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TECHNICAL REPORTS

Organic Compounds in the Environment

Evaluation of Fumigation and Surface Seal Methods on Fumigant Emissions in an Orchard Replant Field

^a Water Management Research Unit, San Joaquin Valley Agricultural Sciences Center, USDA-ARS, Parlier, CA 93648
^b Water Management Research Unit, USDA-ARS, Fort Collins, CO, 80526-8119
^c National Program Staff, USDA-ARS, Beltsville, MD 20705-5139

^* Corresponding author (sgao{at}fresno.ars.usda.gov).

Received for publication February 15, 2007.

	ABSTRACT

TOP NOTES ABSTRACT INTRODUCTION Materials and Methods Results And Discussions Conclusions REFERENCES

 
Soil fumigation is an important management practice for controlling soil pests and enabling successful replanting of orchards. Reducing emissions is required to minimize the possible worker and bystander risk and the contribution of fumigants to the atmosphere as volatile organic compounds that lead to the formation of ground-level ozone. A field trial was conducted in a peach orchard replant field to investigate the effects of fumigation method (shank-injection vs. subsurface drip-application treatments) and surface treatments (water applications and plastic tarps) on emissions of 1,3-dichloropropene (1,3-D) and chloropicrin (CP) from shank-injection of Telone C-35 and drip application of InLine. Treatments included control (no water or soil surface treatment); standard high-density polyethylene (HDPE) tarp, virtually impermeable film (VIF) tarp, and pre-irrigation, all over shank injection; and HDPE tarp over and irrigation with micro-sprinklers before and after the drip application. The highest 1,3-D and CP emission losses over a 2-wk monitoring period were from the control (36% 1,3-D and 30% CP) and HDPE tarp (43% 1,3-D and 17% CP) over shank injection. The pre-irrigation 4 d before fumigation and VIF tarp over shank injection had similar total emission losses (19% 1,3-D and 8–9% CP). The HDPE tarp and irrigations over subsurface drip-application treatments resulted in similar and the lowest emission losses (12–13% 1,3-D, and 2–3% CP). Lower fumigant concentrations in the soil-gas phase were observed with drip-application than in the shank-injection treatments; however, all treatments provided 100% kill to citrus nematodes in bags buried from 30 to 90 cm depth. Pre-irrigation and drip application seem to be effective to minimize emissions of 1,3-D and CP.

Abbreviations: MeBr, methyl bromide • CP, chloropicrin • FC, field capacity • HDPE, high-density polyethylene • MITC, methyl isothiocyanate • VIF, virtually impermeable film • 1,3-D, 1,3-dichloropropene

	INTRODUCTION

TOP NOTES ABSTRACT INTRODUCTION Materials and Methods Results And Discussions Conclusions REFERENCES

 
SOIL fumigation is used to control soil pests for several high-value crops. The phase-out of methyl bromide (MeBr) as a soil fumigant due to its contribution to the depletion of stratospheric ozone has increased the demand for alternative fumigants, such as 1,3-dichloropropene (1,3-D), chloropicrin (CP), and methyl isothiocyanate (MITC) generators (California Department of Pesticide Regulation, 2006; Trout, 2006). These alternatives, however, when emitted into the air, can be harmful to workers and bystanders, and their use is regulated to minimize these hazards. Fumigants are also volatile organic compounds that react with nitrogen oxides in sunlight to form ground-level ozone (Segawa, 2005). Ozone is a strong oxidant, so high concentrations near ground level can be harmful to people, animals, crops, and other materials. Regulations have been developed to reduce emissions and their impacts (e.g., use limits and buffer zones), and more stringent regulations on fumigant emissions are likely to be issued in the near future to protect air quality in California (Segawa, 2005; CDPR, 2006). Thus, fumigant emissions to some extent will determine the availability of fumigants for agricultural production in the future.

Methods to reduce fumigant emissions include the use of several types of plastic tarps, water seals (applying water to soil surface with sprinkler systems), amendments that degrade fumigants (e.g., thiosulfate or organic materials), and chemigation (fumigation through drip-irrigation). The most commonly adopted practice is to place standard high-density polyethylene (HDPE) tarp over the soil surface after fumigation. This technology was developed primarily for MeBr but was found not to be effective to control emissions of 1,3-D (Telone) (Wang et al., 1999; Papiernik and Yates, 2002). Low-permeable films, such as virtually impermeable film (VIF), have been tested on minimizing fumigant emissions. The VIF is a typically multilayer film composed of barrier polymers such as ethylene vinyl alcohol or polyamide (nylon) sandwiched between other polymer layers (typically low-density polyethylene) (Noling, 2002). The VIF is generally much less permeable than polyethylene films. The cost for using HDPE tarp is about $2200 ha^–1 in California, including materials (tarp and glue: $1300 ha^–1), application ($650 ha^–1), and removal and disposal ($250 ha^–1).

Water seals were found to be effective in reducing emissions of MITC (Sullivan et al., 2004). Field tests of emissions from shank injection of Telone C35 on a sandy loam soil showed that water seals can be more effective to reduce 1,3-D and CP emissions than HDPE tarp (Gao and Trout, 2007). An initial water seal immediately after fumigation can delay emission and reduce emission peaks, and intermittent water seals can maximize total emission reductions. Major benefits of water seals are substantially reduced costs for materials and no postfumigation material disposal. The cost for a 25-mm water application by sprinklers is in the range of $100 to 800 ha^–1, depending on whether grower owns or rents the sprinkler system (Gao and Trout, 2006).

Soil amendment with chemicals (e.g., ammonium thiosulfate, thiourea, or polysulfides) has been shown to degrade fumigants in soils (Wang et al., 2000; Zheng et al., 2003) and soil columns (Qin et al., 2007) and to reduce emissions from soil columns and small plot tests (Gan et al., 1998a; Zheng et al., 2006). Amendment of soil with organic materials such as manure has also shown effectiveness to degrade fumigants (Dungan et al., 2001; Xu et al., 2003) and has been found to reduce emissions of MeBr and MITC (Gan et al., 1998b) and 1,3-D (McDonald et al., 2007; Ashworth, and Yates, 2007). The use of amendments is not addressed in this study.

Drip application of fumigants is an effective fumigation practice for strawberry raised beds (Ajwa and Trout, 2004). Trout et al. (2003) found that subsurface drip application of fumigants before orchard replant provided good efficacy. Many orchards are irrigated with micro-irrigation systems, so drip application of fumigants may also be a viable option. Subsurface drip irrigation with fumigants was shown to give lower emissions than shank injections from soil column and small plot tests (Gan et al., 1998c; Wang et al., 2001).

For peach and some other orchard replanting, fumigation methods must be cost-effective. Because trees are planted in widely spaced rows, strip application of fumigants is an option to effectively treat target areas. Effective fumigation of replant orchards may require treatment to 1.5 m depth to kill pests that reside in roots of the previous crop (McKenry, 1999).

To test fumigation options for replanting orchards, a field trial was established in a field being prepared to replant a peach orchard. The effects of fumigation treatments on tree response (not reported in this paper) and emissions were investigated. The specific objective of this study was to determine the effects of soil fumigation methods (shank injection vs. drip application) and surface treatments associated with water applications and plastic tarps on emissions of 1,3-D and CP. Surface treatments tested included plastic tarps (HDPE or VIF), soil moisture condition (dry and pre-irrigated), and water applications (i.e., water seals).

	Materials and Methods

TOP NOTES ABSTRACT INTRODUCTION Materials and Methods Results And Discussions Conclusions REFERENCES

 
Field Trial and Treatment
The fumigation field trial was conducted in fall (26 Oct.–8 Nov.) 2005 in a 1.8-ha peach replant orchard near Parlier (36° 35' 36.74'' N; 119° 30' 48.71'' W) in the San Joaquin Valley, CA. The soil is a Hanford sandy loam (coarse-loamy, mixed, superactive, nonacid, thermic Typic Xerorthents). The soil had a bulk density ranging from 1.45 to 1.65 g cm^–3 and a particle size distribution of 548, 396, and 56 g kg^–1 sand, silt, and clay, respectively. The soil had a pH of 7.2 and electrical conductivity of 0.30 dS m^–1 in 1:1 soil water extracts, a cation exchange capacity of 6.8 cmol_c kg^–1, and an organic matter content of 7.2 mg kg^–1. At 33 kPa suction, the soil water content is about 0.17 g g^–1 or 0.26 cm³ cm^–3 (v/v) (Skaggs et al., 2004). Mature peach trees were removed from this field 3 mo before fumigation. The field was cultivated (deep ripped) to 75-cm depth, disked, and land planed, and all visible root pieces were removed. The field was dry, with water content varying from about 0.02 cm³ cm^–3 near the soil surface to 0.10 cm³ cm^–3 at 1.2 m depth after preparation, as is common for orchard replant conditions in the arid-to-semiarid climate of the San Joaquin Valley. During the field trial, the daily maximum and minimum air temperatures were in the range of 13 to 27°C and 3 to 12°C, respectively.

The fumigation trial was originally designed in replicated complete block to investigate the performance of replant peach trees for several years after fumigation with alternative fumigants (1,3-D, CP, and methyl iodide). This paper does not report tree responses after fumigation; rather, we focus on treatments designed to minimize emissions during fumigation. For all the treatments, fumigation was applied to the center 3.2-m strip of each row (53% coverage of field), which is a common practice in the area. Fumigation by shank-injection was applied by Tri-Cal, Inc. (Hollister, CA) and drip-application by USDA-ARS (Parlier, CA).

Selected row subsections from the field trial were chosen and modified to meet our purposes for emission studies. Two rows from one replication were chosen that included shank-injection of Telone C35 (61% 1,3-D, 35% CP, and 4% inert ingredients) and drip-application of InLine (61% 1,3-D, 33% CP, and 6% inert ingredient). Additional soil surface treatments were made to subsections of each row as described in Table 1 . Four treatments were tested with shank injection: control (dry soil with no water application or surface treatment), HDPE tarp over dry soil, VIF tarp over dry soil, and pre-irrigation. Two treatments were tested over subsurface drip application: HDPE tarp and water seals. Standard (1 mil or 0.025 mm thickness) HDPE film (Tyco Plastics, Princeton, NJ) and Bromostop VIF (0.025 mm thickness; Bruno Rimini Corp., London, UK) were used. Each treatment area was about 9 x 3.2 m. Plastic tarps were applied to the strip immediately after shank injection and before fumigation for the drip application. For the pre-irrigation treatment, the strip was irrigated with microspray sprinklers 4 d before fumigation to achieve soil water content near field capacity (FC) to 25 cm depth, which required about 40 mm of water. All shank treatments were disked and harrowed immediately after fumigation and before tarping. For the water seals over drip-application treatment, 12 mm of irrigation water was applied with microsprinklers just before and after fumigant application.

Sampling and Measurement
Fumigant emissions and distribution in the soil-gas phase were monitored for 2 wk after fumigation. Soil samples were taken at the end of the trial for determining residual fumigants in the soil. Sampling methods and procedures are similar to those described in Gao and Trout (2007), so only summary information is provided here. For efficacy monitoring, because the field did not have significant native parasitic nematode populations, bagged samples of citrus nematode (Tylenchulus semipenetrans)–infested soil were prepared and buried at depths of 30, 60, and 90 cm the day before fumigation in all the treatments and were retrieved 4 wk later and analyzed for living nematodes.

Emission samples were collected using closed, passive (open-bottom) chambers assembled from inverted galvanized steel buckets (Leaktite Co., Leominster, MA). At the top center of the chamber, a sampling port with a Teflon-faced silicone rubber septum (3 mm thick; Supelco Inc., Bellefonte, PA) was installed for withdrawing gas samples. For treatments with plastic tarps, the chamber bottom was sealed to the plastic film with silicone rubber sealant. For treatments with no plastic tarp, the chamber bottom was pushed into the soil about 3 cm. Thirty minutes after the chamber was placed on the soil or tarp, a 120-mL gas sample from inside the chamber was withdrawn using a gas-tight syringe through the sampling port and through an ORBO 613, XAD 4 80/40-mg (Supelco, Bellefonte, PA) tube for trapping 1,3-D and CP. The XAD resin traps 1,3-D and CP efficiently at sampling flow rates below 200 mL min^–1 (Gao et al., 2006). The sampling tubes were immediately capped at both ends, stored on dry-ice in the field, stored in a freezer (–18°C) in the laboratory, and extracted within 6 wk for fumigant analysis using the procedures described in Gao and Trout (2006). Thirty minutes was chosen for chamber capture time to accumulate fumigant concentrations within the chamber high enough to be detected throughout the field trial period. Duplicate measurements were made for each treatment. Samples were collected every 2 to 3 h for the first 36 h and every 4 h thereafter during the day.

Based on the fumigant concentration within the chamber, capture time, chamber volume, and surface area, the average emission rate (flux) during the capture time was calculated and compared among treatments. The average emission rates obtained likely underestimate actual emission rates, especially when emissions were high, because of the decrease of diffusion rates into the chamber as concentrations increased inside the chamber (Yates et al., 2003). After the first 36 h after fumigation, no measurements were made at night, when emissions were expected to be very low. An emission flux measurement early in the morning was used to estimate emission loss during the night, which may lead to a slight overestimation of emission losses. Data from all the treatments were treated the same, and comparisons between treatments for relative effectiveness on emission reduction should be valid. Cumulative emissions of 1,3-D and CP were estimated by summing the products of the average of two consecutive emission flux values and the time interval between the two measurements over the time span of the study. Because the actual application rates for shank injection (745 kg ha^–1) were 20% higher than the drip applications (629 kg ha^–1), direct comparison of absolute emission values between shank injection and drip application was not appropriate. Total emissions were calculated as a percent of applied to reduce this bias.

Sampling probes for fumigants in the soil-gas phase were installed after fumigation and surface treatments. The probes were stainless steel tubing (0.1 mm i.d.) inserted with the lower ends at depths of 10, 30, 50, 70, and 90 cm below the soil surface. A set of five probes was installed in each treatment plot at Location "a" adjacent to shank-injection lines or drip tapes and Location "b" between shank-injection lines or drip tapes. A 50-mL soil gas sample at each depth was withdrawn through an ORBO 613, XAD 4 80/40-mg tube using a custom-made sampling apparatus. This apparatus was able to collect 10 samples at a time. During sample analysis, we concluded that the apparatus did not collect adequate samples at 50-cm depth at Location a. Thus, fumigant concentration in the soil-gas phase at this depth was estimated based on the distribution pattern of fumigant concentrations at Location b. The gas samples were collected at 6, 12, 24, 30, 36, 48, 72, 120, 168, 216, and 336 h after fumigation. Processing of the sampling tubes for analysis was the same as the emission samples.

Soil samples were taken at the end of the field trial at 20-cm-depth intervals to 100 cm to determine residual fumigants. Samples were collected with an auger (5 cm i.d.) and immediately mixed, from which a portion was taken and placed in a screw-top glass jar on dry ice in the field. This process was done as quickly as possible to minimize fumigant losses. The jars were stored in a freezer (–18°C) in the laboratory until analyzed.

Sample Extraction and Analysis
The XAD sampling tubes were extracted with hexane in a 10-mL, clear, crimp-top vial using the procedure described in Gao and Trout (2006). The 1,3-D and CP in the extracts were analyzed using a GC-µECD (6890N Network GC system with a micro electron capture detector [µECD]; 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 for separation of fumigants. 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 as follows: initially 45°C, increasing at 2.5°C min^–1 to 75°C, then increasing at 99°C min^–1 to 110°C and held for 7 min. The retention times for cis-1,3-D, trans-1,3-D, and CP were 8.6, 9.6, and 10.7 min, respectively. The detection limits (three times the SD of the background noise level) were 0.01, 0.01, and 0.001 mg L^–1 for cis-1,3-D, trans-1,3-D, and CP, respectively, when an injection volume of 1 µL solution was used. The total 1,3-D emissions, soil gas concentrations, and residual concentrations in soils reported in the results are the sum of cis- and trans-1,3-D. For the fumigant analysis, high purity of cis- and trans-1,3-D (purity of 98.9%; Dow AgroSciences, Indianapolis, IN) and CP (purity of 99.9%; Niklor Chemical Co., Inc., Mojave, CA) were used to prepare standards.

Extractions of fumigants from the soil samples followed the procedures from Guo et al. (2003) and detailed in Gao and Trout (2007). Ethyl acetate was used to extract soil samples, and anhydrous Na₂SO₄ (10–60 mesh, ACS grade; Fisher Scientific, Tustin, CA) was used to absorb water in the soils (at a 7:1 w/w ratio of Na₂SO₄/water). The extracts were analyzed using the GC-uECD system using ethyl acetate matrix for standard and samples. The vials were stored in a freezer (–18°C) for no more than 4 wk before analysis.

	Results And Discussions

TOP NOTES ABSTRACT INTRODUCTION Materials and Methods Results And Discussions Conclusions REFERENCES

 
Emissions
Emission Flux
Emission rates of 1,3-D and CP are shown in Fig. 1 . The control (shank, dry soil) resulted in the earliest and highest emission rates (76 µg m^–2 s^–1 for 1,3-D and 53 µg m^–2 s^–1 for CP), followed by the HDPE tarp over dry soil with shank injection for 1,3-D (up to 71 µg m^–2 s^–1). The HDPE tarp resulted in a much lower CP emission peak (19 µg m^–2 s^–1) than 1,3-D relative to the control. The emission peaks occurred at 15 h for the control and 48 h for the HDPE tarp after fumigation. This emission delay with HDPE was not found in an earlier study performed during high summer temperatures (Gao and Trout, 2007). High temperatures under the HDPE tarp in the summer may have caused earlier emissions. The overall results indicate that HDPE tarp is not effective in reducing 1,3-D emissions even under relatively cool temperature conditions, although the emission peak may be delayed. Pre-irrigation before shank injection resulted in a much lower 1,3-D peak emission rate (26 µg m^–2 s^–1) than the dry soil control or HDPE tarp over dry soil. This illustrates that irrigation before fumigation to produce a moist soil profile can reduce fumigant emissions more effectively than HDPE tarp. The VIF tarp over the dry soil profile resulted in relatively low but variable emission rates (1–26 µg m^–2 s^–1).

View larger version (32K): [in this window] [in a new window]   Fig. 1. Effects of application methods and surface seal treatments on emission flux of (a) 1,3-dichloropropene (1,3-D) and (b) chloropicrin (CP) from Telone C35 (shank-injection) and InLine (drip-irrigation) applications. Error bars are SD of duplicate measurements. HDPE, high-density polyethylene; VIF, virtually impermeable film.

Emission rates of CP (Fig. 1b) showed similar trends as 1,3-D (Fig. 1a), except with lower values below 20 µg m^–2 s^–1 for all treatments except the control. The amount of CP applied was about 57% of 1,3-D on a weight basis, whereas emission rates were generally <50% 1,3-D emissions. The HDPE tarp over dry soil with shank injection resulted in much lower CP emission rates than 1,3-D, indicating that HDPE is more effective in reducing CP emissions than 1,3-D. Similar results were observed in an earlier study (Gao and Trout, 2007). Chloropicrin is slightly less volatile and has a shorter half-life than 1,3-D in soils. These contribute to its relatively low emissions.

Cumulative Emissions
Cumulative emission losses of 1,3-D and CP and the total losses as a percentage of applied over a 2-wk monitoring period are shown in Fig. 2 and Table 2 , respectively. As with emission rates, the control resulted in the earliest and highest 1,3-D emission losses in the first week, after which emission loss from the HDPE-tarped treatment exceeded the control. Warmer soil temperature under the tarp than bare soil may contribute to this pattern. Total measured losses of 1,3-D were 36% of applied for the control with a large cumulative SD (7.1%) and 43% (SD, 3%) for the HDPE tarp. Total emission losses from the pre-irrigated and VIF tarp over shank-injection applications (19%) were about half of those from the control and HDPE tarp. Emission loss for VIF tarp over shank application was lower than the pre-irrigation treatment initially but increased steadily until the end of the monitoring. The VIF tarp seemed to retain fumigants under the tarp, but the warm temperature under the tarp (see below) might have caused emission increases in the later time. Subsurface drip applications under HDPE tarp and micro-sprinkler water applications before and after drip application resulted in the lowest and similar emission losses (12–13% for 1,3-D and 2–3% for CP). These similar results indicate that small amounts of surface water applications before and after fumigation through subsurface drip irrigation can reduce emissions as effectively as HDPE tarp. If micro-sprinkler systems are available at orchards, using the systems to apply water before or after drip fumigation will provide equivalent emission reductions to plastic tarps and reduce total fumigation cost.

View larger version (26K): [in this window] [in a new window]   Fig. 2. Cumulative emission of (a) 1,3-dichloropropene (1,3-D) and (b) chloropicrin (CP) from surface seal treatments. Plotted data are averages of duplicate measurements with cumulative SD listed in Table 2. HDPE, high-density polyethylene; VIF, virtually impermeable film.

Emission results from this orchard field trial confirmed the ineffectiveness of HDPE tarp on 1,3-D emission reductions. This supports the current Telone label that does not require tarp but does require soil moisture above the injection point. Irrigation with enough water to produce a moist surface soil profile to 25 cm depth before soil fumigation effectively reduced 1,3-D and CP emission peaks and total emission losses by 50% for 1,3-D and 70% for CP relative to the control. Pre-irrigation plus HDPE tarp was as effective as intermittent water seals after fumigation in reducing emissions as reported previously (Gao and Trout, 2007).

Drip applications resulted in high soil water content (see below) and greatly reduced emissions by 70% for 1,3-D and 90% for CP, respectively, in comparison with the dry-soil control. Pre- and postfumigation surface water applications resulted in the same low emissions from subsurface drip-applied fumigants as HDPE. The HDPE tarp over moist soils traps moisture under the tarp and prevents surface drying. High soil water content created by pre-fumigation surface water application greatly reduces soil water suction and thus capillary rise of water with fumigant from the subsurface drip tape to soil surface. Postfumigation replenishment of surface water application further delays any capillary rise to the soil surface of water with fumigant. The current InLine label requires use of HDPE tarps for drip application. These results indicate that tarp is not necessary if the fumigant is applied 20 cm below the surface and light pre- and postfumigation sprinkler irrigations are applied. Orchard growers with micro-irrigation systems can use their irrigation delivery systems (pump, filters, and pipelines) to deliver water to microsprinklers to apply water to the soil surface and subsequently to subsurface drip tape installed to apply fumigants.

Fumigation in this orchard replant field trial was applied in strips covering about half of the field areas. Fumigating target areas where trees would be planted can be an effective strategy to reduce emissions. The total emission losses of fumigants would be reduced in proportion to the decrease of fumigated areas. For example, in this field trial, a further 50% emission reduction was expected compared with when the whole field was fumigated.

Fumigants in Soil-Gas Phase
Figure 3 shows the distribution of 1,3-D in the soil-gas phase after fumigation. Distribution patterns of CP were similar to 1,3-D (data not shown). The ratio of 1,3-D to CP in the soil gas was similar to previous observations discussed in Gao and Trout (2007). The application ratio of CP:1,3-D was 1:1.7. The initial ratio of CP:1,3-D in the soil gas near the surface was 1:1.4, and this ratio decreased with time and depth as CP dissipated.

View larger version (44K): [in this window] [in a new window]   Fig. 3. Distribution of 1,3-dichloropropene (1,3-D) in the soil-gas phase after fumigation under various surface treatments. Sampling was located (a) adjacent to fumigant injection line or (b) between injection lines. HDPE, high-density polyethylene; VIF, virtually impermeable film.

The VIF tarp used in this study did not have a large effect on fumigant concentration in the soil-gas phase in the initial 48 h or toward the end of the trial, although relatively higher concentrations of fumigants were observed for intermediate times compared with other treatments. A single 3.7-m-wide plastic sheet was used to cover the treatment area, and some fumigant might have dissipated under the tarp edges. Fumigant distribution in the soil gas for the pre-irrigation over shank-injection treatment was not measured in this field trial. Measurements in a previous field trial indicated no differences in fumigant concentrations in this soil between the control and pre-irrigated soil plus HDPE tarp (Gao and Trout, 2007) or between the control and pre-irrigated soil profile (unpublished data) when 56 mm of water was sprinkler applied 48 h before fumigation in this soil.

Fumigant concentrations in soil gas throughout the profile in drip applications were generally lower than shank-injection treatments. However, the fumigant concentrations in drip applications declined relatively more slowly. Toward the end of the trial, considerably higher 1,3-D concentrations in the soil profile were measured in drip-application treatments (average, 1.3–1.5 mg L^–1) in comparison to shank-application treatments (average, 0.2–0.7 mg L^–1). The addition of water reduced soil-air volume and would have increased fumigant contained in the liquid phase. High soil water content generally decreases diffusion of fumigants in soils. Thomas et al. (2003) showed that 1,3-D diffusion and emissions in a sandy soil were very high in air-dried soil but were greatly reduced by high soil water content. For fine-textured soils, the effect of water content on fumigant diffusion was greatest when soils had water contents that resulted in soil water tension below 50 kPa at 30 cm depth (McKenry and Thomason, 1974). With drip application and surface water application, soil gas fumigant concentrations near the soil surface were very low, which contributed to the low emissions. This may result in poor pest control, especially for weeds, at the surface. Weed control is not a primary concern for orchard replant.

Residual Fumigant in Soil
Residual 1,3-D and CP extracted from samples taken at the end of the field trial are shown in Fig. 4 . The data represent total fumigants in the soil (mostly in the solid and liquid phase). Similar patterns were observed for 1,3-D and CP except lower concentrations of CP than 1,3-D. The results indicate that fumigants were detectable 2 wk after fumigant application, although in low concentrations. The highest concentrations were from soils under VIF tarp over shank injection, followed by HDPE tarp over shank injection. This indicates that VIF may retain higher fumigant concentrations in the soil, although this was not indicated by the soil gas measurements. Although drip application seemed to maintain fumigant in the water phase longer than shank application, by the end of the trial, very little residual fumigant was measured in the drip treatments.

View larger version (17K): [in this window] [in a new window]   Fig. 4. Residual fumigants (a) 1,3-dichloropropene (1,3-D) and (b) chloropicrin (CP) extracted from soil samples collected 14 d after fumigation. Horizontal bars are the SD of duplicate measurements. HDPE, high-density polyethylene; VIF, virtually impermeable film.

Soil water content measured at the end of the trial (i.e., 2 wk after fumigation) is shown in Fig. 5 . The presented data indicate overall soil water content after significant water redistribution; initial soil water content distribution through the profile would be concentrated near the soil surface. The soil bulk density ranged from 1.45 to 1.65 g cm^–3, and FC water content was 0.17 g g^–1, so the volumetric water content at the FC is about 0.26 cm³ cm^–3. Figure 5 shows that soil water content from all the treatments were significantly below the FC after 2 wk of redistribution and surface evaporation. With a total porosity of 0.45 cm³ cm^–3, air volume in the soil would be about 0.20 cm³ cm^–3 at FC. It would be somewhat lower near the soil surface immediately after water applications and higher at later times after water redistribution. The air volume in the surface soil may play an important role in fumigant movement and emissions.

View larger version (27K): [in this window] [in a new window]   Fig. 5. Soil water content measured 2 wk after fumigation under various surface treatments. Horizontal bars are the SD of duplicate measurements. HDPE, high-density polyethylene; VIF, virtually impermeable film.

The large amount of water used for drip-application treatments resulted in significant increase of soil water content to 1.5 m depth. Figure 5 indicates that about 120 mm of water was stored in the 1-m profile, and additional soil sampling measured similar soil water content to 1.5 m depth. This likely contributed to the low emissions from these treatments. The high soil water content also resulted in lower fumigant concentration measured in the soil gas phase, but it is not clear if the reduced gas-phase fumigant concentration resulted from more partitioning into the liquid phase. Fumigant concentrations in the gas phase and liquid phase are often assumed to be at equilibrium obeying Henry's Law Constant, but there was not enough information to verify if this is true in soils under field conditions. This equilibrium may depend on water content (or thickness of water film surrounding soil particles), which affects total amount of fumigants in and diffusion through the liquid phase.

There were large differences among treatments in soil temperature measured 10 cm below the soil surface (Fig. 6 ). The maximum temperature measured on 4 November indicated that tarping with HDPE over drip irrigation yielded the highest soil temperature (25°C), which was 8°C higher than the untarped moist soil with the lowest soil temperature. The temperature followed the trend HDPE/drip > tarp (HDPE or VIF)/shank > control > pre-irrigated soil > water applications/drip. The warmer temperature under tarp with shank applications may have contributed to the higher emissions observed from the HDPE and VIF tarp and to the large emission variation from VIF tarp at later monitoring times. The results of HDPE tarp from this cool fall season trial showed similar total emission losses (43% over 2 wk) as measured in a hot summer field trial (33% over 9 d of monitoring) (Gao and Trout, 2007).

View larger version (25K): [in this window] [in a new window]   Fig. 6. One-day soil temperature measurements at 10 cm depth during fumigant emission monitoring period. Horizontal bars are the SD of duplicate measurements. HDPE, high-density polyethylene; VIF, virtually impermeable film.

	Conclusions

TOP NOTES ABSTRACT INTRODUCTION Materials and Methods Results And Discussions Conclusions REFERENCES

 
Results from the orchard field trial indicate that soil water content is one of the most critical factors to control fumigant emissions. Applying water to the soil surface has consistently shown more effectiveness than HDPE tarp alone (i.e., from dry soil) on fumigant emission reductions. This effectiveness has been illustrated from sprinkler irrigation to the soil surface before fumigation (pre-irrigation) and after fumigation. The pre-irrigation treatment resulted in 1,3-D and CP emission loss near or below 50% of those from a dry soil profile. The moist profile can be achieved by irrigation or precipitation a few days before fumigation (enough to create adequate soil moisture without hindering the application process). The water applied in this trial was designed to moisten the top 25 cm of soil to FC. Pre-irrigation has the practical advantage because there are no fumigant exposure risks to workers. Applying water to the field after fumigation would increase this exposure risk to workers. Compared with using HDPE tarp, using water costs much less and has no material disposal required. Portable sprinkler or microspray irrigation systems are often available in areas where horticultural crops are grown. Further research is needed to define the optimum water content range for different types of soil that can reduce emissions while maintaining good efficacy. Fumigation through drip irrigation (with HDPE or water applications before and after fumigation) resulted in generally lower emissions than shank fumigation in this field trial, but additional tests are needed to confirm its effectiveness on emission reductions and on soil pest and pathogen control.

	ACKNOWLEDGMENTS

 
Funding for this research was provided by California Dep. of Food and Agriculture, Almond Board of California, and Fruit Tree, Nut Tree, and Grapevine Improvement Advisory Board. TriCal Inc. (Hollister, CA) provided fumigant, fumigation equipment, and personnel to accomplish the field fumigation. Telone C35 was provided by Dow AgroSciences (Indianapolis, IN). Technical assistance for this research was received from Mr. Robert Shenk, Ms. Aileen Hendratna, Mr. Ernie Leyva, Mrs. Nancy Goodell, Mr. Jim Gartung, and Mr. Tom Pflaum of the Water Management Research Unit, USDA-ARS, Parlier, CA.

	NOTES

TOP NOTES ABSTRACT INTRODUCTION Materials and Methods Results And Discussions Conclusions REFERENCES

 
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TOP NOTES ABSTRACT INTRODUCTION Materials and Methods Results And Discussions Conclusions REFERENCES

 

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