Behavior of the Soil Fumigant Methyl Isothiocyanate in Repacked Soil Columns

Abstract

AbstractThe environmental fate of the soil fumigant methyl isothiocyanate (MITC) is of concern in areas with groundwater aquifers sensitive to contamination. The objective of this study was to examine the partitioning of soil‐applied MITC among the processes of degradation, adsorption, volatilization to the atmosphere, and leaching from the root zone. Methyl isothiocyanate was injected into repacked soil columns in which leaching loss, volatilization loss, and soil‐extractable MITC were measured, and degradation was calculated by mass balance. The effects of three soil water regimes and four soils with differing clay and organic C contents on MITC behavior were examined. Methyl isothiocyanate degradation was rapid; >90% of applied MITC was degraded in 83 d. Methyl isothiocyanate volatilization loss was positively correlated to soil air‐filled porosity and was suppressed by water application, indicating volatilization was strongly influenced by gas‐phase MITC transport. Under conditions favorable for volatilization, up to 30% of applied MITC volatilized over 35 d. Methyl isothiocyanate retention compared with bromide was relatively small. Methyl isothiocyanate leaching increased with increasing frequency of water application or decreasing delay of water application after MITC injection, and with decreasing degradation and volatilization losses. Under most field conditions, MITC should be removed from the soil root zone by degradation and volatilization before significant leaching occurs. However, because volatilization and degradation losses were very sensitive to soil properties and water regime, site specific soil and climatic conditions immediately after application will have a large influence on MITC leaching potential.