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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 March 15, 2006.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Soil surface barriers with plastic tarps, such as standard high density polyethylene (HDPE), are commonly used to control fumigant emissions. The HDPE, however, does not effectively reduce 1,3-D emissions because of high permeability to this compound (Wang et al., 1999; Papiernik and Yates, 2002). The HDPE tarp is expensive (about $2000 ha–1 over shank broadcast applications in California for purchase, placement, removal, and disposal of the tarp). Another potentially effective tarp to reduce emissions is virtually impermeable film (VIF), which has much lower permeability to most fumigants than HDPE (Wang et al., 1999; Noling, 2002a; Thomas et al., 2004, 2006). Virtually impermeable film, however, costs substantially more than HDPE. Anecdotal reports indicate that maintaining the low permeability property of the film in large field applications may be difficult due to stretching and inadequate materials for gluing sheets together.
Some studies showed that high water content in surface soil provided a more effective barrier to 1,3-D movement than HDPE tarp (Gan et al., 1998; Thomas et al., 2003). Laboratory column and small field plot tests showed that water applications to the soil surface in combination with HDPE tarp greatly reduced MeBr emissions (Jin and Jury, 1995; Wang et al., 1997). Water seals, especially with intermittent applications after shank injection, showed promising results to reduce methyl isothiocyanate (MITC) emissions from applications of metam-sodium (MITC generator) although mixed results were obtained from intermittent water caps over sprinkler chemigation in field studies (Sullivan et al., 2004).
We tested the potential of using water application to the soil surface to reduce 1,3-D emissions from soil columns (Gao and Trout, 2006). The results showed that spraying water on the soil surface can reduce 1,3-D emissions more effectively than HDPE tarp. Water seals reduced peak emissions more effectively than cumulative emissions, mainly due to the abrupt reduction of emission rate after each water application. Initial water application immediately after fumigant injection reduced peak emissions and delayed the emission peak time, which is important to protect workers and bystanders during fumigation. Water application to the soil surface or water seal (costs of $100 to $700 ha–1) is more economical than plastic tarps in field applications.
The objective of this study was to determine if water applications to the soil surface can effectively reduce emissions of 1,3-D and CP from shank application of Telone C35 under field conditions. Water application treatments were compared to plastic tarps (HDPE and VIF) and the combination of water application and HDPE tarp.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Field Trial and Treatment
A field trial with small plots (9 x 3 or 9 m depending on treatment) was conducted in summer 2005, at the USDA-ARS San Joaquin Valley Agricultural Science Center, Parlier, CA (36°35'36.74'' N; 119°30'48.71'' W). The soil is a Hanford sandy loam (coarse-loamy, mixed, superactive, nonacid, thermic Typic Xerorthents). The selected soil properties are reported in Table 1. The soil was cultivated to 75-cm depth before fumigation. The soil surface was dry (30 g kg–1 or 3% water content) before treatments were applied. A dry soil surface is typical for fall fumigations before orchard or vineyard planting. About 13 mm of water was sprinkled onto the field 1 wk before the field trial and the surface was disked to break down large soil clods. During the field trial, the maximum and minimum air temperature ranged from 37 to 41°C and 21 to 24°C, respectively.
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Individual plots were 9 x 9 m for water application treatments and 9 x 3 m for the control and tarped treatments. Tarps with a width of 3.7 m were placed on the fumigated soil immediately after postfumigation tillage using a standard fumigation rig with the shanks removed. Tarp application was by a single pass perpendicular to the fumigation direction (across the plots) and tarp edges were inserted 20 cm deep into the soil. Sprinkler water was applied to each plot with four Hunter PGP rotary sprinklers set for quarter circle application placed in the corners of each plot (9-m spacing, 8.5 mm h–1 application rate). Each rotary sprinkler was adjusted depending on wind direction changes to achieve uniform water application into each plot. Water applied to the plots was measured by water meters. The 19 mm of water in the initial applications was sufficient to wet the soil to near field capacity to a 10-cm depth. The intermittent 4.2 mm of water applied was sufficient to replace evaporation loss and return the surface soil to field capacity. The fumigated area was divided into 3 blocks. Treatments were tested with three replicates in a randomized complete block design. A 3-m-wide buffer zone was left between blocks and treatments with and without water applications.
Sampling and Measurement
Sampling for air emissions and distribution of applied fumigants in the soil gas phase was continued for 9 d. Soil samples were taken at the end of the sampling period for residual fumigants in the soil. Soil water content was determined for the control and water application plots on the first day of fumigation and at the end of the field trial. Soil temperature at 10-cm depth was measured during the last day of the trial.
Emission samples were collected using closed, passive (open bottom) gas chambers assembled from inverted Leaktite galvanized steel buckets (Leaktite Co., Leominster, MA). The dimension of the emission chambers were 18.6 (top i.d.) x 15.0 (height) x 20.7 cm (bottom i.d.). The volume of the chamber and the surface area it covers were 4.6 L and 337 cm2, respectively. At the top center of the chamber, a sampling port with a Teflon-faced silicone rubber septum (3 mm thick, Supelco, Bellefonte, PA) was installed for withdrawing gas samples. For treatments with plastic tarps, the chamber was sealed to the plastic film with silicone rubber sealant. Our preliminary tests showed that there was no interference of the sealant with the analysis of the fumigants, i.e., compounds volatilized from the sealant and captured in the chamber had substantially different retention time in gas chromatography columns in comparison with 1,3-D and CP. For treatments with no plastic tarp, the chamber bottom was pushed a few centimeters into the soil, depending on the hardness of the soil. For example, when surface water was applied after fumigation, a surface crust formed that did not allow the chamber to be pushed into the soil more than 1 cm deep. In this case, after good contact between the chamber and the soil was created, the chamber edge was covered with more surrounding soil.
The gas sampling chambers were placed on soil or tarp for 30 min. At the end of the 30-min period, 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 80/40mg (Supelco, Bellefonte, PA) tube for trapping both 1,3-D and CP. We determined in preliminary tests that the XAD resin could trap 1,3-D as efficiently as CP (95 ± 6%) under the sampling conditions, i.e., flow rate 
100 mL min–1. The sampling tubes were immediately capped at both ends, stored on dry ice in the field and in a freezer (–18°C) in the laboratory, and extracted within 6 wk for fumigant analysis using the procedures described below. 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. One gas sampling chamber was used for each plot. Samples were collected every 2 to 3 h for the first 48 h and every 4 h thereafter during the day. No sampling was done at night (2100 to 0600 h for the first two nights and 1700 to 0800 h thereafter). For water application treatments, sampling was conducted before and after each water application. Field blank samples were taken at about 500 m away from the field site during the trial.
The passive or closed chamber method allows direct measurement of gas volatilization from soil to the atmosphere (Yates et al., 2003). Upon placing the chamber over the soil surface, the concentration of fumigants in the chamber increases with time as the chemical moves from the soil matrix into the chamber. The concentrations of 1,3-D and CP within the chamber at the end of the 30 min. capture time were determined. 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. Because diffusion rate of the fumigants into the chamber is expected to decrease as a function of time (due to concentration gradient decreases), the average emission rate measured was likely lower than the initial rate (representative of the rate without the chamber). Thus, average emission rates obtained likely underestimate actual emission rates (Yates et al., 2003). 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.
Probes for sampling fumigants in the soil-gas phase were installed in one replicate of the treatments following fumigation and surface treatments. These probes were stainless steel tubing with 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 50-mL soil gas sample was withdrawn through an ORBO 613, XAD 4 80/40mg tube using a custom-made sampling apparatus. Soil gas samples were collected at 6, 12, 24, 30, 36, 48, 72, 120, 168, and 216 h following fumigation. Processing of the sampling tubes was the same as the emission samples.
For residual fumigants in the soil, soil samples were taken at the end of the field trial at 20-cm depth intervals to 100 cm. Samples were collected with an auger (7 cm i.d.), mixed, and placed in a screw-top glass jar on dry ice in the field, and stored in a freezer (–18°C) in the laboratory until analyzed.
Sample Extraction and Analysis
The XAD sampling tubes were broken in the middle and all materials in the tube were transferred into a 10-mL clear crimp-top vial. Five mL of hexane was added to the vial and after crimp sealed the vials were shaken for 2 h on a reciprocating shaker at 120 strokes min–1. After settling, a portion of the extract was transferred to a 2-mL amber GC vial and the vials were stored for no more than 4 wk in a freezer (–18°C) until analysis. Our test showed that 95% of the fumigant was recovered in the solvent after storing in the freezer for 4 wk.
The 1,3-D and CP in the extracts were analyzed using a GC-µECD (Agilent Technology 6890N Network GC system with a micro electron capture detector [µECD]; Agilent Technology, Palo Alta, CA). A DB-VRX capillary column (30-m length x 0.25 mm i.d. x 1.4 µm film thickness, Agilent Technologies, Palo Alto, CA) 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, 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 at 99°C min–1 to 110°C and held for 7 min. The retention time 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 standard deviation of the background noise level) of our methods 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.
Soil sample extractions followed the procedures from Guo et al. (2003). Before defrosting, 8 g equivalent dry weight of soil was weighed into a 21-mL crimp-top extraction vial. Eight mL of ethyl acetate was added to the vial that contained various amounts of Na2SO4 to adsorb soil moisture (at a 7:1 ratio of Na2SO4/water depending on the soil water content). The vial was crimp-sealed with aluminum caps and Teflon-faced butyl-rubber septum, mixed, and incubated at 80°C in a water bath overnight (18 h). This method extracts more than 95% of the fumigant in soils. After settling, a portion of the supernatant was transferred into a 2-mL amber GC vial for fumigant analysis using the GC-µECD as described above, except using ethyl acetate as the standard and sample solvent. The vials were stored in a freezer (–18°C) for no more than 4 wk before analysis.
Statistics
For statistical analysis, SAS Version 9.1 (Littell et al., 2002) was used. Data were analyzed with the two-way factorial analysis of variance (ANOVA) and means were separated using Tukey's HSD (honestly significant difference) test.
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RESULTS AND DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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The HDPE tarp over the pre-irrigated soil resulted in a flatter emission curve (Fig. 1b) and generally lower emission rate peak (40 µg m–2 s–1 for 1,3-D) than the control or HDPE tarp alone (66 µg m–2 s–1). The VIF tarp showed the lowest emission rates with most values below 10 µg m–2 s–1 for 1,3-D (Fig. 1b) and 3 µg m–2 s–1 for CP (Fig. 2b). Variations of emission rates among VIF tarp replicates were observed especially after 2 d and may represent nonuniform permeability of the VIF.
The emission fluxes of CP (Fig. 2) followed similar patterns as 1,3-D but the emission rates were lower. The amount of CP applied was about 57% of 1,3-D on a weight basis. The emission rates for CP were all below 20 µg m–2 s–1 for both the control (Fig. 2a) and HDPE tarp treatments (Fig. 2b), with peak values less than 25% of the 1,3-D fluxes. At the end of the monitoring period, CP emissions in most plots were nondetectable.
Cumulative Emissions
Cumulative emission losses of 1,3-D and CP are shown in Fig. 3 and summarized in Table 2. Emission rate data indicated that the peak emissions for the control treatment were likely missed during the first night. Previous column studies had shown that higher emission peaks and earlier peak times were expected for the control than with tarp (e.g., Gan et al., 1998; Gao and Trout, 2006). These studies also indicated that the peak emission rate for the control was about 1.5 times of that from HDPE tarp. By assuming this peak value was reached during the first night for the control, the total cumulative emission could increase 25% from the estimated value in Table 2. The modified total emission loss, however, is still substantially lower than the total emission loss from HDPE tarp. We highly suspect that the passive chamber method might underestimate fumigant emissions from a bare and dry soil surface because a perfect seal between the chamber and the soil was difficult to form during the 30 min capture time. Thus, cumulative emission for the control was believed to be substantially underestimated as evidenced by its lower total emissions estimated from measurements than several surface seal treatments (Table 2). For other treatments, the peak emissions were observed at or after 22 h of fumigation and the estimated total emission losses in Table 2 would increase about 3 to 8% by assuming the high emission rates observed early on the second morning occurred earlier. For this reason, our discussion below focuses on comparisons between surface seal treatments.
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The HDPE tarp was the poorest and VIF was the most effective barrier to 1,3-D and CP among the surface seal treatments. Pre-irrigated soil plus HDPE tarp and the intermittent water applications were relatively effective methods to reduce total emissions. The continuous evaporation and condensation of water under the plastic film may form an effective moisture barrier to fumigants at the soil surface. The single sprinkler water application following fumigation reduced total emissions, but not significantly different from the HDPE tarp over dry soil. The water seal may be more effective to reduce CP than 1,3-D emissions based on data in Fig. 3 and due to its shorter half-life.
Our estimate of total emissions based on measurements for the control was lower than the HDPE tarp based on measured emissions (Table 2) and the reasons for this underestimate were discussed above. Tests on emissions from column experiments at room temperature (22°C) using the same soil showed that the emission peak of 1,3-D under HDPE tarp was 87% of the dry, nontarped control with a delayed peak time of 1 to 2 h (Gao and Trout, 2006). Gan et al. (1998) also showed that the peak volatilization rates of 1,3-D from soil columns after injection at 20 cm was higher for nontarped control (1100 µg h–1) than the HDPE tarped treatment (750 µg h–1) with a few hours earlier peak time. Their results showed that total emission loss of 1,3-D was 64% of applied in the control compared with 58% in HDPE tarped treatment. We conducted a further field trial on the same soil and measured emission rate differences between the nontarped control and HDPE tarped treatment by continuously sampling the air just above the soil surface. Results (not shown) clearly showed initially higher fumigant concentrations and earlier peak time above nontarped soil surface than HDPE tarp.
Fumigants in Soil Gas Phase
Distribution of 1,3-D and CP in the soil gas phase is shown in Fig. 4 and 5, respectively. Similar patterns were observed between the two fumigants except that concentrations of CP were lower than 1,3-D. The differences in the concentration range and peaks between CP (maximum 25 mg L–1) and 1,3-D (maximum 33 mg L–1) were greater than their application ratio (CP/1,3-D = 1:1.7; i.e., higher portion of CP was measured in soil gas compared with 1,3-D). Recall that relative CP emissions were less than the application ratio. These results indicated that CP remained in the soil proportionally higher initially than 1,3-D. By the end of the field trial, however, CP dissipated from the soil to very low levels (<0.1 mg L–1) in most treatments compared with 1,3-D concentrations that were up to 0.6 mg L–1. This was due to the lesser amount of CP applied as well as its more rapid degradation rate than 1,3-D.
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Measured concentrations of both 1,3-D and CP in the soil gas phase were much lower with intermittent water application compared with other treatments. This was not observed in our previous column experiments when 13 mm water was applied intermittently to soil surface (Gao and Trout, 2006). Although the total amount of water applied in the field trial was 40 mm, we cannot conclude that the amount of water applied resulted in the lower fumigant concentrations in the field trial. The intermittent water application treatment included the initial water application immediately following fumigation and was followed by the second water application 8 h after fumigation. Before 8 h, measured fumigant concentrations in the intermittent water application plot were substantially lower than in the initial water application plot. The reason for the lower measured fumigant concentration in the intermittent water application could not be determined but may be due to soil variability (note that soil gas samples were not replicated). Nonetheless, it should be noted that as soil water content increases, the diffusion of fumigants in soils generally decreases. Excessive water could limit the diffusion of fumigants in the soil and reduce fumigation efficacy. Thomas et al. (2003) observed in a sandy soil that fumigant diffusion was negligible in near-water-saturated soil and fumigant diffusion in near-field capacity soil was between the rates of dry soil and near-saturated soil. For fine-textured soils, the effect of water content on fumigant diffusion was most striking when soils had water contents in excess of 50 kPa moisture tension at 30-cm depth (McKenry and Thomason, 1974). Thus, an optimum soil water content is needed to prevent rapid emissions while maintaining adequate pest control.
The ability of VIF tarp to retain higher 1,3-D concentrations in the soil gas was relatively less in this field trial than was observed in the previous column tests. Although significantly higher fumigant concentrations under the VIF tarp than other treatments were observed during early intermediate time periods (e.g., 36 and 48 h), there were no significant differences in fumigant concentrations under VIF tarp compared with other treatments at the end of the measurement period. The soil was cultivated to about 76-cm depth before fumigation and downward movement of fumigant below the lowest sampling depth is indicated by the fumigant concentrations measured at the 90-cm depth at 36 and 48 h (Fig. 4 and 5). The lower than expected fumigant concentrations under VIF may indicate lateral movement because the VIF tarp used was only 3 m wide and fumigants may have degraded significantly in the soil over 9 d. Others have found that VIF maintained high fumigant concentrations under the tarp, which can improve and may allow lower fumigation rates (Noling, 2002b; Gilreath et al., 2005; Santos et al., 2005).
Residual Fumigant in Soil
Residual fumigants in the soil (solid and liquid phase combined) were extracted from soil samples taken at the end of the field trial and the results for 1,3-D are shown in Fig. 6. Nondetectable or extremely low CP concentrations (<0.01 mg kg–1) were measured. This indicates that CP had degraded in the soils within 9 d, as was corroborated by the soil gas data. For 1,3-D, detectable concentrations were found in soils in the upper 40 cm in most treatments and to 60 cm for the VIF. Concentrations of 1,3-D were relatively higher in shallow soil layers for all the treatments. The highest 1,3-D concentrations (average 0.5 mg kg–1) were observed under the VIF tarp (significantly higher than all other treatments at 
= 0.05). There were no significant differences between other treatments.
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Soil Temperature
Tarping and water application have large effects on soil temperature. The maximum soil temperature measured 10 cm below the soil surface during the last day of the field trial is shown in Table 3. Pre-irrigated soil plus HDPE tarp resulted in the highest temperature (47°C) among the treatments followed by the dry soil with either HDPE or VIF tarp. Higher temperatures would result in a higher Henry's law constant and diffusion rate, a higher permeability of tarps, and a lower solubility of fumigants. As a result, tarping may not help reduce emissions. The high temperature may also result in a benefit for controlling soil pests from the solarization effects as reported by others (e.g., Minagawa et al., 2004; Sharma et al., 2004). Thus, tarping over a moist soil may improve the overall soil pest control at high temperature conditions and at the same time, reduce 1,3-D emissions. However, tarps are an expensive alternative compared with water seals alone.
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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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