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a USDA-ARS, Water Management Research Unit, San Joaquin Valley Agricultural Sciences Center, Parlier, CA 93648
b USDA-ARS, Water Management Research Unit, Fort Collins, CO 80526-8119
* Corresponding author (sgao{at}fresno.ars.usda.gov)
Received for publication August 31, 2005.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Abbreviations: 1,3-D, 1,3-dichloropropene • HDPE, high density polyethylene • MeBr, methyl bromide • MITC, methyl isothiocyanate • VIF, virtually impermeable film
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Emissions from soil fumigation are affected by soil texture and water content, weather, and surface barriers as well as fumigant properties. Methods to reduce fumigant emissions include application methods such as chemigation (through drip irrigation), chemical remediation (e.g., use of thiosulfate to degrade fumigants), and surface barriers (plastic tarps and water seals) (Yates et al., 2002). Chemigation was shown to reduce fumigant emissions compared to shank injections in some cases (Gan et al., 1998b) but not in others (Sullivan et al., 2004). Chemical remediation at the surface (e.g., using thiosulfate) can reduce emissions substantially by reacting with and destroying the fumigants at the surface (Gan et al., 1998a). This method has not yet been tested in the field or evaluated economically. Surface plastic tarp (mulch) reduces fumigant emissions, depending on the chemical, and is generally expensive (about $2000 per ha for purchase, placement, removal, and disposal of high density polyethylene [HDPE] tarps over shank applications). High density polyethylene film is a poor barrier for 1,3-D, but is more effective with MeBr and chloropicrin (Wang et al., 1999; Papiernik and Yates, 2002). Plastic tarp was found to improve pest treatment of surface soil—especially weed control (Shem-Tov et al., 2005). Use of virtually impermeable film (VIF) is more effective in reducing 1,3-D as well as other fumigant emissions than standard HDPE tarp (e.g., Noling, 2002; Thomas et al., 2004, 2006; Wang et al., 1999), but this method is even higher cost and practical application methods that preserve the high barrier characteristics are still being developed. High surface soil water content was found to create a more effective barrier to 1,3-D movement than HDPE (Gan et al., 1998b; Thomas et al., 2003).
Surface water seal (applying water uniformly to soil surface) is a technique that has shown potential to reduce fumigant emissions by forming a high water content layer at the surface that serves as a diffusion barrier. Fumigant diffusion rate in the liquid phase is much lower than through the gas phase. High soil water content and slight compaction at the soil surface reduces emissions also by reducing air-filled pore space. Water application to surface soil in column studies and small plots showed substantial reduction of MeBr emissions especially when combined with use of plastic tarp (Jin and Jury, 1995; Wang et al., 1997). More recent field trials studied use of surface water sprinkler irrigation (water seal) to reduce MITC emissions and found that intermittent water sealing was particularly effective to minimize off-gassing during night time periods when atmospheric dispersion conditions were relatively poor (Sullivan et al., 2004). This technique will usually cost less than using plastic tarps in areas where irrigation water is available, especially on farms where sprinkler systems are used.
The objective of this study was to determine the potential of using surface water applications to reduce 1,3-D emissions in soil columns. Although soil columns cannot fully represent field conditions, they allow us to efficiently test several treatments. Our purpose was to identify the effective treatments to reduce fumigant emissions for further tests in field trials.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Soil Column Experiment
Soil was packed into close-bottomed stainless steel columns (63.5-cm height x 15.5-cm i.d.). Soil was packed to a total depth of 61.5 cm leaving the top 2 cm empty in the column and allowing surface water application when needed. The columns were packed in 5-cm increments to a uniform bulk density of 1.4 g cm–3. Sampling ports for soil gases were installed at depths of 0 (under plastic tarp when applied), 10, 20, 30, 40, 50, and 60 cm below the soil surface. A Teflon-faced silicone rubber septum (3 mm thick; Supelco, Bellefonte, PA) was installed in each sampling port. The septum was replaced with a new one after each sampling. A Teflon tube attached to the inside of each sampling port extended to the center of the column.
For emission measurement, a flow-through gas sampling chamber (4.5 cm deep with the same diameter as the soil column) was placed on the top of the soil column and sealed to the column with a sealant-coated aluminum tape to avoid any gas leakage. After the whole column was assembled and treatment was applied, a continuous flow rate of 110 ± 10 mL min–1 through the chamber was maintained by vacuum. The chamber inlet port was sized such that pressure inside the chamber should be no more than 0.6% below atmospheric pressure. A flow meter was used to adjust the air-flow rate after sampling tubes were replaced and occasionally between sampling times. The flow rate usually stabilized within 5 min to the set range. The column experiments were conducted at laboratory room temperature (22 ± 3°C). Monitoring and sampling were done for 2 wk.
One-hundred microliters of liquid cis-1,3-D (120 mg) was injected into the column center at the 30-cm depth through an injection port connected to a Teflon tube extending to the center of the column. Similar behavior of degradation, emission, and plastic tarp permeability (e.g., HDPE) was observed between the isomers, that is, cis- and trans-1,3-D (e.g., Gan et al., 1999; Wang et al., 1999; Papiernik and Yates, 2002). Cis-1,3-dichloropropene was chosen in this study to predict what will happen when the commercial 1,3-D fumigant Telone II (mixture of the isomers) is applied to soil. Some studies showed that cis-1,3-D diffused slightly faster than trans-1,3-D through HDPE film or soil (e.g., Noling, 2002; Thomas et al., 2003).
To evaluate the potential of using surface water applications to reduce 1,3-D emissions, the following treatments were applied to soil columns:
For surface water application, tap water was quickly applied (within 5 min) to the soil surface using a spray bottle. Eight millimeters of water usually disappeared from the surface within 20 min reflecting a fast infiltration rate. For water applications at 12 and 24 h, the top chamber was removed to spray water onto the soil surface. This resulted in loss of the fumigant from the headspace. The total volume of air in the headspace above the soil surface was 1.22 L. The air-flow rate created one headspace air exchange every 11 min or about three exchanges each 30-min sampling interval. The effect of the chamber removal on the 1,3-D sampling and estimate of emissions was adjusted, that is, measured emission rate for the first sample after water application was increased by 60% to correct for fumigant loss due to removal of the chamber. After the first ORBO tube was replaced, measured emissions should not have been affected by the removal of the chamber. When used, plastic tarps were sealed to the top edge of the stainless steel columns using silicone sealant to minimize leakage.
Treatments A through D were repeated. Because the same trends and similar results were obtained for all the repeated treatments (duplicate results are reported in Table 1), treatments with plastic tarps were not repeated.
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The ORBO tubes were extracted immediately or frozen for up to 14 d for later extraction. After breaking each tube, all materials were transferred into a 10-mL clear headspace vial. Five milliliters of hexane were added to the vial and the vial was then shaken for 2 h. After settling, a portion of the solvent in the vial was transferred to a 2-mL amber gas chromatography (GC) vial. The GC vials were stored in a freezer before analysis. Fumigants in the extracts were analyzed by a GC–mass spectrometry (MS) system (6890 Network GC system, 7683 AutoInjector, and 5973 Inert Mass Selective Detector; Agilent Technologies, Palo Alto, CA). A DB-VRX capillary column (30-m length x 0.25-mm i.d. x 1.4-µm film thickness; Agilent Technologies) was used. The injected volume was 2 µL splitless. The carrier gas (He) flow rate and inlet temperature were set at 1.3 mL min–1 and 140°C, respectively. The mass selective detector temperatures were set as follows: transfer line 260°C, source 230°C, and quad 150°C. The oven temperature program was as follows: initially 45°C, increasing at 2.5°C min–1 to 75°C, and then at 99°C min–1 to 110°C and held for 7 min. The retention time for cis-1,3-D was 8.6 min.
For determining fumigant concentration in soil-gas phase, 0.5-mL volume of soil gas was withdrawn from the sampling ports with a gas-tight syringe at times of 3, 6, 12, 24, 36, and 48 h, and 3, 4, 7, 9, 11, and 14 d after fumigant injection. The gas sample was injected into a 21-mL clear headspace vial and the vials were crimp-sealed immediately with an aluminum caps and Teflon-faced butyl-rubber septum (Supelco). This method was quantitative and reproducible (Gan et al., 1997). To avoid moisture effect on the fumigant stability, 0.2 g sodium sulfate was added to each vial before sample injections. If analysis could not be performed immediately, the vials were stored in an ultra freezer at a temperature of –44°C. All samples were analyzed within 72 h, a stable period of time for fumigants under laboratory conditions (Guo et al., 2004b). The analysis was performed using a GC–µECD (Agilent Technologies 6890N Network GC system with a micro electron capture detector) and an automated headspace sampler (Agilent Technologies G1888 Network Headspace Sampler) system. A DB-VRX capillary column was used with the same dimension as the fumigant analysis mentioned above. Conditions for the headspace autosampler were: equilibration temperature, 100°C; equilibration time, 2 min; and sample loop, 1 mL. The GC carrier gas (He) flow rate, inlet temperature, and detector temperature were set at 2.0 mL min–1, 150°C, and 300°C, respectively. The oven temperature program was the same as the GC–MS system described above.
Upon termination of the experiment, soil samples from each column were taken at 10-cm depth intervals, and soil water content and residual 1,3-D in the soil were determined. The extraction procedure for soil samples basically followed Guo et al. (2003). An equivalent dry weight of 8 g of soil was added to a 21-mL clear vial that contained Na2SO4 (amount depended on soil water content at a 7:1 ratio of Na2SO4 to water). Eight milliliters of ethyl acetate were added to the vial. The vial was crimp-sealed with aluminum caps and a Teflon-faced butyl-rubber septum. After the vials were incubated at 80°C in a water bath over night (approximately 18 h), the supernatant was settled and a portion was transferred into a 2-mL amber GC vial for fumigant analysis using the GC–MS as described above. The vials were preserved in the freezer when analysis could not be completed immediately.
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RESULTS AND DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Initial water application plus HDPE tarp provided additional emission reduction over the first 48 h. Similar results were obtained for MeBr when water application plus HDPE tarp reduced MeBr emissions much more efficiently than HDPE tarp alone in soil columns and small plots (Jin and Jury, 1995; Wang et al., 1997). This treatment might be even more effective on reducing emissions in the field where diurnal temperature fluctuations cause periodic water evaporation and condensation under the tarp, which helps maintain the surface soil moisture as an effective barrier to fumigants.
The maximum reduction of 1,3-D emissions from our treatments was with VIF tarp, which showed very low emission rates (<1.5 µg m–2 s–1). Although VIF had extremely low emissions under laboratory conditions, damage to the film during field application and difficulties in joining (gluing) VIF sheets together may result in significantly greater emissions under field conditions than we observed in the column studies.
All the treatments except VIF had similar emissions beyond 72 h (data not shown). Most treatments showed very low emission rates (0.0–0.5 µg m–2 s–1) at the end of the experiments (2 wk).
Total Emissions
The cumulative emissions from the column treatments during the 2-wk experimental period are shown in Fig. 2. The closed bottom of the soil columns would limit downward movement of fumigant, which may result in more upward movement compared to longer columns. The relative difference among soil surface treatments should not be affected significantly. For the total 2 wk, the emission percentages of total applied were 51% for the control, 46% for the initial water application, 45% for the initial water plus a second water application (2.6 mm) at 12 h, 41% for the initial water plus water applications at 12 h and 24 h, 45% for HDPE tarp only, 38% for the initial water application plus HDPE tarp, and 10% for VIF tarp. The VIF was clearly the most effective barrier to reduce 1,3-D emissions among all the treatments. The three water applications were more effective in reducing total emissions than those with fewer water applications and the HDPE tarp with the initial water application was more effective than the HDPE tarp alone, which indicates the important role of soil water in 1,3-D emission reduction.
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These results indicate that surface water applications alone and in combination with HDPE tarp are effective in reducing early emissions, and thereby reducing risks to workers and bystanders during fumigation. Water application just before fumigant injection also showed a lag period of time before emission rates significantly increased compared to the control. Reducing peak flux is important for fumigants for which acute toxicity or odor (e.g., chloropicrin) is the primary concern. Reducing overall emissions is also desired for fumigants, such as 1,3-D, when total exposure (chronic toxicity) is also a concern, or for all fumigants where impact of air-borne organic compounds contribute to air quality (ozone) problems.
Results from our column tests show that the intermittent water applications reduced 1,3-D emissions equally or more effectively than HDPE tarp (Fig. 1 and 2; Table 1). This is not surprising considering the known high permeability of HDPE to 1,3-D. The diffusion rate of 1,3-D dramatically reduced as soil water content increased and as a result, emission of 1,3-D was also reduced and was minimal from the near-water-saturated soil (Thomas et al., 2003).
The cost for using standard HDPE tarp over shank applications is about $2000 ha–1 ($800 acre–1) in the San Joaquin Valley of California. This estimate includes cost of the materials (tarp and glue: $1240 ha–1), application ($470 ha–1), and removal and disposal ($270–$440 ha–1). For surface water applications using a sprinkler system, cost estimate for a total 25 mm of water (about twice the amount of water used in the column tests) application is in the range of $100 to 700 ha–1 ($40–280 acre–1), depending on whether the grower owns or rents the sprinkler system. This estimate is based on an estimated water cost of $0.04 m–3 ($10 per ha for a 25-mm application), 5 worker-h per ha for sprinkler installation and removal (<$80 per ha), and sprinkler system rental/delivery cost of $600 per ha (depending greatly on the number of times the system is moved and reused). The overall cost of using water is substantially less than using plastic tarp. Thus, using water seal to reduce emissions has a great potential where irrigation water is available. Irrigation system capacity may limit the area that can be fumigated each day.
Two practices may reduce emissions beyond those achieved in these tests. Emission reductions depend on high soil water content in the surface soil. Therefore, very frequent small irrigations may further reduce both peak and total fumigant emission. Since the sprinkler system must be in the field for the treatment, applying small, frequent irrigations is possible. The second technique is to promote the degradation rate of the fumigant in soil surface, such as with the use of thiosulfate or thiourea (Wang et al., 2000; Zheng et al., 2004). A concern with both practices is if fumigant concentration is reduced significantly near the soil surface that resulted in the reduction in dosage exposure (concentration x time) and may reduce fumigation efficacy near the surface. Reduced emissions and the decreased diffusion rate of fumigants in soils with high water content were observed by Thomas et al. (2003). Further investigations are necessary on how to maintain fumigation efficacy while reducing emissions using water or chemicals that are delivered with water.
The results of surface water application on reducing 1,3-D emissions observed in this study may not apply to other fumigants with different chemical properties. Other fumigants may exhibit different emission patterns, such as metam sodium as a generator of MITC (Merricks, 2002). To maximize emission reduction, surface water applications may be different from what we observed for 1,3-D. High density polyethylene tarp provides more resistance to MeBr and chloropicrin than to 1,3-D. Thus, the relative effect of water application or plastic tarps on reducing fumigant emissions will be different for various chemicals. For instance, an 8-mm initial water application with HDPE tarp in our column experiment reduced emissions an additional 8% of applied from that achieved with tarp alone. Jin and Jury (1995) showed that application of 4 mm of water plus 1-mil HDPE tarp reduced MeBr emission an additional 25% of applied. The combination of water application with HDPE tarp on emission reduction bears further investigation for different fumigants.
1,3-Dichloropropene Concentrations in Soil Gas Phase
The distribution of 1,3-D in the soil-gas phase over time is shown in Fig. 3. Note that for the tarped treatments, fumigant concentration at the soil surface (depth = 0 cm) was sampled under the tarp. The main difference observed in 1,3-D concentrations in soil gas was between the VIF tarp treatment and others. The VIF tarp was able to retain much higher 1,3-D concentrations in the soil-gas phase than other treatments, especially at later times (after 24 h). At the first sample time (3 h), all treatments had essentially the same fumigant concentration distribution. The fumigant concentration reached the highest level at the bottom sampling port at or before 24 h for all treatments. After this time, fumigant was dispersed throughout the lower column fairly evenly due to the closed bottom. The concentration of 1,3-D in soil-gas phase decreased to below 0.1 mg L–1 by the end of the experiment in all of the columns except the VIF treatment where about 0.4 mg L–1 1,3-D was detected throughout the profile. The VIF tarp reduced emissions the most and maintained elevated fumigant concentrations in soil profile for the longest time. This may imply that substantially reducing emissions may lead to a relatively longer fumigant residence time in soil profile. The average 1,3-D concentrations in the soil-gas phase throughout the columns at 72 h were 0.8, 0.9, 1.0, 1.1, 1.0, 1.1, and 1.5 mg L–1 (standard deviations ranging from 0.1 to 0.3) for Treatments A–G, respectively. Fumigant concentrations at the 10-cm depth for the first 24 h were similar for water seal and tarped treatments, implying that effects on pest control in that near-surface layer should be similar.
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Water application to the soil and its downward movement would have changed the volume of soil gas in the soil column as a function of time. This change was not monitored during the experimental period. By assuming water movement was very low after 3 d, 1,3-D in the soil-gas phase was estimated by integrating the product of air volume (using the phase distribution measured at the end of the experiment) and the fumigant concentrations in soil columns (Table 2). The control had the least total amount of 1,3-D at 72 and 168 h, but the differences from other treatments were small except for the VIF tarp, which retained the highest amount of 1,3-D throughout the experimental period in the soil column.
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The values in Table 3 show that the primary fate of 1,3-D was either emission to the atmosphere or degradation in soil. By reducing emission rates, longer fumigant residence time in soil may be achieved and degradation of fumigants in soil may be promoted. This balance is important to establish the efficacious fumigant application rate although information is not yet adequate to quantify this balance. It is also unknown if increasing fumigant residence time in soil may increase the risk of fumigant leaching and ground water contamination.
The degradation values of 1,3-D in this study generally agree with reported data. The reported half-life of 1,3-D in soil can range from 4 to 43 d and averaged 11 d (Ajwa et al., 2003). Dungan et al. (2001) studied the effect of temperature, moisture, and organic amendment on 1,3-D degradation in a sandy loam soil and reported that the half-lives of both cis- and trans-1,3-D in unamended soil were 6.3 d at 20°C. The half-life decreased to about 2 d in soils amended with 5% composted steer manure. Gan et al. (1998b) found that the half-life of 1,3-D was between 5 and 7 d for two California soils at 20°C. The half-life of 1,3-D varies as soil temperature and water content change as these factors affect microbial activity and chemical reactions, the major degradation mechanisms. The role of these mechanisms varies depending on soil conditions. Microbial degradation of 1,3-D was found substantially depressed when the temperature was >30°C (Gan et al., 1998b). Hydrolysis of 1,3-D in deionized water is relatively rapid, with a half-life of 9.8 d at 20°C and the process is pH dependent (Guo et al., 2004a).
In our study, the VIF tarp treatment showed the highest amount of 1,3-D degraded although no water was added to the soil and the highest 1,3-D concentrations were retained in the soil-gas phase. The high degraded portion raises an interesting question on how 1,3-D or other fumigant degradation may respond to surface sealing and tarp treatments. Answers to this question can assist in prediction of fumigation efficacy.
Caution should be taken in applying the results from soil columns to the field conditions. The effectiveness of emission reducing treatments may vary under field conditions because water application, the soil surface, atmospheric conditions, and tarp condition will be less uniform and controlled than in the laboratory. Diurnal temperature changes and variable wind can affect emissions. Although the results cannot be directly applied to field conditions, the relative differences among treatments provide information on the potential of treatments that could be developed into effective practices in the field.
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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ACKNOWLEDGMENTS |
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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