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United States Department of Agriculture

Agricultural Research Service

Emission Reduction
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1 - Introduction
2 - Containment
3 - Soil Degradation
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.
 
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Last Modified: 11/4/2009
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