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United States Department of Agriculture

Agricultural Research Service

Emission Reduction
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Many soil-chemical processes affect the fate and transport of soil fumigants. Containment, degradation and soil-gas concentration (i.e., effective dosage) must be controlled to reduce emissions while maintaining adequate pest control. Unless each of these factors is controlled, unacceptable emissions or inadequate pest control will likely occur. Two important factors that may affect fumigant transformation and distribution and its ultimate volatilization into the air are depth of placement and the use of soil surface cover. These two factors have been frequently used to alter a fumigant's distribution to reach adequate control in the specified target zones. Depending on the target pest, the fumigant can be applied at the soil surface under plastic film to as deep as 60-100 cm below the soil surface. Deeper placement consistently results in deeper penetration in the soil. Abdalla et al. (1974) found that MeBr application at 76-81 cm without a soil cover resulted in gas distribution at concentrations sufficient for nematode kill as deep as 244 cm. Kolbezen et al. (1974) detected adequate dosages at 300-360 cm when MeBr was applied at 90 cm. Though these early studies were mostly designed for achieving better nematode control in deep soil layers, they demonstrate that downward diffusion is encouraged by deep application. More recent research has been directed at investigating other methods for controlling emissions.

Containment is necessary to hold the gas at the treatment location and provide sufficient fumigant concentrations and time for pest control. Without adequate containment, a large fumigant quantities will be lost to the atmosphere. Containment is difficult because of high vapor pressures and low boiling points, which result in a large fractions of a fumigant existing in the vapor phase at normal temperatures and pressures. Movement in the vapor phase occurs more rapidly compared to the liquid phase due, in part, to a larger gas-phase diffusion coefficient compared to the liquid phase. Pesticides which have a large vapor pressure easily move through soil (Goring, 1962; Kolbezen et al. 1974; Reible, 1994). Factors that affect containment include: use of plastic, the properties of plastic, injection depth, soil bulk density, soil water content, soil cracking, and other mechanisms which promote or retard movement.
Plastic-Film Barriers
Plastic films are commonly used to improve containment and reduce the amount of fumigant leaving the treated soil. Covering soil with plastic film, or tarping, was introduced early in the development of soil fumigation with MeBr to enhance efficiency and reduce toxicity to workers and residents. Covering the field with plastic can reduce volatilization by inhibiting transport from the soil into the atmosphere. In early studies, it was shown that tarping increases fumigant retention in soil. Kolbezen et al. (1974) reported increased MeBr concentrations in surface soil under tarped conditions, but they later found that, overall, polyethylene film was a poor cover material compared to other plastics (Kolbezen and Abu-El-Haj, 1977). The ineffectiveness of polyethylene tarps in preventing MeBr volatilization was realized later by many other workers (de Heer et al., 1983; Rolston and Glauz, 1982; Jin and Jury, 1995). This ineffectiveness is not only caused by the high permeability of the polyethylene film to MeBr, but also by the short covering time normally used in agricultural fumigations.
Advantages of using films are that the properties and condition of the film are known in advance and films are more uniform in space and time compared to soil. There may be a higher certainty of effective containment when films are used compared to soil-water based methods. Also, the level of containment can be controlled by altering the plastic material used. New plastics are available which are very impermeable to fumigant diffusion (i.e., resists a fumigant passing through the film). This has led to the need for methods to estimate film permeability. The effect of various plastic materials and ambient temperatures on film permeability have been investigated. Low-permeability films have been developed which have permeability hundreds to thousands of times lower than that of 1-mil HDPE. For more information see Film Permeability.
Studies have shown that increasing the soil water content under soil tarps decreases cumulative emissions of soil fumigant compounds. Slower transport from the application depth to the soil surface in soils with high moisture increases soil degradation. In a soil column study where the soil surface was tarped with PE and the column was exposed to diurnal temperature variations, soil water accumulated at the soil surface; this accumulation resulted from condensed water on the tarp being redeposited on the soil surface and from diurnal heat variation resulting in upward flux of water vapor at night (Jury et al., 1996). Increased water content at the soil surface had a large impact on cumulative emissions in PE-tarped columns, so that the application of water under 1-mil HDPE was much more effective at reducing emissions than was surface sealing with 1-mil HDPE alone (Jin and Jury, 1995).
Using a simulation model, Reible (1994) showed that volatilization losses would be reduced from 53 to 33%, following a 25 cm injection, if the cover time was extended from 2 to 7 days. The total volatilization loss would depend heavily on the selected degradation rate. In packed columns, covering the soil surface with 0.025 mm high-density polyethylene film for 2-3 weeks generally reduced volatilization loss by 40% (Gan et al., 1997a). Less permeable plastics have also been tested (Kolbezen and Abu-El-Haj, 1977; de Heer et al., 1983; Gamliel et al., 1997; Chakrabarti et al., 1995; Daponte, 1995; Wang et al., 1997a; Wang et al., 1998; Papiernik et al., 2001), and their usefulness in reducing volatilization has attracted attention over the last few years.
Wang et al. (1997a) demonstrated a large reduction in MeBr volatilization using reduced application rates and high-barrier plastic films. In Hytibar-tarped plots, less than 4% of the applied MeBr volatilized when the tarp remained on the soil surface for at least 10 days. In polyethylene-tarped plots, MeBr emissions were less than 50%(Wang et al., 1997a).
Injection Depth Total Emissions (Measured)(%) Total Degradation(%) Mass Balance(%) Total Emissions Corrected Using Diffusion Model† (%)
Tarped Soil Columns
20 cm 59 36 94 43
30 cm 52 39 91 37
60 cm 45 46 91 26
Non-Tarped Soil Columns
20 cm 91 12 102 82
30 cm 83 15 98 71
60 cm 60 36 96 38

This Table provides a summary of the total MeBr emission in percent of applied MeBr for both tarped and untarped treatments following injection into soil columns (Gan et al., 1997a). These results indicate the importance of injection depth and use of a plastic tarp in reducing MeBr volatilization.
When the soil surface remained bare, MeBr volatilization was extremely rapid (Majewski et al., 1995), with as much as 80-90% of the total loss occurring during the first 24 h. When a tarp was present on the soil surface, the maximum volatilization flux was significantly smaller, with only 30 - 45% of the overall loss occurring during the first 24 h.
Surface Barrier Laboratory Columns Field Plot Experiments Field Experiments
Cover Period (days) Total Emission (%) Cover Period (days) Total Emission (%) Cover Period (days) Total Emission (%)
Bare na 44-90 na 87 na 89
HDPE 8 37-83 15 67 4,5,8 32-67
Hytibar 8 2 15 <5 na na

This Table shows recent estimates of total MeBr emissions for soils left bare and covered with plastic film after application. It is clear that the use of HDPE has a beneficial effect on reducing MeBr emissions, and using virtually impermeable films (Hytibar) appears to hold great promise for reducing emissions to near-zero levels.
Application Depth
The depth of application is an important factor affecting the amount of fumigant escaping into the atmosphere. In laboratory soil columns, when the application depth was increased from 20 to 60 cm, the MeBr emission rates decreased by 54% under untarped conditions, and 40% under tarped conditions (Table 1 above). Combining deep injection with the use of a surface tarp has the potential to significantly reduce MeBr emissions (Table 1 above). This supports the results from a recent field experiment (Yates et al., 1997), where 21% of the applied mass was volatilized when MeBr was injected deep in the soil (68 cm) and the soil surface left uncovered. This is 66% less than the total MeBr emissions rate (62%) observed in an adjacent field for which shallow MeBr injection and a high-density polyethylene tarp was used (Yates et al., 1996a,b,c). The lower emissions rate may be attributable to the deeper injection depth and a cooler average air temperature.
These results are also in agreement with the predictions made by Reible (1994) using a vapor transport model. Under hypothetical conditions, it was estimated that increasing the injection depth from 15 to 45 cm would decrease the MeBr emission rates from 53% to 28% when the soil was tarped. From these findings, it can be concluded that placing MeBr at a greater depth is another effective approach for minimizing its emission into the air during soil fumigation.
Soil Water Content
Increasing soil water content has been considered as a means for controlling fumnigant movement (Goring , 1962; Reible, 1994; Jin and Jury, 1995). The effect of water content on volatilization can be explained by the interactions of soil water content and the retardation factor, R d= (q + K dr b)/K h+ e, and tortuosity factor, (e.g., t = e 10/3/h 2) in gas-phase transport, where h, q, r b, K d, K h, respectively, are the porosity, water content, bulk density, liquid-solid and liquid-gas partition coefficients and the air content is e = h-q.
When the water content in laboratory columns containing Greenfield sandy loam was increased from 0.058 to 0.180 cm 3cm -3(i.e., v/v), R dincreased from 1.21 to 1.58, t decreased from 0.241 to 0.076 and the effective soil diffusion coefficient was reduced by 76%. For volumetric water contents of 0.058 and 0.124 (v/v), the estimated emission loss after correcting for the presence of the column bottom was approximately 77% of the applied MeBr (Gan et al., 1996; Table 9). This indicates that at moderately dry soil conditions, water content does not strongly effect emissions. When the water content was increased to 0.180 (v/v), only 62% of the applied MeBr was lost. As the soil water content increased, the maximum MeBr flux density decreased and the time interval before reaching the maximum flux density increased. Measurements of the MeBr gas concentration in the soil also indicated rapid movement through the soil column for the drier soils. MeBr in these soil columns was completely depleted 54-72 h after the application. For the wetter soil, measurable concentrations remained in the column until 144 h after the application.
In a field experiment (Yates et al., 1997), lower MeBr emissions were observed for bare soil, deep application than for a tarped, shallow application in the same field. Part of this difference may have been attributed to the water content of the soil profile. During the deep-injection study, the average soil water content around the injection point (68 cm below the surface) was 0.223 (v/v), whereas that observed during the shallow-injection study was 0.145 (v/v). Further, light rain occurred during the early part of the experiment which helped to seal the soil pores. Although the measured water content data were not given, Yagi et al. (1995) also attributed the decrease in MeBr emission from 87 in their first study to 34 % in their second study, in part, to soil moisture differences. Similar results were observed in the laboratory by Jin and Jury (1995).
Soil Bulk Density
Soil bulk density can also have an effect on MeBr transport since the pore space decreases as bulk density increases. The bulk density, r b, is related to the porosity, h, from the relationship: h = (1 - r b/r p), where r pis the particle density.
In laboratory columns packed with Greenfield sandy loam (Gan et al., 1996), the corrected cumulative volatilization loss for a column with a bulk density of 1.70 g/cm 3was 53%, significantly lower than the 77% loss from a column with a bulk density of 1.40 g/cm 3. The columns with higher bulk density behaved in a manner similar to the wetter soil column described above. Measurable volatilization continued for 120 h, the maximum flux density was reduced ~60% compared to the low bulk density column, and the time to reach the maximum flux increased to 6.5 h after application (compared to 2.5 h in the low bulk density column).
In the untarped, deep-injection field study (Yates et al., 1997) the field was disked and packed shortly (approximately 5 min) after MeBr was injected into the field. The disking and surface packing closed the openings above the injection fractures and increased the bulk density near the surface. This, along with a higher water content, probably contributed to the reduced total emission compared to the shallow-tarped experiment. Packing the soil surface and carefully closing the soil fractures created during application should be considered for minimizing MeBr volatilization.

Soil Degradation
Hydrolysis and methylation are the principal degradation processes removing MeBr from agricultural soils. Degradation affects fumigant volatilization since it removes the chemical from the soil making it unavailable for transport to the atmosphere. Total degradation is affected by both intrinsic soil degradation and the type and performance of any agricultural barriers or systems used for containment, since this increases soil residence time.
Predicted Emissions
Predicted total MeBr emissions as a function of the degradation half life are shown in the Figure when MeBr is injected at 25 cm depth and the soil surface is covered with either a HDPE or Hytibar film for 5 or 30 days. For a wide range in soil degradation half lives, a significant reduction in emission occurs when using a virtually impermeable film (e.g., Hytibar) together with long cover periods. Such information can aid in reducing emissions by providing a prediction of expected emissions to allow for comparison between various MeBr application methods. This information can also be used to allow specification of target total emissions, and provides information needed to achieve this goal. For example, the dotted line in the Figure gives the soil degradation half life that would be necessary to achieve 20% emissions using Hytibar film and a 30 day cover period. A soil with a shorter half life would not exceed the 20% emissions threshold. It is clearly shown in the Figure that a very short degradation half life would be required to reduce MeBr emissions to 20% for a 25 cm injection with a 5 day HDPE cover period.
MeBr emissions degradation
This Figure shows the total MeBr emissions MeBr after injection at 25 cm and covering the soil surface with HDPE or Hytibar film. Dotted line shows the half life needed to achieve 20% total emissions for MeBr application using Hytibar and a 30 day cover period. (Simulation parameters: Ds = 1450 cm2 d-1, h = 18 and 0.09 cm d-1 for HDPE and Hytibar, respectively)
Soil Organic Matter
The effect of soil organic matter on MeBr volatilization has been investigated in laboratory soil columns using three soil types. One soil (Greenfield sandy loam) had relatively low organic matter (0.92%) and clay contents (9.5%) and is representative of many soil types in the state of California. A second soil (Carsetas loamy sand) had a very high sand content and very low organic matter (0.22%) and clay contents (0.1%). A third soil (Linne clay loam) was relatively rich in organic matter (2.99%) and clay (25.1%). Soil type had a pronounced effect on MeBr volatilization behavior as shown in Table 9.
Soil Type Total Measured Emissions
Total Degradation
Mass Balance
Total Corrected Emissions
(Using Diffusion Model) (%)
Greenfield SL 90 12 102 77
Carsetas LS 90 9 99 77
Linne CL 44 49 94 37
Volatilization of MeBr from untarped Carsetas and Greenfield soil columns following 30-cm injection was very rapid; cumulative emissions were 77% of the applied MeBr for both soil columns. However, under the same conditions with the Linne clay loam, only 37% of the applied MeBr was lost. Analysis of Br¯ concentration in soil at the end of the experiment revealed that 49% of the applied MeBr was degraded to Br¯ in the Linne soil, while the degradation in Carsetas and Greenfield soils accounted for approximately 10% of the applied MeBr mass (Table 9). The enhanced degradation of MeBr in Linne clay loam is due to its higher organic matter content as indicated by earlier work (Brown and Rolston, 1980; Arvieu, 1983; Gan et al., 1994).
Using a gas-phase diffusion model, Reible (1994) predicted that when the soil organic carbon content was increased from 2 to 4%, the MeBr emission rate decreased from 45 to 37% following a tarped (2 days), 25-cm application under the assumed conditions. However, in his simulation, only the effect of soil organic matter on adsorption behavior was considered. From the column experiments, it is clear that enhanced degradation due to higher organic matter content may play an important role in reducing MeBr volatilization in organic-matter-rich soils.
Enhancing Soil Degradation
Increasing the rate of MeBr transformation in soil by addition of organic amendments or nucleophilic compounds has also been demonstrated to reduce MeBr emissions, as demonstrated using laboratory soil columns (Gan et al., 1998b,d). Adding organic amendments, such as composted manure, to soil results in more rapid transformation of MeBr due to an increase in both abiotic and biological transformation rates (Gan et al., 1998b). Incorporation of organic amendments into the surface soil resulted in reduced MeBr emissions (Gan et al., 1998b). With no amendment, cumulative MeBr volatilization losses were 68%. Incorporating composted manure (5%) into the top 5 cm of soil resulted in cumulative MeBr emissions of 56% (Table). The proportion of organic amendment at the soil surface further decreased the volatilization loss, and mixing 20% composted manure into the top 10 cm of soil resulted in cumulative MeBr emissions of 40%.
Table: Cumulative fumigant emissions (% of applied) observed in unamended soil columns and columns that included composted manure (5%) in the surface 5 cm of soil (amended)
Compound Unamended Amended
methyl bromide 68.2 56
1,3-dichloropropene 25-34 14-18
methyl isothiocyanate 21.3 0.3
Thiosulfate-containing Agrochemicals
Addition of nucleophilic compounds such as fertilizer ammonium thiosulfate (ATS) greatly enhances the rate of MeBr degradation in soil (Gan et al., 1998d). Addition of such compounds at the soil surface can provide an effective barrier to volatilization while maintaining adequate MeBr concentrations in the root zone to provide efficacy against soil-borne pests. Total MeBr emissions were reduced from 61% (unamended column) to less than 10% after adding ATS to the soil surface at a 3:1 molar ratio (see Figure). A field study showed that adding ATS (at 640 kg/ha) to the soil surface had no discernible effect on the efficacy of MeBr for controlling nematodes and weeds (Gan et al., 1998d). Emissions of MeBr alternatives were also reduced with application of ATS and organic amendments (Gan et al., 2000a). Wang et al. (2000) found that the byproducts of ATS degradation of fumigant compounds are not toxic. Since ATS is an inexpensive material used as a sulfur fertilizer, this approach has promise for field application.
MeBr emissions reduction after ATS
This Figure shows that the total MeBr emissions are reduced after application of ammonium thiosulfate (ATS) to the soil surface. Thiosulfate is effective in degrading the halogenated soil fumigants (i.e., methyl bromide, methyl iodide, 1,3-dichloropropene, chloropicrin, propargyl bromide). Thiosulfate is not effective in degrading methyl isothiocyanante.

Last Modified: 11/4/2009