METHYL-BROMIDE EMISSIONS FROM AGRICULTURAL FIELDS - BARE SOIL, DEEP INJECTION

The environmental fate and transport of methyl bromide (MeBr) in an ag ricultural field after deep injection (68 cm) is described and compare d to a recent experiment where MeBr was injected at a shallow (25 cm) depth, and the surface was covered with high-density polyethylene plas tic. Three independent methods were used to estimate the total MeBr lo st after application, i.e., the appearance of soil Br-, the flux chamb er, and micrometeorological methods. Peak MeBr volatilization rates oc curred during the first 24 h, but relatively high rates continued for more than 7 days after application. Diurnally, the largest volatilizat ion rates occurred from midnight to early morning during periods of de creasing barometric pressure, and the volatilization rate decreased wh en the barometric pressure was increasing. Due to deep injection, cool er temperatures, and smaller thermal gradients, the total MeBr mass em itted from the field was significantly less than a previous experiment . The total emissions estimate obtained from the Br- data was 239 kg o r 21%. The estimates obtained from the direct flux measurements were f ound 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 dis crepancy between Br- and direct flux measurements was attributed to lo sses 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 minimi zed.