Methyl Bromide Alternatives – Meeting the Deadlines Reducing Fumigant Emissions After Soil Application

Yates, S. R. , Gan, J., Papiernik, S. K. , Dungan, R. , and Wang, D. 2002. Reducing fumigant emissions after soil application. Phytopathology 92:1344-1348. Volatilization and soil transformation are major pathways by which pesticides dissipate from treated agricultural soil. Volatilization is a primary source of unwanted agricultural chemicals in the atmosphere and can significantly affect fumigant efficacy. Volatile pesticides may cause other unique problems; for example, the soil fumigant methyl bromide has been shown to damage stratospheric ozone and will soon be phased out. There is also great concern about the health consequences of inhalation of fumigants by people living in proximity to treated fields. Because replacement fumigants will likely face increased scrutiny in years ahead, there is a great need to understand the mechanisms that control their emission into the atmosphere so these losses can be minimized without loss of efficacy. Recent research has shown that combinations of vapor barriers and soil amendments can be effective in reducing emissions. In this paper, some potential approaches for reducing fumigant emissions to the atmosphere are described. Soil fumigants have been used throughout the world for decades to control soilborne pests prior to planting various food crops. Soil fumigants have been implicated in causing various environmental problems. Methyl bromide (MeBr), one of the widely used soil fumigants, depletes stratospheric ozone and restrictions have been placed on the future production and use of this chemical. Ethylene dibromide and dibromochloropropene (DBCP) contaminate ground water systems, and DBCP remains a problem decades after its use in soil fumigation ceased (24). Estimates of losses as a result of the MeBr withdrawal vary, but the USDA National Agricultural Pesticide Impact Assessment Program (1) determined that it could be in excess of $1.5 billion in annual lost production in the United States (1,6) if suitable MeBr replacements are not found. This has led to an intense search for nonchemical alternatives and chemical replacements for MeBr. The most promising chemical alternatives identified thus far include 1,3-dichloropropene (1,3-D), chloropicrin, and methyl isothiocyanate (MITC). However, these alternatives are less effective than MeBr in controlling plant pathogens (26). Therefore, it is likely that combinations of two or more chemicals in large quantities will have to be used to achieve similar pest control efficacy, and this in turn may lead to significant emissions of these chemicals. Recent field experiments have demonstrated that emissions from soil fumigation are significant and can vary from 20 to 90% of the total applied fumigant (2,3,7,21,25,31–35,38–41,43). These studies have shown that many chemical, soil, and environmental factors affect emission losses. To achieve sufficient emission reduction, these factors must be controlled or mitigated during and after soil fumigation. A thorough understanding of the fate and transport of soil fumigants is necessary to develop efficient emission control measures. Generally, three factors must be balanced to reduce emissions while maintaining pest-control efficacy: containment, degradation, and soil-gas distribution (i.e., effective dosage). This paper describes how containment and enhancing degradation can be used to reduce fumigant emissions to the atmosphere. Containment. Containment is the retention of fumigant in the soil environment long enough for efficient control of pests. Containment is needed due to the high vapor pressure of all agricultural fumigants. Because of the high vapor pressure, a large fraction of the fumigant exists in the vapor phase at temperatures and pressures that normally occur in the field. Without adequate containment, a significant fraction of the fumigant will quickly be lost to the atmosphere. When emission losses are high, the applicator will need to compensate by using larger quantities of fumigant compared with low emission-loss fumigation. Plastic films. One of the most common methods of containment is the use of plastic film to cover the soil surface after fumigation. A variety of physical-chemical properties of the film and environmental factors affect the permeability of plastic film. Some films provide a better diffusion barrier to certain fumigants than others. For example, the commonly used high-density polyethylene (HDPE) provides some resistance to MeBr and chloropicrin, but offers little resistance to 1,3-D diffusion. The permeability of two types of film to several fumigants is shown in Figure 1 in which larger mass transfer coefficient values indicate less resistance to diffusion (29) and, in general, higher emission rates. The mass transfer coefficient, h, shown in Figure 1 is an intrinsic property of the plastic film and is a measure of the permeability (29). The emission rate through film, however, depends on h and the difference in fumigant concentration across the film. The soil emission rate depends on factors affecting fumigant diffusion to the soil surface, the emission rate through the film, and properties of the atmosphere that enhance or inhibit transport away from the soil surface in the air space above and below the film. Impermeable films trap fumigation gases at the soil surface, promoting Corresponding author: S. R. Yates; E-mail address: syates@ussl.ars. usda.gov. Publication no. P-2002-1021-03O This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological