METHYL-BROMIDE EMISSIONS FROM A COVERED FIELD .1. EXPERIMENTAL CONDITIONS AND DEGRADATION IN SOIL

An experiment is described to investigate the environmental fate and t ransport of methyl bromide in agricultural systems. The experiment was designed to determine the dynamics of methyl bromide movement through soil, degradation, and total emissions to the atmosphere. This is of particular interest because it will allow an assessment of the environ mental impacts (i.e., stratospheric ozone depletion) resulting from th e agricultural use of methyl bromide. Methyl bromide was applied at a rate of 843 kg in a 3.5-ha (i.e., 240 kg/ha) field at a depth of 0.25 m and covered with a sheet of 1-mil polyethylene plastic. The maximum methyl bromide concentration in the atmosphere occurred at night betwe en 0200 and 0600 h. During the first 3 d of the experiment, the maximu m daily concentrations at 0.2 m above the soil surface were 30, 5, and 1 mg/(m(3) of air), respectively. The trend of reduced emissions with time continued until the plastic was removed, when a momentary increa se in the methyl bromide emissions occurred. The maximum soil gas conc entration 24 h after injection was 30 g/m(3) located at a 0.25-m depth . When the plastic was removed from the field (at 5.6 d), the maximum soil gas concentration was approximately 2 g/m(3) at a 0.5-m depth. A mass-difference method for estimating the total methyl bromide emissio ns from the soil, based on degradation of methyl bromide to Br-, indic ates that approximately 39% or 325 kg (+/-164 kg) of the applied methy l bromide was converted to Br- and, therefore, 61% or 518 kg (+/-164 k g) was lost via volatilization.