the soils were sterilized, indicating predominance of chemical reactions in MeI degradation. Soil texture, mineralogy, and moisture content in a range of 5-15% 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 ground water contamination.