Methyl Bromide Emissions from Agricultural Fields:  Bare-Soil, Deep Injection

Abstract

The environmental fate and transport of methyl bromide (MeBr) in an agricultural field after deep injection (68 cm) is described and compared to a recent experiment where MeBr was injected at a shallow (25 cm) depth, and the surface was covered with high-density polyethylene plastic. Three independent methods were used to estimate the total MeBr lost after application, i.e., the appearance of soil Br -, the flux chamber, and micrometeorological methods. Peak MeBr volatilization rates occurred during the first 24 h, but relatively high rates continued for more than 7 days after application. Diurnally, the largest volatilization rates oc curred from midnight to early morning during periods of decreasing barometric pressure, and the volatilization rate decreased when the barometric pressure was increasing. Due to deep injection, cooler temperatures, and smaller thermal gradients, the total MeBr mass emitted from the field was significantly less than a previous experiment. The total emissions estimate obtained from the Br - data was 239 kg or 21%. The estimates obtained from the direct flux measurements were found to range from 1.9% to 4.9%. The percent mass recovery ranged from 81% to 84% of the applied mass, with an average value of 82%. The discrepancy between Br - and direct flux measurements was attributed to losses from the shank fracture during or immediately after injection. It is important to understand how deep injection affects MeBr emissions so that the total fumigant loss from agricultural fields can be minimized.