Volatilization and distribution of methyl iodide and methyl bromide after subsoil application

Methyl iodide (MeI) was recently proposed as a potential replacement for methyl bromide (MeBr) as a soil fumigant, but little is known about its fate in the environment. Volatilization of MeI was measured and compared with that of MeBr in packed soil columns with different soils and under various soil surface conditions. Under the same conditions, MeI volatilization loss was greater than that of MeBr, and the enhanced volatilization was attributed to its slower degradation in soil. In a Greenfield sandy loam (coarse-loamy, mixed, thermic Typic Haploxeralfs), the greatest loss occurred in a nontarped application (78% for MeI and 62% for MeBr), with the smallest loss in a high-barrier plastic-tarped treatment (28% for MeI and 24% for MeBr). Covering the soil surface with polyethylene film was ineffective in preventing MeI or MeBr volatilization. Volatilization losses of MeI and MeBr were also influenced by soil type, and were significantly less from soils that were high in organic matter content and capable of rapidly degrading the fumigants. Ten days after a point application (30 cm) in field plots covered with polyethylene film, higher MeI concentrations were detected at most depths. Because of its Longer persistence in soil, MeI may have a greater tendency ten reach groundwater than MeBr.