Agricultural soil fumigation as a source of atmospheric methyl bromide.

Abstract

Methyl bromide (MeBr) is used increasingly as a biocidal fumigant, primarily in agricultural soils prior to planting ofcrops. This usage carries potential for stratospheric ozone reduction due to Br atom catalysis, depending on how much MeBr escapes from fumigated soils to the atmosphere and on details of atmospheric chemical reactions. We present direct field measurements of MeBr escape; 87% of the applied MeBr was emitted within 7 days after a commercial fumigation. Covering the field with plastic sheets retarded MeBr escape somewhat but first-day losses were still 40%; thicker sections of sheets were relatively more effective than thin sections. We also measured gaseous MeBr concentrations versus depth in the soil column; these profiles display diffusion-like evolution. In soil, MeBr is partitioned among gas, liquid, and adsorbed solid phases. The contribution of halons to the bromine flux, F., that enters the stratosphere is relatively well known (7.2 x 106 kg of Br per year in 1990) because the halons are totally synthetic, their industrial production and emission rates are known and they are inert in the troposphere (5, 6).The contribution from MeBr to F,, on the other hand, is poorly quantified at present. Natural sources, principally oceanic surface waters, are known to inject MeBr into surface air but are not well quantified (7,8). Currently, the global atmospheric content, B, of MeBr is 2 + 0.5 x 108 kg. If reaction with tropospheric OH radicals (7-9) is its major sink, the atmospheric residence time, T, for MeBr is 2 + 0.5 years. In an assumed quasi steady state, the total surface source = BIT or 0.6 to 1.7 x 108 kg/year (7)(8)(9). While muchThe publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact.of the MeBr upward flux may be removed by tropospheric OH, thus limiting its contribution to F,, the amount that reaches the stratosphere may still exceed the contributions from CF3Br and CBrClF2.Concern arises over synthetic MeBr because its worldwide industrial usage has increased from 4.2 x 107 to 6.3 x 107 kg/year between 1984 and 1990; these figures exclude MeBr used as a chemical intermediate. Eighty percent of this usage is as an agricultural soil fumigant prior to planting (10). MeBr is a broad-spectrum fumigant biocide against arthropods, weeds, nematodes, fungi, and bacterial pests, apparently cost effective when applied to soils before planting; it is being used in many countries (10). Thus there is a significant and growing potential for atmospheric MeBr to increase, depending on the fraction of MeBr that escapes from soils during and after fumigations and on the size of the natural sources (7,8). In soils, MeBr may be removed by physical hydrolysis, adsorption to soil particles, and microbiological and transport processes (11,12). Moisture and organic matter in soils should enhance dissociation rates but it is very difficult to estimate h...

Fate of Methyl Bromide in Fumigated Soils

“…MeBr is known to be hydrolyzed to Br" and methanol in water (13). Yagi et al (4,21) compared two field fumigation experiments that measured the flux of MeBr and formation of Br" in soil. They observed an increase in Br" and a decrease in MeBr volatility related to a combination of increased soil moisture, pH, organic matter, and injection depth.…”

Large carbon isotope fractionation associated with oxidation of methyl halides by methylotrophic bacteria

“…To determine whether naturally occurring populations of soil bacteria can fractionate elevated (fumigation) levels of methyl bromide, we deployed flux-chamber collars over experimental soil plots (University of California Bay Area Research and Extension Center, Santa Clara, CA), into which we injected MeBr (34 g m Ϫ2 at 12 cm depth). The chamber collars were polyvinylchloride (PVC) cylinders (diameter ϭ 30 cm, height ϭ 14 cm) with removable lids constructed of PVC rings fitted with 0.0025-cm-thick polyethylene covers stretched as a drum skin (20). The chamber lids were placed over the collars and, after an accumulation period of 4 h, samples of the gas phase were collected 4 times over the next 24 h.…”

Evaluation of permeation rates of soil fumigants through plastic films by the cup method

“…Such films typically remain intact in the field for 1-2 weeks, after which the film is removed for planting or punctured for placement of seedlings. Polyethylene (PE) films are most commonly used in soil fumigation, but these films are reportedly permeable by MeBr [7][8][9][10][11][12] and other soil fumigants. 12,13) To reduce emissions and to develop fumigation management practices that protect the environment while providing adequate pest control, they must also maintain their impermeability to fumigant vapors under field conditions.…”

“…Experiments to assess the volatilization of soil fumigants from a soil surface covered with plastic film have been conducted in the laboratory for MeBr, MeI, 12) 1,3-D, and CP. 13) Field trials have also been conducted to measure the volatilization of MeBr, [10][11][12]14,15) MeI, 12) and 1,3-D 16) from the soil surface when covered with different plastic films.…”