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Reducing Methyl Bromide Emissions
Scott Yates and colleagues from
ARS and the
University of
California-Riverside have been exploring ways to decrease the
amount of methyl bromide that escapes from fields into the atmosphere.
Yates, a soil scientist with the ARS-
U.S. Salinity Laboratory at
Riverside, and colleagues completed studies in 1993 and 1994 that
measured methyl bromide emission from a field located at the University of
California's Moreno Valley Field Station.
To reduce the amount of methyl bromide released into the air from
fields, Yates and colleagues recommend that the chemical be applied
relatively deep in organics-rich, moist soil under tarpaulins when
temperatures are cool. Further packing the surface soil immediately after
application and covering with high-barrier plastic should reduce emissions
even more.
"Following current practices, we applied methyl bromide at a
shallow depth and covered the field with a sheet of 1 millimeter
polyethylene plastic," Yates reports. "About 61 percent of the
methyl bromide we put on the field escaped into the atmosphere."
"In our second experiment, when methyl bromide was placed
deeper in wetter soil and the daily temperatures were cooler, only 21
percent was lost."
Many soil-chemical processes affect the fate of any fumigant,
including methyl bromide, Yates says. But an adequate balance of
containment, degradation, and effective dosage must be maintained to lower
emissions without sacrificing efficacy.
Containment
"Perfect containment in the absence of degradation will not
produce lower emissions unless the field remains covered
indefinitely," he says. "Ideally, degradation will destroy
methyl bromide in the soil before the plastic is removed but after
achieving adequate pest control."
Since methyl bromide has a high vapor pressure, it moves through the
soil easily. But injection depth, bulk density, water content, cracking of
the soil, and the use of plastic all greatly affect how methyl bromide
moves in the soil after application and how much is lost to the
atmosphere.
Plastic films can reduce the amount of methyl bromide that escapes,
Yates says, and new, impermeable materials are now available that can
control containment even further.
He recommends that plastic be used rather than leaving the soil
surface uncovered. "Recent experiments showed that, when injected at
a shallow level, nearly all of the methyl bromide applied leaves the
treated soil after a few days." Since the chemical stays in the soil
much longer under lower permeability films, the application rate could be
reduced without sacrificing efficacy. Reducing the rate translates into
reduced emissions.
Yates' research showed that the depth at which methyl bromide is
applied affects the amount that escapes into the atmosphere. Placing the
compound at a greater depth can minimize its emission into the air during
soil fumigation.
The studies also showed that disking and surface-packing closed the
cracks in the soil above the injection sites. This, along with
increasing the water content at the surface, helped reduce total methyl
bromide emissions.
Degradation
Degradation, or the breaking down, of methyl bromide in the soil keeps
it from escaping into the atmosphere. Yates and colleagues found that
less methyl bromide escaped from soils high in organic matter. Organic
matter enhances degradation by providing a way for the chemical to
uncouple. Once this occurs, the part of primary co cern, bromine, remains
in the soil as bromide. Therefore, increasing the organic matter in the
soil allows less of the chemical to escape.
Effective Dosage
If new ways to enhance methyl bromide's effectiveness could be
developed, the quantity needed for agricultural uses would be reduced,
thereby reducing the amount released into the atmosphere, Yates says.
In general, the amount and depth of injection depends on soil
conditions and the type and distribution of target organisms. For
strawberries, pests live fairly close to the soil surface because
strawberries have shallow plant roots; but pests that attack grape roots
are deeper in the soil. Therefore, the ideal injection depth for pest
control differs for these two crops. For emission reduction, however,
deep injection is preferred. For some conditions, like coarse textured
soils, good pest control can be obtained with deep placement of the
chemical, and pest efficacy can be further increased with high-barrier
plastics.
"We recently injected methyl bromide at 60 cm and covered the soil
with polyethylene and high-barrier films to control citrus nematodes,
Rhizoctonia solani fungi and yellow nutsedge seeds," Yates
reports. "In soil covered with polyethylene, we got poor results
from the methyl bromide, but we got good pest control in soil covered with
high-barrier plastic."
To ensure sufficient levels of pest control from new
emission-reduction technology, Yates and colleagues plan to test their
ideas in typical regions, soils, and environmental conditions.
[April 1996 Table of Contents]
[Newsletter Issues
Listing]
[Methyl Bromide Home
Page]
[ARS Home Page]
[USDA Home Page]
Last Updated: October 15, 1996
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