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Impact of hairy vetch cover crop on herbicide transport under field and laboratory conditions
Chemosphere 44 (2001) 109±118
Impact of hairy vetch cover crop on herbicide transport under ®eld and laboratory conditions Ali M. Sadeghi *, Allan R. Isensee USDA, Agricultural Research Service, BARC-West, Environmental Chemistry Lab., B-001, R-220, 10300, Baltimore Avenue, Beltsville, MD 20705, USA Received 30 April 2000; received in revised form 21 June 2000; accepted 22 June 2000
Abstract This study was conducted to evaluate the eect of hairy vetch cover crop residue on runo losses of atrazine and metolachlor under both no-till corn ®eld plots and from a laboratory runo system. A 2-year ®eld study was conducted in which losses of atrazine and metolachlor from vetch and non-vetch ®eld plots were determined from the ®rst runo event after application (5 and 25 days after application in 1997 and 1998, respectively). A laboratory study was conducted using soil chambers, designed to simulate ®eld soil, water, vegetation, and herbicide treatment conditions, subjected to simulated rain events of 5, 6, 20 and 21 days after application, similar to the rainfall pattern observed in the ®eld study. Atrazine losses ranged from 1.2 to 7.2% and 0.01 to 0.08% and metolachlor losses ranged from 0.7 to 3.1% and 0.01 to 0.1% of the amount applied for the 1997 and 1998 runo events, respectively. In the laboratory study, atrazine runo losses ranged from 6.7 to 22.7% and 4.2 to 8.5% and metolachlor losses ranged from 3.6 to 9.8% and 1.1 to 4.7% of the amount applied for the 5±6 and 20±21 day events, respectively. The lower losses from the ®eld study were due to smaller rainfall amounts and a series of small rains prior to the runo event that likely washed herbicides o crop residue and into soil where adsorption could occur. Runo losses of both herbicides were slightly higher from non-vetch than vetch ®eld plots. Losses from the laboratory study were related to runo volume rather than vegetation type. Ó 2001 Elsevier Science Ltd. All rights reserved. Keywords: Soil chamber; Runo simulation; Herbicide leaching; Rainfall simulation
1. Introduction Cover crops, such as hairy vetch (Vicia villosa Roth.) are increasingly being used in no-till corn production because they provide signi®cant amounts of nitrogen and early season weed control, and function as a mulch to retard soil moisture loss (Smith et al., 1987; Teasdale, 1993). The hairy vetch is normally killed, mechanically
* Corresponding author. Tel.: +1-301-504-6693; fax: +1-301504-7976. E-mail address: [email protected] (A.M. Sadeghi).
or with herbicides, a few days before corn is planted, resulting in a thick mass of crop residue covering the soil surface. Herbicides applied after corn planting may be largely intercepted by the hairy vetch crop residue. Previous studies have shown that 15±80% of an applied pesticide will be intercepted, depending on the amount and type of crop residue and vegetation (Martin et al., 1978; Ghadiri et al., 1984; Banks and Robinson, 1986; Sorenson et al., 1991). Pesticides are usually washed o crop residue by rainfall, but the quantity that reaches soil is dependent on the rainfall amount and timing (Shipitalo et al., 1990; Sorenson et al., 1991). Many studies have measured runo losses of herbicides under ®eld conditions (Baker and Johnson, 1979;
0045-6535/01/$ - see front matter Ó 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 0 4 5 - 6 5 3 5 ( 0 0 ) 0 0 2 0 7 - 1
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A.M. Sadeghi, A.R. Isensee / Chemosphere 44 (2001) 109±118
Isensee and Sadeghi, 1993; Southwick et al., 1993). These ®eld runo studies provide an accurate measurement of herbicide transport for the environmental conditions that exist at the time of each runo event. For any one site, studies need to be conducted over a series of years to obtain a representative range of rainfall and antecedent moisture conditions, since climatic variables cannot be controlled in ®eld experiments. Therefore, studies have been set up using small ®eld plots and rain simulators to evaluate rainfall parameters (Kenimer et al., 1987; Sauer and Daniel, 1987). These studies provide needed control of rain parameters but may still be limited to the capacity of the simulator, existing slope, and soil moisture conditions. Laboratory runo simulation systems have been developed because most of the parameters can be controlled (Burgoa et al., 1993; Truman and Bradford, 1995; Isensee and Sadeghi, 1999). However, the diculties associated with packing soil in runo containers and the ability to have adequate replications for large-sized soil chambers have limited laboratory system use primarily to soil erosion studies. The recently developed runo system by Isensee and Sadeghi (1999) includes in its design soil chambers that can simulate most of the soil structural and vegetative conditions that exist in the ®eld, allowing for simulation of actual ®eld situations. The objectives of this study were: (1) to evaluate the eect of hairy vetch cover crop on runo loss of atrazine and metolachlor under ®eld and laboratory conditions; and (2) to examine the comparativeness of laboratory vs ®eld-derived runo data. 2. Materials and methods 2.1. Field studies The ®eld runo evaluations were conducted in an ongoing research site at the Beltsville Agricultural Research Center in Beltsville, MD. The experimental set-up for the site has been described by Sadeghi and Isensee (1997). In brief, the site was divided into eight no-till corn production plots (0.1±0.23 ha) with a slope ranging from 2 to 6%. Each plot was separated with berms that direct surface water ¯ow through ¯umes, each instrumented with an automated ¯ow-meter and water sampler. Field experiments were set up for the 1997 and 1998 corn-growing seasons. In the fall of 1996 and 1997, after corn harvest, hairy vetch was planted at 28 kg ha 1 with a no-tillage grain drill in half of the plots. In 1997, all the plots were sprayed with paraquat (1,10 dimethyl-4-40 -bipyridylium dichloride) on 23rd May, corn was planted on 27th May and atrazine [6-chloroN-ethyl-N-(1-methylethyl)-1, 3,5-triazine-2,4-diamine] and metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2methoxy-1-methylethyl)acetamide] were applied on
28th May. In 1998, all the plots were sprayed with paraquat on 15th May, corn was planted on 17th May and atrazine and metolachlor applied on 19th May. Paraquat, atrazine and metolachlor were applied at 0.56, 1.7 and 2.0 kg ha 1 , respectively. Rainfall and runo amounts and timing are shown in Fig. 1. The automatic samplers were triggered by the ¯ow through the ¯umes and were programed to collect runo samples every 100 gallons during each rain event. Runo samples, collected from each of the eight plots, were taken to the laboratory, placed in glass bottles and stored at 4°C until analyzed. All samples were extracted and analyzed as previously described (Nash, 1990). In brief, atrazine and metolachlor were extracted from water by solid phase extraction (C18 cartridges), eluted with ethyl acetate, and analyzed by gas±liquid chromatography using a nitrogen±phosphorus detector. 2.2. Laboratory studies The laboratory study was set up to roughly simulate the ®eld conditions associated with the 1997 and 1998 ®eld runo events. The rainfall±runo simulation system used in this study has been described by Isensee and Sadeghi (1999). In brief, the system consists of: (1) an adjustable rainfall simulator. This simulator consists of two oscillating linear dripper units that provide simulated rain between radii of 54±112 cm at 0° and 180° over the turntable. Two independent peristaltic pumps supply water to the dripper units; (2) a 2.4-m diameter, 1000-kg capacity, 1-rpm turntable that supports and rotates the soil chambers under the rain simulator; (3) an adjustable elevating system designed to support the soil chambers at the desired slope (0± 20%); (4) tipping bucket gauges and data logger to
Fig. 1. Rainfall amount and timing prior to herbicide application and up to the ®rst runo event for 1997 and 1998.
A.M. Sadeghi, A.R. Isensee / Chemosphere 44 (2001) 109±118
measure and record runo; and (5) soil chambers (described below) for measuring runo and leaching of the herbicides. 2.3. Soil chambers and experimental procedures 16 soil chambers were constructed similar to those described by Isensee and Sadeghi (1999). In brief, the chambers were constructed using 15-mm thick plywood with inside dimensions of 100-cm length by 35cm width by 25-cm depth with top edges cut at a 45° angle to ensure that rain falling on the edges would be directed away from the interior. The chambers were assembled and water-proofed with epoxy resin. Runo from the chambers was collected from a 13-mm i.d., height adjustable drain located on one end of each chamber and leachate is collected from three bottom drains (6-mm i.d.) located at 1, 34, and 67 cm upslope from the runo outlet. Partitions (1:3 1:3 35 cm pieces of aluminum angle) were attached (with silicon cement) to the bottom of the chambers downslope from the 34- and 67-cm drains to aid leachate collection. The soil used in this study is a Hatboro silt loam (®ne-loamy, mixed, non-acidic, mesic, Typic Fluvaquents) and was obtained from a ®eld at the Beltsville Agricultural Research Center, Beltsville, MD. About 1200 kg of surface soil, 0±15 cm depth, was ®eld sieved (1.25 cm) and stored until use (average gravimetric moisture content was 10.3%). This soil had a pH of 6.5, an organic carbon content of 0.7% and a sand, silt and clay content of 34.5%, 49.5%, 16%, respectively. Soil chambers were packed as follows: screen (1 mm2 openings) was placed over the drains and 7 kg of sand (0.5±1.0 mm) was placed at the bottom to a depth of 1.3 mm (top of the partitions). The soil was added to each chamber in nine 7-kg increments plus a ®nal 5-kg increment for a total of 68 kg. After an increment of soil was added to all chambers, they were leveled and packed with a pressure of about 0.15 kg cm 2 before the next addition. This resulted in an estimated bulk density of 1.44±1.52 g cm 3 . After packing, the three drains in each chamber were connected together with tubing and the soil slowly wetted from below until the soil surface was uniformly moist. The tubing was then removed from the drains to allow excess water to drain. 1 day after wetting the soil, the height-adjustable gates were positioned so that the center of the drain was in level with the soil surface and sealed in place with silicone sealant. Five days after wetting the soil, hairy vetch was planted in seven rows across the width of the chambers (63 seeds each chamber) to simulate ®eld seeding methods. Eight chambers were planted to vetch and eight were left fallow. Volunteer vegetative growth was
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allowed to develop in the fallow chambers as it occurs in a no-till ®eld. Fifteen days after planting the vetch, 200 g of corn stover was uniformly spread on the surface of all 16 chambers to simulate crop residue conditions of no-till ®elds. All chambers were watered as needed. At the time of spraying, the hairy vetch averaged 25 cm in height and provided 100% coverage of the soil surface while the volunteer vegetation in the non-vetch chambers averaged 15 cm in height and provided 75± 90% coverage of the soil surface. Prior to spraying, all chambers had developed cracks between the soil and chamber walls. These cracks were widened to about 3 mm by repetitiously plunging a thin lath between the soil and chamber wall around the perimeter of each chamber. This crack was then ®lled with a 1:1 mixture of sand and bentonite clay to prevent water from ¯owing down along the wall. All chambers were sprayed with paraquat at the rate of 0.56 kg ha 1 (19.6 mg each chamber), 89 days after planting vetch. 4 days after killing the vegetation with paraquat, all the chambers were treated with atrazine and metolachlor as follows: an aqueous solution containing 5.44 mg ml 1 atrazine and 6.58 mg ml 1 metolachlor was prepared by diluting 4.25 ml of the commercial formulation Bicept II (containing 320 g l 1 atrazine and 387.1 g l 1 metolachlor) to 250 ml. 10 ml of this solution was spray applied to the upper 35 by 90 cm area of each chamber which equals the ®eld application rate of 1.7 and 2.1 kg ha 1 atrazine and metolachlor, respectively. Runo experiments were conducted to simulate 5- and 20-day time periods between application of atrazine and metolachlor and the ®rst rainfall/runo event. However, since the turntable could accommodate only four chambers at a time, actual runo runs were conducted on 5, 6, 20 and 21 days after treatment. It was assumed that data obtained from two consecutive days would be adequately similar to represent one time period. Two soil cores, 2-cm diameter by 8-cm depth were taken from each chamber before each run to determine the soil moisture content and the holes were back®lled with untreated soil. For each run, two randomly chosen vetch and two non-vetch chambers were placed on the turntable, elevated to 5% slope and subjected to rain at 24.3 mm h 1 for 2 h (days 5 and 6), and at 23.0 mm h 1 for 2.5 h (days 20 and 21). The longerduration rain events for days 20 and 21 were needed to obtain volumes of runo similar to those obtained for the 5- and 6-day events. Runo from each chamber was channeled into custom-built tipping bucket gauges that delivered 80 ml each tip. Each tip was recorded by a data logger so that a hydrograph could be developed later. The discharge from the gauges ¯owed into beakers that were used to obtain samples. Runo was collected in 80-ml increments (one tip) for samples 1±5, about 320 ml for samples 6±10 and about 480 ml for all remaining sam-
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3. Results and discussion
ples. Time (after the start of the rain simulator) was recorded for each sample to determine the location of each sample on the hydrograph. Leachate was collected in about 300-ml increments. Subsamples (about 20 ml) were taken from all runo and leachate samples for later analysis. After each runo event, two 82 cm2 crop residue samples were taken from each chamber and frozen for later analysis of herbicide residues. Soil cores (0±6 and 6±12 cm deep) were also taken for moisture determinations. Crop-residue samples were extracted and analyzed as previously described (Nash, 1990). In brief, samples were shake-extracted with methanol:water (4:1), ®ltered, and the ®ltrate reduced to about 15 ml in a rotary evaporator. The atrazine and metolachlor were extracted from the aqueous ®ltrate by solid-phase adsorption using C18 cartridges, eluted from the cartridge with ethyl acetate and analyzed by gas±liquid chromatography using a nitrogen±phosphorus detector. Runo and leachate samples were processed as follows: 5 ml water samples were vortex-mixed with 2 ml ethyl acetate in 15-ml centrifuge tubes for 1 min. The ethyl acetate layer was removed with a Pasteur pipette, dried with anhydrous sodium sulfate, and analyzed by GC. Recovery eciencies from solutions containing atrazine at 0.02±3.04 lg l 1 and metolachlor at 0.03±3.87 lg l 1 were 65 and 104%, respectively. Results were not adjusted for recovery.
3.1. Field study The ®rst runo events for 1997 and 1998 occurred between 5 and 25 days after herbicide application, respectively (Fig. 1). In 1997, runo resulted from 30.5 mm rain, 5 days after application while in 1998, runo followed 18.5 mm rain, 25 days after application. In general, the volume of runo in 1997 was greater than in 1998 (data not shown) whereas runo expressed as percent of the rain falling on each plot was similar for the 2 years (Table 1). These results were unexpected based on the rainfall patterns (Fig. 1). Between 1 and 8 days before the 1997 runo event, a total of 78 mm of rain fell compared to only 24 mm rain in the 25 days before the 1998 runo event. However, in 1998, the soil was very wet prior to herbicide application (77 mm rain between 1st and 12th May) and all vegetation was killed with paraquat on 15th May, greatly reducing evapotranspiration losses. Thus, the residual soil moisture level at the time of the runo event was likely high. The amount of atrazine and metolachlor lost in runo in 1997 was much higher than the loss in 1998 (Table 1). Atrazine losses ranged from 1.2 to 7.2% and 0.01 to 0.08% and metolachlor losses ranged from 0.7 to 3.1% and 0.01 to 0.1% for 1997 and 1998, respectively. The much lower losses in 1998 than in 1997 are
Table 1 Runo and herbicide loss from hairy vetch and non-vetch corn production plots Plot no.
Vetch 1 2 3 4
Non-vetch 1 2 3 4 a
Plot size (ha)
0.16 0.23 0.16 0.12
0.16 0.23 0.16 0.10
1997a
1998a
Runob %
Atrazinec %
Metolachlor %
Runo %
Atrazine %
Metolachlor %
nsd ns 12.9 25.7
nde nd 2.8 5.0
nd nd 1.0 2.0
10.0 4.5 12.9 ns
0.03 0.01 0.04 nd
0.01 0.01 0.03 nd
0.03 0.02f
0.02 0.01
0.02 0.03 0.01 0.08
0.03 0.06 0.02 0.10
0.04 0.03
0.05 0.04
8.3 18.3 21.6 29.0
1.2 3.1 4.5 7.2
0.7 1.7 2.8 3.1
4.0 2.5
2.1 1.1
10.5 12.2 5.6 33.8
Runo event occurred between 5 and 25 days after herbicide application in 1997 and 1998, respectively. Runo expressed as percent of rainfall on each plot based on 30.5 and 18.5 mm rains for 1997 and 1998, respectively. c Herbicide loss expressed as a percentage of the amount applied to the ®eld. d No sample collected due to ¯ow-meter's malfunction. e No data. f Mean standard deviation. b
A.M. Sadeghi, A.R. Isensee / Chemosphere 44 (2001) 109±118
likely due to a combination of (1) longer time between application and the ®rst-runo event during which time more dissipation could occur, and (2) a succession of small rainfall events that likely washed herbicides o the crop residue and into soil where adsorption would reduce the amount available for transport. 3.2. Laboratory study In general, the total discharge as runo plus leachate from the soil chambers within each time period was similar (Table 2). However, runo varied widely between chambers in direct proportion to the amount of leaching. The experimental conditions of this study (modest slope [5%] and dense mat of dead vegetation) slowed overland ¯ow and provided adequate time for water in®ltration. However, these parameters were similar to actual ®eld conditions experienced in the ®eld study as described above. Runo from the soil chambers began about 10 min after the start of simulated rain for the 5±6 day treatments, but was delayed until 20±30 min for the 20±21 day events (Fig. 2). This was primarily the result of soil being drier (nearly 3% less water content) at 20±21 day runs compared to the 5±6 day runs. In addition, the relative amount of runo vs leachate is indicated by the shape of the hydrograph. A hydrograph that shows a rapid initiation followed by a uniform-plateau ¯ow is indicative of high runo whereas, when initiation is gradual and variable over the rain event, then a combination of runo and leachate is indicated (vetch vs non-vetch hydrographs for day 5 in Fig. 2). With the exception of one vetch hydrograph, higher runo was observed from the vetch than non-vetch chambers (Fig. 2). Larger amounts (nearly twice) of both atrazine and metolachlor were recovered in runo and leachate during the 5±6 day rain events than from the 20±21 day rain events (Table 3). In the vetch and non-vetch treatments, average atrazine losses ranged from 15.6 to 19.4% and 7.7 to 9.1% of applied for the 5±6 and 20±21 day events, respectively. Similarly, average metolachlor losses ranged from 7.8 to 8.3% and 2.6 to 5.9% of applied for the same rain events. The lower recovery for the 20±21 day events is probably due in part to increased foliage washo and soil in®ltration. In the 20±21 day chambers, crop residue was subjected to rain for 10±20 min longer than in the 5±6 day chambers before runo started. During this time, more of the atrazine and metolachlor that was washed o crop residue could be leached into the soil where adsorption could occur. In a related study using soil cores, a decrease in soil moisture from 32 to 24 g 1 resulted in a decrease of 11% in the amount of atrazine that was washed o crop residue and leached through intact soil cores (Sigua et al., 1995). In addition, more dissipation would have occurred during the addi-
113
tional 15 days so that the amount of atrazine and metolachlor that was available for runo and leaching losses would be reduced. Further, the relative amounts of both herbicides lost in runo appeared to be related to volume of runo rather than to the vegetation type. For the 5±6 day events, average losses of runo, atrazine and metolachlor were 12.3 l, 16.6% and 7.3% for vetch and 8.7 l, 8.9% and 5.1% for the non-vetch, respectively (Fig. 3). For the 20±21 day events, average losses of runo, atrazine and metolachlor were 7.4 l, 4.6% and 1.6% for the vetch and 10.8 l, 6.7% and 4.0% for the non-vetch, respectively. The amount of atrazine and metolachlor recovered in discharge from the chambers was correlated to the volume of runo and leachate (Fig. 3). Correlations were generally higher (r2 0:89 or greater) for the 5±6 day events while correlations ranged from r2 0:52 to r2 0:93 for the 20±21 day events. The generally lower correlations for the 20±21 day events likely re¯ect the increased washo and in®ltration into soil during the initial time of the rain events. The slope of the metolachlor regression was higher than atrazine for all rain events, indicating higher concentrations of atrazine compared to metolachlor in runo. This observation suggests that metolachlor is less soluble than atrazine, while the reverse is true (metolachlor's water solubility is 530 lg l 1 compared to 33 lg l 1 for atrazine). It appears that the formulation (BiceptIIÒ) that these herbicides were applied has aected the relative solubilities of atrazine and metolachlor. The highest concentrations of both atrazine and metolachlor were reached during the ®rst 500 ml of runo and then decreased during the rest of the rainfall event (Fig. 4). Maximum concentrations of atrazine and metolachlor were 1.9 and 1.1 lg l 1 and 1.4 and 0.7 lg l 1 for the 5±6 day vetch and non-vetch treatments, respectively. For the 20±21 day vetch and non-vetch treatments, maximum concentrations for atrazine and metolachlor decreased to 0.6 and 0.2 lg l 1 and 0.9 and 0.4 lg l 1 , respectively. These consistently higher concentrations of atrazine compared to metolachlor in runo were most likely due to solubility dierences and relatively higher Koc value for metolachlor (500) than for atrazine (150). Formulation may also have aected the amount of herbicide remaining on crop residue after the simulated rain event (Table 4). After the rain event, the vetch and non-vetch crop residues retained 2.5±4 and 5±7 times more metolachlor than atrazine, respectively. The experimental conditions that were established for the 5±6 and 20±21 day laboratory studies were designed to simulate many of the conditions that were encountered prior to the ®rst runo event in 1997 and 1998, respectively. In the laboratory study, soil type, vegetative biomass, initial soil moisture level, killing of the vegetation with paraquat, atrazine and metolachlor formulation and application rate, and time between
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Table 2 Runo, leachate, change in soil-matrix water, and percent of simulated rain applied to each chamber following simulated rainfall events 5±6 and 20±21 days after application of atrazine and metolachlor to hairy vetch and non-vetch soil chambers Treatment
Runo (ml)
Leachate (ml)
Matrix water (ml)
Total (ml)
Rain %
Vetch
Days 5 5 6 6
12250 14863 13160 8990
2240 351 1235 5957
1028 930 1346 962
15518 16144 15741 15927
91.2 94.9 92.5 93.6
Non-vetch
5 5 6 6
6840 6570 10728 10658
6570 6925 3520 1380
2004 1710 2048 2665
15414 15204 16296 14703
90.6 89.4 95.8 86.04
Vetch
20 20 21 21
10780 7105 1944 9928
3585 7655 11260 2905
3900 4842 4615 3900
18265 19602 17819 16733
90.8 97.4 88.5 83.1
Non-vetch
20 20 21 21
11376 9928 10220 11890
3955 5475 3195 365
2990 3640 3185 4550
18321 19043 16600 16805
91.0 94.6 82.5 85.5
Fig. 2. Runo from vetch and non-vetch soil chambers at 5% slope and subjected to 24.3 mm h mm h 1 for 2.5 h (20 and 21 day events).
application and the ®rst runo event were the same or similar to the ®eld conditions. The amounts of atrazine and metolachlor lost in runo during the 5±6 day study were generally within
1
from 2 h (5 and 6 day events) and 23
an order of magnitude of the 1997 ®eld values while amounts lost in the 20±21 day study were much larger (30±800 times higher) than the 1998 ®eld values (Tables 1 and 3). Several factors are likely responsible for the
A.M. Sadeghi, A.R. Isensee / Chemosphere 44 (2001) 109±118
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Table 3 Atrazine and metolachlor recovered as percent of total applied in runo and leachate from soil chambers with and without hairy vetch cover crops Treatment
Days
Herbicide recovered Atrazine Runo
Vetch
5 5 6 6
16.9(13.0)a 22.7(17.5) 17.4(14.0) 9.5(7.3)
Metolachlor Leachate 1.9 0 1.3 7.7
Total
Runo
Leachate
Total
18.8 22.7 18.7 17.2
7.1(5.2) 9.8(7.2) 8.8(6.4) 3.6(2.6)
0.6 0 0.4 2.8
7.7 9.8 9.2 6.4
19.4 2.4 Non-vetch
5 5 6 6
6.7(5.1) 7.5(5.7) 9.3(7.1) 12.1(9.4)
11.3 11.4 3.4 0.7
18.0 18.9 12.7 12.8
8.3 1.5 3.4(2.5) 3.4(2.5) 5.8(4.2) 7.6(5.5)
4.5 4.2 1.9 0.2
15.6 3.3 Vetch
20 20 21 21
7.0(2.0) 4.2(1.2) 0.2(0.06) 2.5(0.7)
3.3 4.9 7.5 1.1
10.3 9.1 7.7 3.6
7.8 0.1 2.1(0.46) 1.7(0.37) 0.1(0.02) 1.1(0.24)
0.9 2.0 2.2 0.3
7.7 2.9 Non-vetch
20 20 21 21
8.5(2.4) 6.9(1.0) 6.0(1.7) 5.4(1.5)
2.6 4.8 1.9 0.1
11.1 11.7 7.9 5.5 9.1 2.9
7.9 7.6 7.7 7.8
3.0 3.7 2.3 1.4 2.6 1.0
4.7(1.03) 4.1(0.90) 4.3(0.95) 2.7(0.60)
0.9 2.2 1.0 0.1
5.6 6.3 5.3 2.8 5.0 1.5
a
Amount of atrazine and metolachlor lost in runo from the ®rst 30.5 mm and 18.5 mm of rain for the 5±6 and 20±21 day events, respectively. Rain amounts equal 1997 and 1998 rains, respectively.
dierences between the laboratory and ®eld studies. First, more simulated rain was applied to the soil chambers (48.6 and 57.5 mm for the 5±6 and 20±21 day events, respectively) than that which fell during the ®eld runo events (30.5 and 18.5 mm for 1997 and 1998, respectively). For comparative purposes, an average of 77 and 73% of the atrazine and metolachlor, respectively, that was lost during the 5±6 day events occurred in the ®rst 30.5 mm rain and an average of 28% and 22% of the atrazine and metolachlor, respectively, that was lost during the 20±21 day events occurred in the ®rst 18.5 mm rain. Adjustment of the values in Table 3 to these loss rates narrows the differences between the laboratory and ®eld values, especially when comparing the 5±6 day and 1997 results. The adjusted means for the 5±6 day data were about twice as high as the 1997 means, but were not statistically dierent. Second, small rain events prior to the runo event, was probably even more important than rainfall amounts in accounting for the dierences between ®eld and laboratory runo losses. In 1997, a 2 and 16 mm rain fell 2 and 4 days after application and in 1998, a series of six 2±6 mm rains fell 5±24 days
after application (Fig. 1). These small rain events likely washed some of the atrazine and metolachlor o the crop residue and into soil, thus reducing the amount available for transport. A recent study has shown that 5±10% of the atrazine applied to several types of crop residue will be washed o in the ®rst 2 mm of simulated rain (Isensee et al., 1998). Another study (Shipitalo et al., 1990) demonstrated the eect of a small initial rain on later transport. They reported that a single 5 mm, non-leaching rain prior to a high-intensity rain reduced by 50%, the amount of atrazine leached through blocks of soil compared to blocks of soil receiving only the high-intensity rain. In addition to the direct eects of the rains, the wetting-drying of the tissue and soil may have enhanced microbial degradation and increased volatilization losses. Variations in herbicide volatilization rates of an order of magnitude or more were found to occur due to diurnal changes in soil-surface moisture levels (Glotfelty et al., 1984). The coecient of variation in the runo losses between plots for both years ranged from 53 to 88 while the variation in the runo losses from the laboratory were 34±48. The lower variability associated with the laboratory
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Fig. 3. Correlations between the amount of atrazine and metolachlor recovered in the runo and leachate volumes.
Fig. 4. Concentrations of atrazine and metolachlor in runo from vetch and non-vetch chambers for the 5±6 and 20±21 day events. Each data point represents mean standard deviation of the four replications.
A.M. Sadeghi, A.R. Isensee / Chemosphere 44 (2001) 109±118 Table 4 Atrazine and metolachlor remaining on crop residue as percent of total applied after simulated rainfall Days after application
Vegetation
Amount Recovered Atrazine
Metolachlor
5±6
Vetch Non-vetch
3.3 1.9
12.8 9.6
20±21
Vetch Non-vetch
2.7 0.7
6.7 4.6
runo losses was expected because each chamber was the same size and at the same slope whereas, each ®eld plot was slightly dierent in size, and slope varied from 2 to 6%. Runo variability could have been reduced even further in the laboratory study if the drains had been closed so that only runo was measured.
4. Conclusions We developed a laboratory system, using large-scale soil chambers and rainfall simulation, for simulating actual ®eld conditions. Based on the laboratory ®ndings and ®eld results, the following, both positive and negative, correlations can be drawn: 1. Although we used relatively large-sized boxes as the experimental units, compared to the size of the ®eld plots, there are still dramatic dierences in scale. In the boxes, the entire surface area is contributing to herbicide runo, whereas in the ®eld, for the rainfall intensities we observed, it seems likely that only the down-slope portion (lower) of the ®eld was contributing to the herbicide runo. 2. Signi®cantly lower concentrations of both atrazine and metolachlor were observed, respectively, from the day 25 event compared to day 5 of the ®eld studies, and from the day 20±21 events compared to day 5±6 of the laboratory simulations. 3. On an average, there were no signi®cant dierences in the amount of runo and the losses of atrazine and metolachlor with respect to vetch or non-vetch, both in the ®eld, from the runo event occurring on day 5 in 1997, and in the laboratory, from the 5±6 day runo simulations. Therefore, it appears that vetch cover does not provide additional runo-retarding properties in excess of what is aorded by the other volunteer vegetation cover.
Acknowledgements The authors thank Ute Klingebiel and Kerry Laddbush for their assistance in sample collection,
117
processing and analysis. Special thanks to Kerry Laddbush for operating the rainfall simulation and automatic datalogger system and also in helping with producing the graphs.
References Baker, J.L., Johnson, H.P., 1979. Eect of tillage systems on pesticides in runo from small watersheds. Trans. ASAE 22, 554±559. Banks, B.A., Robinson, E.F., 1986. Soil reception and activity of acetochlor, alachlor and metolachlor as aected by wheat (Triticum aestivum) straw and irrigation. Weed Sci. 34, 607± 611. Burgoa, R., Hubbard, R.K., Wauchope, R.D., Davis-Carter, J.G., 1993. Simultaneous measurement of runo and leaching losses of bromide and phosphate using tilted beds and simulated rainfall. Commun. Soil Sci. Plant Anal. 24, 2689±2699. Ghadiri, H., Shea, P.J., Wicks, G.W., 1984. Interception and retention of atrazine by wheat (Triticum aestivum L.) stubble. Weed Sci. 32, 24±27. Glotfelty, D.E., Taylor, A.W., Turner, B.C., Zoller, W.H., 1984. Volatilization of surface-applied pesticides from fallow soil. J. Agric. Food Chem. 32, 638±646. Isensee, A.R., Sadeghi, A.M., 1993. Impact of tillage practice on runo and pesticide transport. J. Soil Water Conserv. 48, 523±527. Isensee, A.R., Sadeghi, A.M., 1999. Quanti®cation of runo in laboratory-scale chambers. Chemosphere 38, 1733±1744. Isensee, A.R., Sadeghi, A.M., Mylavarapu, R.O., 1998. Impact of burn-down herbicides on atrazine washo from vegetation. Chemosphere 36, 13±19. Kenimer, A.L., Mostaghimi, S., Young, R.W., Dillaha, T.A., Shanholtz, V.O., 1987. Eects of residue cover on pesticide losses from conventional and no-tillage systems. Trans. ASAE 30, 953±959. Martin, C.D., Baker, J.L., Erbach, D.C., Johnson, H.P., 1978. Washo of herbicides applied to corn tissue. Trans. Am. Soc. Agric. Eng. 33, 335±345. Nash, R.G., 1990. Solid-phase extraction of carbofuran, atrazine, simazine, alachlor and cyanazine from shallow well water. J. Assoc. O. Anal. Chem. 73, 434±442. Sadeghi, A.M, Isensee, A.R., 1997. Alachlor and cyanazine persistence in soil under dierent tillage and rainfall regimes. Soil Sci. 162, 430±438. Sauer, T.J., Daniel, T.C., 1987. Eect of tillage system on runo losses of surface-applied pesticides. Soil Sci. Soc. Am. J. 51, 410±415. Shipitalo, M.J., Edwards, W.M., Dick, W.A., Owens, L.B., 1990. Initial storm eects on macropore transport of surface-applied chemicals in no-till soil. Soil Sci. Soc. Am. J. 54, 1530±1536. Sigua, G.C., Isensee, A.R., Sadeghi, A.M., Im, G.J., 1995. Distribution and transport of atrazine as in¯uenced by surface cultivation, earthworm population and rainfall pattern. Chemosphere 31 (10), 4237±4243. Smith, M.S., Frye, W.W., Varco, J.J., 1987. Legume winter cover crops. Adv. Soil Sci. 7, 95±139.
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Sorenson, B.A., Shea, P.J., Roeth, F.W., 1991. Eects of tillage application time and rate on metribuzin dissipation. Weed Res. 31, 335±345. Southwick, L.M., Willis, G.H., Bengtson, R.L., 1993. Runo losses of nor¯urazon: Eect of runo timing. J. Agric. Food Chem. 41, 1503±1506.
Teasdale, J.R., 1993. Contribution of cover crops to weed management in sustainable agricultural systems. J. Prod. Agric. 9, 475±479. Truman, C.C., Bradford, J.M., 1995. Laboratory determination of interrill soil erodibility. Soil Sci. Soc. Am. J. 59, 519±526.
Impact of hairy vetch cover crop on herbicide transport under ®eld and laboratory conditions Ali M. Sadeghi *, Allan R. Isensee USDA, Agricultural Research Service, BARC-West, Environmental Chemistry Lab., B-001, R-220, 10300, Baltimore Avenue, Beltsville, MD 20705, USA Received 30 April 2000; received in revised form 21 June 2000; accepted 22 June 2000
Abstract This study was conducted to evaluate the eect of hairy vetch cover crop residue on runo losses of atrazine and metolachlor under both no-till corn ®eld plots and from a laboratory runo system. A 2-year ®eld study was conducted in which losses of atrazine and metolachlor from vetch and non-vetch ®eld plots were determined from the ®rst runo event after application (5 and 25 days after application in 1997 and 1998, respectively). A laboratory study was conducted using soil chambers, designed to simulate ®eld soil, water, vegetation, and herbicide treatment conditions, subjected to simulated rain events of 5, 6, 20 and 21 days after application, similar to the rainfall pattern observed in the ®eld study. Atrazine losses ranged from 1.2 to 7.2% and 0.01 to 0.08% and metolachlor losses ranged from 0.7 to 3.1% and 0.01 to 0.1% of the amount applied for the 1997 and 1998 runo events, respectively. In the laboratory study, atrazine runo losses ranged from 6.7 to 22.7% and 4.2 to 8.5% and metolachlor losses ranged from 3.6 to 9.8% and 1.1 to 4.7% of the amount applied for the 5±6 and 20±21 day events, respectively. The lower losses from the ®eld study were due to smaller rainfall amounts and a series of small rains prior to the runo event that likely washed herbicides o crop residue and into soil where adsorption could occur. Runo losses of both herbicides were slightly higher from non-vetch than vetch ®eld plots. Losses from the laboratory study were related to runo volume rather than vegetation type. Ó 2001 Elsevier Science Ltd. All rights reserved. Keywords: Soil chamber; Runo simulation; Herbicide leaching; Rainfall simulation
1. Introduction Cover crops, such as hairy vetch (Vicia villosa Roth.) are increasingly being used in no-till corn production because they provide signi®cant amounts of nitrogen and early season weed control, and function as a mulch to retard soil moisture loss (Smith et al., 1987; Teasdale, 1993). The hairy vetch is normally killed, mechanically
* Corresponding author. Tel.: +1-301-504-6693; fax: +1-301504-7976. E-mail address: [email protected] (A.M. Sadeghi).
or with herbicides, a few days before corn is planted, resulting in a thick mass of crop residue covering the soil surface. Herbicides applied after corn planting may be largely intercepted by the hairy vetch crop residue. Previous studies have shown that 15±80% of an applied pesticide will be intercepted, depending on the amount and type of crop residue and vegetation (Martin et al., 1978; Ghadiri et al., 1984; Banks and Robinson, 1986; Sorenson et al., 1991). Pesticides are usually washed o crop residue by rainfall, but the quantity that reaches soil is dependent on the rainfall amount and timing (Shipitalo et al., 1990; Sorenson et al., 1991). Many studies have measured runo losses of herbicides under ®eld conditions (Baker and Johnson, 1979;
0045-6535/01/$ - see front matter Ó 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 0 4 5 - 6 5 3 5 ( 0 0 ) 0 0 2 0 7 - 1
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Isensee and Sadeghi, 1993; Southwick et al., 1993). These ®eld runo studies provide an accurate measurement of herbicide transport for the environmental conditions that exist at the time of each runo event. For any one site, studies need to be conducted over a series of years to obtain a representative range of rainfall and antecedent moisture conditions, since climatic variables cannot be controlled in ®eld experiments. Therefore, studies have been set up using small ®eld plots and rain simulators to evaluate rainfall parameters (Kenimer et al., 1987; Sauer and Daniel, 1987). These studies provide needed control of rain parameters but may still be limited to the capacity of the simulator, existing slope, and soil moisture conditions. Laboratory runo simulation systems have been developed because most of the parameters can be controlled (Burgoa et al., 1993; Truman and Bradford, 1995; Isensee and Sadeghi, 1999). However, the diculties associated with packing soil in runo containers and the ability to have adequate replications for large-sized soil chambers have limited laboratory system use primarily to soil erosion studies. The recently developed runo system by Isensee and Sadeghi (1999) includes in its design soil chambers that can simulate most of the soil structural and vegetative conditions that exist in the ®eld, allowing for simulation of actual ®eld situations. The objectives of this study were: (1) to evaluate the eect of hairy vetch cover crop on runo loss of atrazine and metolachlor under ®eld and laboratory conditions; and (2) to examine the comparativeness of laboratory vs ®eld-derived runo data. 2. Materials and methods 2.1. Field studies The ®eld runo evaluations were conducted in an ongoing research site at the Beltsville Agricultural Research Center in Beltsville, MD. The experimental set-up for the site has been described by Sadeghi and Isensee (1997). In brief, the site was divided into eight no-till corn production plots (0.1±0.23 ha) with a slope ranging from 2 to 6%. Each plot was separated with berms that direct surface water ¯ow through ¯umes, each instrumented with an automated ¯ow-meter and water sampler. Field experiments were set up for the 1997 and 1998 corn-growing seasons. In the fall of 1996 and 1997, after corn harvest, hairy vetch was planted at 28 kg ha 1 with a no-tillage grain drill in half of the plots. In 1997, all the plots were sprayed with paraquat (1,10 dimethyl-4-40 -bipyridylium dichloride) on 23rd May, corn was planted on 27th May and atrazine [6-chloroN-ethyl-N-(1-methylethyl)-1, 3,5-triazine-2,4-diamine] and metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2methoxy-1-methylethyl)acetamide] were applied on
28th May. In 1998, all the plots were sprayed with paraquat on 15th May, corn was planted on 17th May and atrazine and metolachlor applied on 19th May. Paraquat, atrazine and metolachlor were applied at 0.56, 1.7 and 2.0 kg ha 1 , respectively. Rainfall and runo amounts and timing are shown in Fig. 1. The automatic samplers were triggered by the ¯ow through the ¯umes and were programed to collect runo samples every 100 gallons during each rain event. Runo samples, collected from each of the eight plots, were taken to the laboratory, placed in glass bottles and stored at 4°C until analyzed. All samples were extracted and analyzed as previously described (Nash, 1990). In brief, atrazine and metolachlor were extracted from water by solid phase extraction (C18 cartridges), eluted with ethyl acetate, and analyzed by gas±liquid chromatography using a nitrogen±phosphorus detector. 2.2. Laboratory studies The laboratory study was set up to roughly simulate the ®eld conditions associated with the 1997 and 1998 ®eld runo events. The rainfall±runo simulation system used in this study has been described by Isensee and Sadeghi (1999). In brief, the system consists of: (1) an adjustable rainfall simulator. This simulator consists of two oscillating linear dripper units that provide simulated rain between radii of 54±112 cm at 0° and 180° over the turntable. Two independent peristaltic pumps supply water to the dripper units; (2) a 2.4-m diameter, 1000-kg capacity, 1-rpm turntable that supports and rotates the soil chambers under the rain simulator; (3) an adjustable elevating system designed to support the soil chambers at the desired slope (0± 20%); (4) tipping bucket gauges and data logger to
Fig. 1. Rainfall amount and timing prior to herbicide application and up to the ®rst runo event for 1997 and 1998.
A.M. Sadeghi, A.R. Isensee / Chemosphere 44 (2001) 109±118
measure and record runo; and (5) soil chambers (described below) for measuring runo and leaching of the herbicides. 2.3. Soil chambers and experimental procedures 16 soil chambers were constructed similar to those described by Isensee and Sadeghi (1999). In brief, the chambers were constructed using 15-mm thick plywood with inside dimensions of 100-cm length by 35cm width by 25-cm depth with top edges cut at a 45° angle to ensure that rain falling on the edges would be directed away from the interior. The chambers were assembled and water-proofed with epoxy resin. Runo from the chambers was collected from a 13-mm i.d., height adjustable drain located on one end of each chamber and leachate is collected from three bottom drains (6-mm i.d.) located at 1, 34, and 67 cm upslope from the runo outlet. Partitions (1:3 1:3 35 cm pieces of aluminum angle) were attached (with silicon cement) to the bottom of the chambers downslope from the 34- and 67-cm drains to aid leachate collection. The soil used in this study is a Hatboro silt loam (®ne-loamy, mixed, non-acidic, mesic, Typic Fluvaquents) and was obtained from a ®eld at the Beltsville Agricultural Research Center, Beltsville, MD. About 1200 kg of surface soil, 0±15 cm depth, was ®eld sieved (1.25 cm) and stored until use (average gravimetric moisture content was 10.3%). This soil had a pH of 6.5, an organic carbon content of 0.7% and a sand, silt and clay content of 34.5%, 49.5%, 16%, respectively. Soil chambers were packed as follows: screen (1 mm2 openings) was placed over the drains and 7 kg of sand (0.5±1.0 mm) was placed at the bottom to a depth of 1.3 mm (top of the partitions). The soil was added to each chamber in nine 7-kg increments plus a ®nal 5-kg increment for a total of 68 kg. After an increment of soil was added to all chambers, they were leveled and packed with a pressure of about 0.15 kg cm 2 before the next addition. This resulted in an estimated bulk density of 1.44±1.52 g cm 3 . After packing, the three drains in each chamber were connected together with tubing and the soil slowly wetted from below until the soil surface was uniformly moist. The tubing was then removed from the drains to allow excess water to drain. 1 day after wetting the soil, the height-adjustable gates were positioned so that the center of the drain was in level with the soil surface and sealed in place with silicone sealant. Five days after wetting the soil, hairy vetch was planted in seven rows across the width of the chambers (63 seeds each chamber) to simulate ®eld seeding methods. Eight chambers were planted to vetch and eight were left fallow. Volunteer vegetative growth was
111
allowed to develop in the fallow chambers as it occurs in a no-till ®eld. Fifteen days after planting the vetch, 200 g of corn stover was uniformly spread on the surface of all 16 chambers to simulate crop residue conditions of no-till ®elds. All chambers were watered as needed. At the time of spraying, the hairy vetch averaged 25 cm in height and provided 100% coverage of the soil surface while the volunteer vegetation in the non-vetch chambers averaged 15 cm in height and provided 75± 90% coverage of the soil surface. Prior to spraying, all chambers had developed cracks between the soil and chamber walls. These cracks were widened to about 3 mm by repetitiously plunging a thin lath between the soil and chamber wall around the perimeter of each chamber. This crack was then ®lled with a 1:1 mixture of sand and bentonite clay to prevent water from ¯owing down along the wall. All chambers were sprayed with paraquat at the rate of 0.56 kg ha 1 (19.6 mg each chamber), 89 days after planting vetch. 4 days after killing the vegetation with paraquat, all the chambers were treated with atrazine and metolachlor as follows: an aqueous solution containing 5.44 mg ml 1 atrazine and 6.58 mg ml 1 metolachlor was prepared by diluting 4.25 ml of the commercial formulation Bicept II (containing 320 g l 1 atrazine and 387.1 g l 1 metolachlor) to 250 ml. 10 ml of this solution was spray applied to the upper 35 by 90 cm area of each chamber which equals the ®eld application rate of 1.7 and 2.1 kg ha 1 atrazine and metolachlor, respectively. Runo experiments were conducted to simulate 5- and 20-day time periods between application of atrazine and metolachlor and the ®rst rainfall/runo event. However, since the turntable could accommodate only four chambers at a time, actual runo runs were conducted on 5, 6, 20 and 21 days after treatment. It was assumed that data obtained from two consecutive days would be adequately similar to represent one time period. Two soil cores, 2-cm diameter by 8-cm depth were taken from each chamber before each run to determine the soil moisture content and the holes were back®lled with untreated soil. For each run, two randomly chosen vetch and two non-vetch chambers were placed on the turntable, elevated to 5% slope and subjected to rain at 24.3 mm h 1 for 2 h (days 5 and 6), and at 23.0 mm h 1 for 2.5 h (days 20 and 21). The longerduration rain events for days 20 and 21 were needed to obtain volumes of runo similar to those obtained for the 5- and 6-day events. Runo from each chamber was channeled into custom-built tipping bucket gauges that delivered 80 ml each tip. Each tip was recorded by a data logger so that a hydrograph could be developed later. The discharge from the gauges ¯owed into beakers that were used to obtain samples. Runo was collected in 80-ml increments (one tip) for samples 1±5, about 320 ml for samples 6±10 and about 480 ml for all remaining sam-
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3. Results and discussion
ples. Time (after the start of the rain simulator) was recorded for each sample to determine the location of each sample on the hydrograph. Leachate was collected in about 300-ml increments. Subsamples (about 20 ml) were taken from all runo and leachate samples for later analysis. After each runo event, two 82 cm2 crop residue samples were taken from each chamber and frozen for later analysis of herbicide residues. Soil cores (0±6 and 6±12 cm deep) were also taken for moisture determinations. Crop-residue samples were extracted and analyzed as previously described (Nash, 1990). In brief, samples were shake-extracted with methanol:water (4:1), ®ltered, and the ®ltrate reduced to about 15 ml in a rotary evaporator. The atrazine and metolachlor were extracted from the aqueous ®ltrate by solid-phase adsorption using C18 cartridges, eluted from the cartridge with ethyl acetate and analyzed by gas±liquid chromatography using a nitrogen±phosphorus detector. Runo and leachate samples were processed as follows: 5 ml water samples were vortex-mixed with 2 ml ethyl acetate in 15-ml centrifuge tubes for 1 min. The ethyl acetate layer was removed with a Pasteur pipette, dried with anhydrous sodium sulfate, and analyzed by GC. Recovery eciencies from solutions containing atrazine at 0.02±3.04 lg l 1 and metolachlor at 0.03±3.87 lg l 1 were 65 and 104%, respectively. Results were not adjusted for recovery.
3.1. Field study The ®rst runo events for 1997 and 1998 occurred between 5 and 25 days after herbicide application, respectively (Fig. 1). In 1997, runo resulted from 30.5 mm rain, 5 days after application while in 1998, runo followed 18.5 mm rain, 25 days after application. In general, the volume of runo in 1997 was greater than in 1998 (data not shown) whereas runo expressed as percent of the rain falling on each plot was similar for the 2 years (Table 1). These results were unexpected based on the rainfall patterns (Fig. 1). Between 1 and 8 days before the 1997 runo event, a total of 78 mm of rain fell compared to only 24 mm rain in the 25 days before the 1998 runo event. However, in 1998, the soil was very wet prior to herbicide application (77 mm rain between 1st and 12th May) and all vegetation was killed with paraquat on 15th May, greatly reducing evapotranspiration losses. Thus, the residual soil moisture level at the time of the runo event was likely high. The amount of atrazine and metolachlor lost in runo in 1997 was much higher than the loss in 1998 (Table 1). Atrazine losses ranged from 1.2 to 7.2% and 0.01 to 0.08% and metolachlor losses ranged from 0.7 to 3.1% and 0.01 to 0.1% for 1997 and 1998, respectively. The much lower losses in 1998 than in 1997 are
Table 1 Runo and herbicide loss from hairy vetch and non-vetch corn production plots Plot no.
Vetch 1 2 3 4
Non-vetch 1 2 3 4 a
Plot size (ha)
0.16 0.23 0.16 0.12
0.16 0.23 0.16 0.10
1997a
1998a
Runob %
Atrazinec %
Metolachlor %
Runo %
Atrazine %
Metolachlor %
nsd ns 12.9 25.7
nde nd 2.8 5.0
nd nd 1.0 2.0
10.0 4.5 12.9 ns
0.03 0.01 0.04 nd
0.01 0.01 0.03 nd
0.03 0.02f
0.02 0.01
0.02 0.03 0.01 0.08
0.03 0.06 0.02 0.10
0.04 0.03
0.05 0.04
8.3 18.3 21.6 29.0
1.2 3.1 4.5 7.2
0.7 1.7 2.8 3.1
4.0 2.5
2.1 1.1
10.5 12.2 5.6 33.8
Runo event occurred between 5 and 25 days after herbicide application in 1997 and 1998, respectively. Runo expressed as percent of rainfall on each plot based on 30.5 and 18.5 mm rains for 1997 and 1998, respectively. c Herbicide loss expressed as a percentage of the amount applied to the ®eld. d No sample collected due to ¯ow-meter's malfunction. e No data. f Mean standard deviation. b
A.M. Sadeghi, A.R. Isensee / Chemosphere 44 (2001) 109±118
likely due to a combination of (1) longer time between application and the ®rst-runo event during which time more dissipation could occur, and (2) a succession of small rainfall events that likely washed herbicides o the crop residue and into soil where adsorption would reduce the amount available for transport. 3.2. Laboratory study In general, the total discharge as runo plus leachate from the soil chambers within each time period was similar (Table 2). However, runo varied widely between chambers in direct proportion to the amount of leaching. The experimental conditions of this study (modest slope [5%] and dense mat of dead vegetation) slowed overland ¯ow and provided adequate time for water in®ltration. However, these parameters were similar to actual ®eld conditions experienced in the ®eld study as described above. Runo from the soil chambers began about 10 min after the start of simulated rain for the 5±6 day treatments, but was delayed until 20±30 min for the 20±21 day events (Fig. 2). This was primarily the result of soil being drier (nearly 3% less water content) at 20±21 day runs compared to the 5±6 day runs. In addition, the relative amount of runo vs leachate is indicated by the shape of the hydrograph. A hydrograph that shows a rapid initiation followed by a uniform-plateau ¯ow is indicative of high runo whereas, when initiation is gradual and variable over the rain event, then a combination of runo and leachate is indicated (vetch vs non-vetch hydrographs for day 5 in Fig. 2). With the exception of one vetch hydrograph, higher runo was observed from the vetch than non-vetch chambers (Fig. 2). Larger amounts (nearly twice) of both atrazine and metolachlor were recovered in runo and leachate during the 5±6 day rain events than from the 20±21 day rain events (Table 3). In the vetch and non-vetch treatments, average atrazine losses ranged from 15.6 to 19.4% and 7.7 to 9.1% of applied for the 5±6 and 20±21 day events, respectively. Similarly, average metolachlor losses ranged from 7.8 to 8.3% and 2.6 to 5.9% of applied for the same rain events. The lower recovery for the 20±21 day events is probably due in part to increased foliage washo and soil in®ltration. In the 20±21 day chambers, crop residue was subjected to rain for 10±20 min longer than in the 5±6 day chambers before runo started. During this time, more of the atrazine and metolachlor that was washed o crop residue could be leached into the soil where adsorption could occur. In a related study using soil cores, a decrease in soil moisture from 32 to 24 g 1 resulted in a decrease of 11% in the amount of atrazine that was washed o crop residue and leached through intact soil cores (Sigua et al., 1995). In addition, more dissipation would have occurred during the addi-
113
tional 15 days so that the amount of atrazine and metolachlor that was available for runo and leaching losses would be reduced. Further, the relative amounts of both herbicides lost in runo appeared to be related to volume of runo rather than to the vegetation type. For the 5±6 day events, average losses of runo, atrazine and metolachlor were 12.3 l, 16.6% and 7.3% for vetch and 8.7 l, 8.9% and 5.1% for the non-vetch, respectively (Fig. 3). For the 20±21 day events, average losses of runo, atrazine and metolachlor were 7.4 l, 4.6% and 1.6% for the vetch and 10.8 l, 6.7% and 4.0% for the non-vetch, respectively. The amount of atrazine and metolachlor recovered in discharge from the chambers was correlated to the volume of runo and leachate (Fig. 3). Correlations were generally higher (r2 0:89 or greater) for the 5±6 day events while correlations ranged from r2 0:52 to r2 0:93 for the 20±21 day events. The generally lower correlations for the 20±21 day events likely re¯ect the increased washo and in®ltration into soil during the initial time of the rain events. The slope of the metolachlor regression was higher than atrazine for all rain events, indicating higher concentrations of atrazine compared to metolachlor in runo. This observation suggests that metolachlor is less soluble than atrazine, while the reverse is true (metolachlor's water solubility is 530 lg l 1 compared to 33 lg l 1 for atrazine). It appears that the formulation (BiceptIIÒ) that these herbicides were applied has aected the relative solubilities of atrazine and metolachlor. The highest concentrations of both atrazine and metolachlor were reached during the ®rst 500 ml of runo and then decreased during the rest of the rainfall event (Fig. 4). Maximum concentrations of atrazine and metolachlor were 1.9 and 1.1 lg l 1 and 1.4 and 0.7 lg l 1 for the 5±6 day vetch and non-vetch treatments, respectively. For the 20±21 day vetch and non-vetch treatments, maximum concentrations for atrazine and metolachlor decreased to 0.6 and 0.2 lg l 1 and 0.9 and 0.4 lg l 1 , respectively. These consistently higher concentrations of atrazine compared to metolachlor in runo were most likely due to solubility dierences and relatively higher Koc value for metolachlor (500) than for atrazine (150). Formulation may also have aected the amount of herbicide remaining on crop residue after the simulated rain event (Table 4). After the rain event, the vetch and non-vetch crop residues retained 2.5±4 and 5±7 times more metolachlor than atrazine, respectively. The experimental conditions that were established for the 5±6 and 20±21 day laboratory studies were designed to simulate many of the conditions that were encountered prior to the ®rst runo event in 1997 and 1998, respectively. In the laboratory study, soil type, vegetative biomass, initial soil moisture level, killing of the vegetation with paraquat, atrazine and metolachlor formulation and application rate, and time between
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Table 2 Runo, leachate, change in soil-matrix water, and percent of simulated rain applied to each chamber following simulated rainfall events 5±6 and 20±21 days after application of atrazine and metolachlor to hairy vetch and non-vetch soil chambers Treatment
Runo (ml)
Leachate (ml)
Matrix water (ml)
Total (ml)
Rain %
Vetch
Days 5 5 6 6
12250 14863 13160 8990
2240 351 1235 5957
1028 930 1346 962
15518 16144 15741 15927
91.2 94.9 92.5 93.6
Non-vetch
5 5 6 6
6840 6570 10728 10658
6570 6925 3520 1380
2004 1710 2048 2665
15414 15204 16296 14703
90.6 89.4 95.8 86.04
Vetch
20 20 21 21
10780 7105 1944 9928
3585 7655 11260 2905
3900 4842 4615 3900
18265 19602 17819 16733
90.8 97.4 88.5 83.1
Non-vetch
20 20 21 21
11376 9928 10220 11890
3955 5475 3195 365
2990 3640 3185 4550
18321 19043 16600 16805
91.0 94.6 82.5 85.5
Fig. 2. Runo from vetch and non-vetch soil chambers at 5% slope and subjected to 24.3 mm h mm h 1 for 2.5 h (20 and 21 day events).
application and the ®rst runo event were the same or similar to the ®eld conditions. The amounts of atrazine and metolachlor lost in runo during the 5±6 day study were generally within
1
from 2 h (5 and 6 day events) and 23
an order of magnitude of the 1997 ®eld values while amounts lost in the 20±21 day study were much larger (30±800 times higher) than the 1998 ®eld values (Tables 1 and 3). Several factors are likely responsible for the
A.M. Sadeghi, A.R. Isensee / Chemosphere 44 (2001) 109±118
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Table 3 Atrazine and metolachlor recovered as percent of total applied in runo and leachate from soil chambers with and without hairy vetch cover crops Treatment
Days
Herbicide recovered Atrazine Runo
Vetch
5 5 6 6
16.9(13.0)a 22.7(17.5) 17.4(14.0) 9.5(7.3)
Metolachlor Leachate 1.9 0 1.3 7.7
Total
Runo
Leachate
Total
18.8 22.7 18.7 17.2
7.1(5.2) 9.8(7.2) 8.8(6.4) 3.6(2.6)
0.6 0 0.4 2.8
7.7 9.8 9.2 6.4
19.4 2.4 Non-vetch
5 5 6 6
6.7(5.1) 7.5(5.7) 9.3(7.1) 12.1(9.4)
11.3 11.4 3.4 0.7
18.0 18.9 12.7 12.8
8.3 1.5 3.4(2.5) 3.4(2.5) 5.8(4.2) 7.6(5.5)
4.5 4.2 1.9 0.2
15.6 3.3 Vetch
20 20 21 21
7.0(2.0) 4.2(1.2) 0.2(0.06) 2.5(0.7)
3.3 4.9 7.5 1.1
10.3 9.1 7.7 3.6
7.8 0.1 2.1(0.46) 1.7(0.37) 0.1(0.02) 1.1(0.24)
0.9 2.0 2.2 0.3
7.7 2.9 Non-vetch
20 20 21 21
8.5(2.4) 6.9(1.0) 6.0(1.7) 5.4(1.5)
2.6 4.8 1.9 0.1
11.1 11.7 7.9 5.5 9.1 2.9
7.9 7.6 7.7 7.8
3.0 3.7 2.3 1.4 2.6 1.0
4.7(1.03) 4.1(0.90) 4.3(0.95) 2.7(0.60)
0.9 2.2 1.0 0.1
5.6 6.3 5.3 2.8 5.0 1.5
a
Amount of atrazine and metolachlor lost in runo from the ®rst 30.5 mm and 18.5 mm of rain for the 5±6 and 20±21 day events, respectively. Rain amounts equal 1997 and 1998 rains, respectively.
dierences between the laboratory and ®eld studies. First, more simulated rain was applied to the soil chambers (48.6 and 57.5 mm for the 5±6 and 20±21 day events, respectively) than that which fell during the ®eld runo events (30.5 and 18.5 mm for 1997 and 1998, respectively). For comparative purposes, an average of 77 and 73% of the atrazine and metolachlor, respectively, that was lost during the 5±6 day events occurred in the ®rst 30.5 mm rain and an average of 28% and 22% of the atrazine and metolachlor, respectively, that was lost during the 20±21 day events occurred in the ®rst 18.5 mm rain. Adjustment of the values in Table 3 to these loss rates narrows the differences between the laboratory and ®eld values, especially when comparing the 5±6 day and 1997 results. The adjusted means for the 5±6 day data were about twice as high as the 1997 means, but were not statistically dierent. Second, small rain events prior to the runo event, was probably even more important than rainfall amounts in accounting for the dierences between ®eld and laboratory runo losses. In 1997, a 2 and 16 mm rain fell 2 and 4 days after application and in 1998, a series of six 2±6 mm rains fell 5±24 days
after application (Fig. 1). These small rain events likely washed some of the atrazine and metolachlor o the crop residue and into soil, thus reducing the amount available for transport. A recent study has shown that 5±10% of the atrazine applied to several types of crop residue will be washed o in the ®rst 2 mm of simulated rain (Isensee et al., 1998). Another study (Shipitalo et al., 1990) demonstrated the eect of a small initial rain on later transport. They reported that a single 5 mm, non-leaching rain prior to a high-intensity rain reduced by 50%, the amount of atrazine leached through blocks of soil compared to blocks of soil receiving only the high-intensity rain. In addition to the direct eects of the rains, the wetting-drying of the tissue and soil may have enhanced microbial degradation and increased volatilization losses. Variations in herbicide volatilization rates of an order of magnitude or more were found to occur due to diurnal changes in soil-surface moisture levels (Glotfelty et al., 1984). The coecient of variation in the runo losses between plots for both years ranged from 53 to 88 while the variation in the runo losses from the laboratory were 34±48. The lower variability associated with the laboratory
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A.M. Sadeghi, A.R. Isensee / Chemosphere 44 (2001) 109±118
Fig. 3. Correlations between the amount of atrazine and metolachlor recovered in the runo and leachate volumes.
Fig. 4. Concentrations of atrazine and metolachlor in runo from vetch and non-vetch chambers for the 5±6 and 20±21 day events. Each data point represents mean standard deviation of the four replications.
A.M. Sadeghi, A.R. Isensee / Chemosphere 44 (2001) 109±118 Table 4 Atrazine and metolachlor remaining on crop residue as percent of total applied after simulated rainfall Days after application
Vegetation
Amount Recovered Atrazine
Metolachlor
5±6
Vetch Non-vetch
3.3 1.9
12.8 9.6
20±21
Vetch Non-vetch
2.7 0.7
6.7 4.6
runo losses was expected because each chamber was the same size and at the same slope whereas, each ®eld plot was slightly dierent in size, and slope varied from 2 to 6%. Runo variability could have been reduced even further in the laboratory study if the drains had been closed so that only runo was measured.
4. Conclusions We developed a laboratory system, using large-scale soil chambers and rainfall simulation, for simulating actual ®eld conditions. Based on the laboratory ®ndings and ®eld results, the following, both positive and negative, correlations can be drawn: 1. Although we used relatively large-sized boxes as the experimental units, compared to the size of the ®eld plots, there are still dramatic dierences in scale. In the boxes, the entire surface area is contributing to herbicide runo, whereas in the ®eld, for the rainfall intensities we observed, it seems likely that only the down-slope portion (lower) of the ®eld was contributing to the herbicide runo. 2. Signi®cantly lower concentrations of both atrazine and metolachlor were observed, respectively, from the day 25 event compared to day 5 of the ®eld studies, and from the day 20±21 events compared to day 5±6 of the laboratory simulations. 3. On an average, there were no signi®cant dierences in the amount of runo and the losses of atrazine and metolachlor with respect to vetch or non-vetch, both in the ®eld, from the runo event occurring on day 5 in 1997, and in the laboratory, from the 5±6 day runo simulations. Therefore, it appears that vetch cover does not provide additional runo-retarding properties in excess of what is aorded by the other volunteer vegetation cover.
Acknowledgements The authors thank Ute Klingebiel and Kerry Laddbush for their assistance in sample collection,
117
processing and analysis. Special thanks to Kerry Laddbush for operating the rainfall simulation and automatic datalogger system and also in helping with producing the graphs.
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