Laboratory leaching studies of oryzalin and diuron through three undisturbed vineyard soil columns

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Laboratory leaching studies of oryzalin and diuron through three undisturbed vineyard soil columns

David Landry

^*

, Sylvie Dousset, Francis Andreux

Centre des Sciences de la Terre, Universiteede Bourgogne, UMR GeeoSol––Microbiologie des Sols INRA A111, 6, Boulevard Gabriel, 21000 Dijon, France

Received 5 February 2003; received in revised form 22 May 2003; accepted 14 August 2003

Abstract

The leaching of diuron and oryzalin through undisturbed soil columns was studied in the laboratory using three vineyard soils from Vosne-Romaneee (Burgundy): a rendosol, a calcosol and a vegetated calcosol. After 845 mm of simulated rainfall in 15 days, soil leachates contained higher amounts of diuron (3.2%, 11.8% and 18.8% of applied diuron, respectively) than oryzalin (0.2%, 4.9%, 3.7%, respectively). A greater proportion of soil extractable residues was obtained for diuron (42.5%, 26.8% and 32.2%, respectively) than for oryzalin (14.7%, 12% and 15.5%, respectively).

The greater mobility of diuron might be related to its higher water solubility (36.4 mg l¹compared with 2.6 mg l¹for oryzalin) and smaller adsorption coefficient (400 l kg¹, compared with 700–1100 l kg¹ for oryzalin). The mobility of the two herbicides was greater in the two calcosols than in the rendosol, not only due to different organic carbon contents but also different soil textures and structures.

Keywords:Diuron; Oryzalin; Herbicide leaching; Vineyard soil; Structured soil; Column experiments

1. Introduction

Diuron and oryzalin may contaminate natural waters, due to their high rate of application. Many studies have reported the presence of diuron in rivers (Gar- mouma et al., 1997; De Almeida Azevedo et al., 2000) and groundwater (Zhang et al., 1997; Schweinsberg et al., 1999) at concentrations higher than 0.1 lg l¹, which is the European maximum threshold limit for drinking water (EEC Directive 80/778). Consequently, agricultural institutions have advised farmers to use oryzalin in vineyards because few studies have reported water contamination by oryzalin. A study by Futch and Singh (1999) showed a higher mobility of diuron than

that of oryzalin, but there has been little additional research comparing the transport of the two molecules in soils. Further research is therefore needed to justify the preferential use of oryzalin.

The present study aims to compare the mobility of diuron and oryzalin through undisturbed vineyard soils under controlled laboratory conditions. Undisturbed soil columns approximate ﬁeld movement of water and solutes better than columns with repacked soils (Smith et al., 1985; Starrett et al., 1996; Sadeghi et al., 2000;

Singh et al., 2002). The use of laboratory columns allowed us to control parameters such as temperature, and water content, and thus study the influence of soil type on herbicide migration while eliminating the cli- matic variation which would have been present at the different sites. The concentrations of parent molecules and metabolites are monitored in the column leachates during the experiment, and within the soil profile at the end of the experiment. It is very important to measure

*Corresponding author. Tel.: +33-3-80-39-3724; fax: +33-3- 80-39-6387.

E-mail address:[email protected](D. Landry).

doi:10.1016/j.chemosphere.2003.08.039

www.elsevier.com/locate/chemosphere

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not only the parent molecules, but also the products of degradation, because some metabolites such as DCPMU and DCPU may be more toxic than diuron (Tixier et al., 2000).

This work was performed with three calcareous vineyard soils of Burgundy chosen for their diﬀerent textures, structures and organic carbon (OC) contents.

These results will aid in assessing the risk of groundwater contamination by the two herbicides under different soil physical and chemical parameters.

2. Materials and methods 2.1. Herbicides

Diuron [3-(3,4-dichlorophenyl)-1,1-dimethylurea]

and its main degradation products, DCPMU [N⁰-3, 4-dichlorophenyl-N-methylurea] and DCPU [3,4-di- chlorophenylurea], and oryzalin [4-(dipropylamino)-3,5- dinitrobenzenesulphonamide] were purchased from Dr.

Ehrenstorfer, GmbH. The main physico-chemical properties of diuron and oryzalin are listed in Table 1.

2.2. Soils and lysimeter sampling

Three vineyard soils from Vosne-Romaneee (Bur- gundy, France) were chosen along a topolithosequence:

a rendosol at the top, a calcosol in the middle of the slope and a vegetated calcosol at the bottom (SRBS, 1998) (or a leptic calcosol and two calcaric cambisol, FAO, 1994). The last treatments of oryzalin were applied in the ﬁeld on April 1999. The three soils, were sampled in the 0–20 cm layer. The main characteristics of the three soils, with a pH of 8, are given in Table 3.

Column extraction of the three soils occurred on 27 of April 2000 and was facilitated by the use of a backhoe to carefully excavate the surrounding soil. A 25 cm long polyvinyl chloride (PVC) pipe with an internal diameter of 20 cm was placed over the soil cylinder, and minimal- expansion foam was injected into the gap between the soil and PVC pipe (to reduce artiﬁcial sidewall ﬂow), and allowed to cure overnight. The soil columns were

20 cm long and 15 cm in diameter. The column was then removed by digging under the PVC pipe, and placing a nylon mesh at the bottom of the column to retain the soil base. Two undisturbed soil columns were extracted for each soil.

2.3. Chemical application and experimental set-up

All the soil columns were brought back to the laboratory and placed on a support base. The base of each column rested on a PVC plate perforated with 2 mm diameter holes. Diuron or oryzalin was applied to each column by pipetting 30 ml of a methanolic solution containing 212 mg l¹of diuron or 452 mg l¹of oryzalin onto the surface of the dry soil to simulate an application rate of 3.6 kg ha¹ active ingredient (a.i.) and 7.7 kg ha¹ a.i. respectively (i.e., the equivalent of twice the agricultural dose). The two herbicides were not applied to the same column in order to avoid possible interac- tions between them. The leaching experiment started 48 h after treatment. Distilled water was applied to the top of each lysimeter at a constant flow rate of 4 ml min¹ (1.3 cm h¹) using a peristaltic pump. A glass fiber filter was placed on the soil surface to ensure homogeneous distribution of the water over the soil surface. After 15 days and the application of 845 mm of simulated rain, an equivalent of 10 pore volumes (20 year yearly average rainfall at Vosne-Romaneee), the irrigation was stopped.

Simulated rain was applied discontinuously during the 15 days of the experiment at 20 C. There was no ponding at the soil surface. The distribution of the water applied is given in Table 2. The pore volume of each soil column was measured as the diﬀerence in weight between the saturated and dried soil.

2.4. Leachate collection and analysis

Eﬄuents were collected at 24 h intervals in high- density polyethylene bottles. Leachate volumes were determined gravimetrically, then water samples were ﬁltered through 0.45lm Millipore membranes. Pesticide residues contained in the leachates were concentrated by solid-phase extraction with a LC-18 bonded silica car- tridge (2 g, Supelclean, Supelco). Before processing the water samples, the cartridges were conditioned with 10 ml of methanol followed by 10 ml of distilled water. The pesticide residues adsorbed by the LC-18 cartridges were eluted using 5 ml of methanol, and evaporated to dryness in a rotary evaporator at 30C. The residues were then dissolved in 3 ml of methanol and stored at)18C prior to analysis. Recovery rates were 86 ± 2.7% for diuron, 84.6 ± 0.4% for DCPMU, 89.6 ± 0.4% for DCPU and 83.3 ± 0.5% for oryzalin. All the sample concentrations have been corrected based on these recovery values. The samples were analyzed for diuron, its three main metabolites, and oryzalin, using a Waters HPLC Table 1

Main physico-chemical properties of diuron and oryzalin (Tomlin, 1997)

Diuron (phenylurea)

Oryzalin (dinitroaniline) Water solubility (25C)

(mg l¹)

36.4 2.6

Adsorption coeﬃcientKoc

(l kg¹)

400 700–1100

Half-life DT50(month) 4–8 1.2

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with a Diode Array Detector with a 25 cm·4.6 mm C18-column packed with Kromasil 5 lm. The mobile phase was acetonitrile–water at 70:30, v/v for the phenylureas and 50:50, v/v for oryzalin. The ﬂow rate of mobile phase was 0.8 ml min¹ for diuron and its me-

tabolites, and 1.2 ml min¹ for oryzalin. UV detection was performed at 251 nm for diuron and DCPMU, 247 nm for DCPU, and 254 nm for oryzalin. Minimum de- tectable levels were 0.3lg l¹ for diuron, DCPMU and DCPU, and 0.75lg l¹for oryzalin.

Table 2

Monthly precipitation at Vosne-Romaneee (20 year average: 1975–1994), day and amount of water applied on the soil column

Month Jan. Feb. Mar. Apr. May Jun. Jul. Aug. Sept. Oct. Nov. Dec.

Precipitations (mm)

74 60 62 59 95 81 55 59 81 77 63 79

Day 1 2 3 4 7 8 9 10 11 12 13 14

Water applied (ml)

1302 1056 1091 1038 1672 1425 955 1038 1425 1355 1108 1390

Table 3

Main soil characteristics for soil columns treated with diuron or oryzalin

Depth (cm) CM^a(%) Sand^b(%) Silt^b(%) Clay^b(%) CaCO3(%) OC^c (%)

Soils treated with diuron Rendosol

0–5 38.3 34.5 38.0 27.5 39.8 2.7

5–10 47.0 36.9 36.9 26.2 42.9 2.1

10–15 45.8 35.5 36.6 27.9 41.4 1.8

15–20 45.7 37.4 34.2 28.4 39.3 1.6

Calcosol

0–5 20.4 20.9 42.5 36.6 16.8 1.8

5–10 41.6 24.0 40.5 35.5 20.8 1.5

10–15 33.3 22.8 40.7 36.5 24.7 1.1

15–20 36.9 24.4 39.7 35.9 25.2 1.0

Vegetated calcosol

0–5 24.0 20.5 44.7 34.8 22.8 2.2

5–10 26.2 22.1 39.4 38.5 22.8 1.6

10–15 27.9 21.0 39.3 39.7 22.6 1.4

15–20 30.2 20.1 39.4 40.5 19.1 1.3

Soils treated with oryzalin Rendosol

0–5 36.7 35.3 38.1 26.6 41.8 2.6

5–10 44.3 35.4 37.7 26.9 42.6 1.9

10–15 43.4 35.4 36.7 27.9 41.0 2.1

15–20 47.8 35.7 35.8 28.5 41.9 1.7

Calcosol

0–5 27.5 20.4 43.5 36.1 16.6 1.1

5–10 28.8 27.4 38.1 34.5 22.8 1.3

10–15 20.6 15.8 42.6 41.6 9.8 0.8

15–20 19.0 17.6 41.8 40.6 11.2 0.8

Vegetated calcosol

0–5 13.8 20.1 46.0 33.9 23.9 2.3

5–10 23.1 21.6 40.7 37.7 23.0 1.4

10–15 25.1 22.2 39.5 38.3 21.8 1.4

15–20 23.8 20.2 40.5 39.3 21.6 1.9

aCM: coarse materials in percentage of the total weight of the air-dried soil.

bGranulometric analysis with carbonates in percentage of the weight of the 2 mm-sieved soil.

cOC: organic carbon.

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2.5. Soil core extraction and analysis

At the end of the percolation period, the soil columns were divided into 5 cm-long sections, air-dried, weighed, and sieved to 2 mm. A 50 g-subsample was taken from each section and stored at )18 C prior to pesticide analysis. To extract the pesticide residues from the soil, 500 ml polypropylene vials containing 50 g of soil and 100 ml of 80/20 (v/v) methanol/water were agitated on a rotary shaker for 10 h. The suspensions were then cen- trifuged for 20 min at 6700g. The soil slurries remaining in the vials were repeatedly extracted 3 times. The three successive extracts were combined and evaporated to dryness in a rotary evaporator at 30 C. The residues were then dissolved in 3 ml of methanol and analysed by HPLC as described above. Recovery rates varied from 94.5% to 100% for diuron, 87.2% to 94.4% for DCPMU, 73.7% to 81.9% for DCPU and from 86.8% to 92.6%

for oryzalin, depending on the soils. The detection limits expressed as a function of soil dry weight (mg kg¹) were 6·10³ lg g¹ for diuron, DCPMU and DCPU, and 0.015lg g¹ for oryzalin.

3. Results and discussion

3.1. Concentrations of herbicide residues in leachates

The mean pore volumes of soil columns were 1150, 1242 and 1355 ml resulting in calculated mean porosities of 32.5%, 35.1% and 38.3% for the rendosol, calcosol and vegetated calcosol, respectively. The mean apparent densities of soils were 1.8, 1.9 and 2.0, respectively. The height of water eluted from the three soil types ranged from 765 to 778 mm. The cumulative percentages of diuron and oryzalin eluted, as a function of the heights of percolated water, are given in Fig. 1.

Seventy-ﬁve percent of the rendosol leachates contained diuron residues, compared to 92% of the calcosol and 100% of the vegetated calcosol leachates. Diuron was detected in the rendosol, calcosol and vegetated calcosol leachates after 218, 110 and 58 mm water, corresponding to 3.3, 1.6 and 0.8 pore volumes, had percolated through the columns. Diuron mobility was greatest in the vegetated calcosol. At the end of the percolation period, 3.2%, 11.8% and 18.8% of the diuron applied was recovered in the leachates of the rendosol, calcosol and vegetated calcosol respectively (Fig. 2). These values are counter to those of Gonzales- Pradas et al. (1998) who detected no diuron in the percolates from a soil with 0.2% OC, even after 200 cm of water was applied. No DCPU was detected in the percolates of any soil. DCPMU was not detected in the percolates of the rendosol, but was measured in the leachates of the calcosol and the vegetated calcosol 7 and 4 days, respectively after diuron application. At the

end of the experiment, 0.6% and 0.8% of DCPMU (based on the amount of diuron applied) were recovered in the leachates of the less organic soils, the calcosol (1.6% OC in the 0–10 cm layer) and the vegetated calcosol (1.9% OC); whereas, no DCPMU was detected in the rendosol leachates (2.4% OC). Despite having greater >2 mm coarse fraction and sandy fraction, less diuron was transported through the rendosol than the two calcosols. Its higher average OC content in the top 10 cm of soil (2.4%) compared to that of the calcosol (1.7%) and the vegetated calcosol (1.9%) could enhance the adsorption of diuron and its metabolite DCPU in the upper layers (Fig. 2). Gonzales-Pradas et al. (1998) also found that diuron is more mobile in a soil with a low OC content. DCPMU was detected earlier and in a slightly higher proportion in the percolates of the vegetated calcosol (0.8%) than in those of the calcosol (0.6%). In the vegetated calcosol, the degradation of diuron might be enhanced by increased microbial ac- tivity in the rhizosphere. This hypothesis is supported by the ﬁndings of Benoit et al. (1999) who showed that the Fig. 1. Cumulative amount of leached diuron (––), DCPMU ( ) and oryzalin (– – –) (% herbicide applied) as a function of cumulative water heights in the rendosol (j), the calcosol (

⁾

and the vegetated calcosol (N_).

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phenylurea herbicide isoproturon has been degraded more quickly in a grassed soil than in a cropped soil. In addition, the vegetated calcosol could enhance the soil structure and subsequently explain the greater concen- tration of diuron in the leachates of this soil, compared to the two others studied soils.

Oryzalin has been shown to adsorb strongly to soil organic matter. Giry and Ayele (1998) measured adsorption coefficients (Kd) of 38.4 and 10.5 l kg¹ in two soils containing 2.4% and 0.7% OC, respectively. In addition, Jacques and Harvey (1979) showed that oryzalin adsorption coefficients increased with soil OC content. Thus, it is not surprising that the amounts of oryzalin recovered in the leachates (0.2%, 3.7% and 4.9%, respectively for the rendosol, vegetated calcosol and calcosol) were inversely correlated to the OC content of the three soils (2.6%, 2.3% and 1.1%, respectively). Similarly, given its higher sorption coefficients and solubility (Table 1), it is not surprising that greater amounts of diuron than oryzalin were recovered from the leachates. However, traces of oryzalin were detected

in 50% of the rendosol leachates, 92% of the calcosol leachates and 100% of the vegetated calcosol leachates.

Furthermore, oryzalin breakthrough occurred simulta- neously with or before that of diuron (Fig. 1). A hypothesis that requires further investigation is that the transport of oryzalin is being facilitated by dissolved OC. To date, we have not found any other study dem- onstrating the presence of oryzalin in soil proﬁle leachates.

3.2. Distribution of pesticide residues in soil proﬁles

The distribution of diuron and oryzalin throughout the 20 cm length columns of the three soils after 845 mm of simulated rain is shown in Fig. 2. The herbicide residues varied (from top to bottom) between 20.3% and 2.6% of applied diuron, and 17.8% and 0.4% of applied oryzalin (Fig. 2).

Altogether, 26.8%, 32.2% and 42.5% of diuron were recovered in the calcosol, the vegetated calcosol and the rendosol, respectively, compared to 12.0%, 15.5%

et 14.7% for oryzalin (Fig. 3). In every layer of each soil, recovery was greater for diuron than for oryzalin and with the highest recovery occurring in the upper layer (0–

5 cm). A similar observation was made by Hogue et al.

(1981) who reported that 35% the applied diuron remained in the 0–10 cm upper layer of a sandy loam and a loam soil column. Greater percentages of diuron than of oryzalin reached the 15–20 cm soil layer (6·, 3·and 10· in the calcosol, vegetated calcosol and rendosol, respectively). These results are in good agreement with those obtained by Futch and Singh (1999) who reported a greater distance of vertical migration for diuron (16 cm) than for oryzalin (4 cm) in a sandy soil. The higher amounts of diuron measured in the percolates might be related to its greater solubility, which is about 14 times that of oryzalin, and its smaller adsorption coeﬃcient, Fig. 2. Distribution proﬁle of diuron (––), DCPMU ( ) and

oryzalin (– – –) in the three studied soils, after 845 mm of simulated rain.

Fig. 3. Mass balance of herbicide residues in the three studied soils (% herbicide applied).

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which is 2–3 times lower than that of oryzalin. In addition, a faster degradation of oryzalin than of diuron may have occurred in the soils. This hypothesis ﬁts well with the lower half-life of oryzalin (5 days) than of diuron (70 days), found by Giry and Ayele (1998) in a sandy loam soil with a 2.4% OC content.

For the three soils, the pesticide migration with depth seemed to coincide with the vertical distribution of OC, but other parameters could be involved. Dousset et al.

(1995) observed that higher sand contents in calcareous clay soils facilitated the transport of water and solutes such as the herbicide atrazine. Sadeghi et al. (2000) found that a more stable structure facilitated the atrazine mobility. However, in the rendosol used in this study, the greater coarse (>2 mm) and sand fractions (Table 2) did not result in faster migration of either herbicides. In this soil, the relatively high OC content appears to enhance diuron and oryzalin sorption, thus, retarding their migration relative to that in the two calcosols. This hypothesis is supported by the results of Gonzales-Pradas et al. (1998) who found that diuron adsorption coeﬃcients (K_d), (1.3–15.7 l kg¹) in the different horizons of a calcareous soil increased with its OC content (0.2–4.6%). The relationship between the adsorption coeﬃcients of diuron and oryzalin with in- creasing soil OC contents has also been observed by other authors (Madhun et al., 1986; Mallawatantri and Mulla, 1992; Giry and Ayele, 1998). In fact, Jacques and Harvey (1979) state that oryzalin adsorption by soils is more closely related to soil organic matter content than to any other soil property.

The recovered percentages of diuron and oryzalin were greater in the upper soil layer of the vegetated calcosol than in the surface layer of the calcosol. These herbicide percentages could be explained by the higher OC content in the upper layer of the vegetated calcosol.

However the use of undisturbed soil columns might lead to diﬀerent soil structures from one column to another, due to ﬁeld heterogeneity. It is also possible that better structure in the vegetated calcosol, related to root- binding is at least partially responsible for the elevated amounts of diuron in the leachates of this soil.

The only diuron metabolite detected in the soil pro- ﬁles was DCPMU. Although this metabolite remained largely in the upper layes, it migrated to a 15 cm depth in the rendosol, and 10 cm depth in the calcosol. Only in the vegetated calcosol did DCPMU remain in the upper layers (Fig. 2). These results are in good agreement with those obtained by Gomez de Barreda et al. (1993) who report that DCPMU reached the 12–18 cm layer of a sandy soil.

3.3. Mass balance

The amounts of diuron + DCPMU, and oryzalin in the four soil-layer extracts and in the column leachates

were summed for each column (Fig. 3). The total amount of applied herbicide was not recovered. Losses of oryzalin (80.9–85.1%) were greater than those of diuron (44.8–56.5%) in the three soils. The losses could have occurred during handling or could be due to volatilization, mineralization, and/or formation of non- extractable residues.

Volatilization of the herbicide molecules was proba- bly negligible due to their low vapour pressures:

1.1·10³ mPa for diuron and <1.3·10³ mPa for oryzalin. Reported diuron mineralization is low, from 1% (Madhun and Freed, 1987) to 5% (Piutti et al., 2002) in 15 days. The degradation of diuron into metabolites other than DCPMU, DCPU could also explain the observed losses. No data regarding oryzalin mineralization is available in the literature. However, based on its half- life values which ranged from 5 days (Giry and Ayele, 1998) to 24–42 days (Krieger et al., 1998), and even 17–

77 days (Gaynor, 1985), it appears to degrade quickly.

The formation of oryzalin metabolites (not identiﬁed due to the unavailability of analytical standards) could explain most of the losses observed. In addition, non- extractable residues may have formed. Indeed, Golab et al. (1975) and Nelson et al. (1983) found that within 1 month, 13–25% of the total oryzalin applied to a soil was in the form of bound residues.

4. Conclusion

This work has shown that diuron mobility is greater than that of oryzalin. The amount of herbicide leached through the undisturbed 20 cm soil column amounted to 3.2%, 11.8% and 18.8% of the diuron applied and 0.2%, 4.9% and 3.7% of the oryzalin applied, in the rendosol, calcosol and vegetated calcosol, respectively. The amount of diuron measured in the organic matter rich 0–5 cm upper layer represented from 37.4% to 62.9% of the recovered herbicide, depending on the soil, compared to 61.2–78.4% for oryzalin. The greater leaching of diuron and its deeper migration into the soil columns might be due to its lower adsorption and higher solubility relative to oryzalin. The differences in oryzalin mobility between soil types seems related to the OC distribution in the profiles; whereas, differences in diuron mobility are correlated with both OC content and soil structure.

Under our experimental conditions, leaching through the three soils, and therefore the risk of potential groundwater contamination, is greater for diuron than for oryzalin. However, oryzalin was present in the ma- jority (50–100%) of the column leachates. The risk of contamination is diﬀerent for each soil type and is related to the soil’s OC content and structure. Diuron metabolites such as DCPMU, detected in the calcosol

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leachates, may also pose a risk of groundwater contamination. Thus, not only the parent molecules, but also the degradation products should be monitored in groundwater. The rapid degradation of oryzalin could result in the formation of great amounts of metabolites.

Unfortunately, none of them could be identified in our study because the standards are unavailable. This prob- lem is both a scientific and legal deficiency given the new EC directive for drinking water, which requires the ana- lyses of parent molecules and their major metabolites (EEC Directive 98/83).

This laboratory leaching experiment using undisturbed soil columns showed that both diuron and oryzalin may be transported through soils. Additional leaching experiments under natural conditions are being conducted in order to conﬁrm these results.

Acknowledgements

The authors would like to thank the Conseil Re- gional de Bourgogne for the ﬁnancial support of this research. In addition, the PhD grant from the Ministeere de l’Education Nationale, de la Recherche et de la Technologie was greatly appreciated.

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