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a Inst. of Soils, Water, and Environmental Sci., ARO, Volcani Center, P.O.B. 6 Bet Dagan, Israel 50250
b Univ. of California, Dep. of Soil and Environmental Sci., Riverside, CA 92521-0424
* Corresponding author (meni{at}agri.gov.il)
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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Abbreviations: EC, electrical conductivity • DOM, dissolved organic matter • SAR, sodium adsorption ratio • SOM, solid organic matter
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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Atrazine is usually applied to the soil surface, and its fate in soil depends on the interaction between its molecules and soil components. Transport models often predict the fate of chemicals in soil by using distribution coefficients generated from batch adsorption studies. A frequently used method to describe herbicide adsorption in soil is the Freundlich equation
 | [1] |
In mineral soils, two major types of adsorbing surfaces are normally available to herbicides: soil organic matter and clay. Celis et al. (1997) compared the adsorption behavior of atrazine on soil components such as montmorillonite, ferrihdrite, and soil humic acid, that represented the clay, Fe-oxyhydroxide, and organic matter fractions of the soil, respectively. They found that the atrazine adsorption was higher on humic acid than on montmorillonite, and observed no significant adsorption of this herbicide on ferrihydrite. Borggaard and Streibig (1988) and Johnson and Sims (1993) also found that the organic matter content in soil was the major parameter that affected the adsorption of atrazine in soils.
Because of the importance of the organic matter in herbicide adsorption in soil, the coefficient Kd can be normalized for the natural organic C fraction of the soil to give the organic C coefficient, Koc (Chiou, 1989), as described by Eq. [2]:
 | [2] |
Soil organic matter can be divided into solid and water-dissolved fractions, both of which can associate with herbicides. The distribution of herbicide association between these two fractions could significantly affect herbicide adsorption and mobility in the soil. Solid organic matter is complex and has been described as a three-dimensional C-based network that allows for solute inclusion in intramolecular hydrophobic sites of various sizes and shapes. Adsorption of herbicide on the SOM should decrease its transport in the soil profile (Moorman et al., 2001). In contrast, DOM has been shown to move through the soil profile in the absence of preferential flow pathways (Dunnivant et al., 1992). Consequently, the formation of complexes between DOM and herbicides molecules could increase the herbicide solubility in the soil solution, which, in turn, could increase its mobility in the soil (Chiou et al., 1986; Madhun et al., 1986; Gauthier et al., 1987; Lee and Farmer, 1989; Nelson et al., 1998, 2000; Williams et al., 1999, 2000; Letey et al., 2000).
The association of atrazine with DOM in natural estuarine water that was collected from Chesapeake Bay, MD, was studied by Means and Wijayaratne (1982), who found that the association of atrazine with the estuarine DOM per unit mass of organic matter was 10 times greater than that with soil SOM. The association of DOM with atrazine was also studied by Barriuso et al. (1992), who added atrazine to a 0.01 M CaC12 solution and to an extract from a silty loam soil that had a total organic C content of 13.5 g kg-1. These solutions were left for 8 d at 25°C to allow formation of DOM-atrazine complexes, and then the atrazine adsorptions on silty loam soil were determined with the equilibration batch technique. Atrazine adsorption from the soil extract solution was lower than that from the CaC12 solution. Lower atrazine adsorption was also found when the atrazine was prepared in extract solutions of liquid sludge, solid sludge, farm slurry, soil humic acids, and field drainage water (Barriuso et al., 1992); the authors concluded that formation of complexes between atrazine and DOM in the solutions decreased the atrazine adsorption.
Cultivated soils may include various SOM contents, and an increase in the SOM is usually associated with an increase in the DOM, but not necessarily in the same proportion. Consequently, changing the total organic matter content in a soil may affect the distribution of the herbicide associations with SOM and DOM, respectively, which, in turn, could affect the herbicide transport in the soil. In most of the previous studies (e.g., Barriuso et al., 1992; Moorman et al., 2001), which determined the effects of soil DOM on atrazine adsorption using batch equilibrium techniques, the studied soil was mixed with water atrazine solutions or with its own extract that was previously mixed with atrazine. This mixing could simulate the situation of solution flow in soil in which the organic matter distribution with depth is uniform. However, the SOM content in cultivated soil is usually not uniformly distributed with depth: the SOM content in the A horizon is higher than that in the deeper horizons. Dror et al. (1999a)( b) determined the Kd values of atrazine at various depths in a cultivated field, and found that the highest Kd value was in the 0- to 0.1-m soil layer and the lowest in the 0.7- to 1.2-m layer. In this soil, the SOM content was highest (1.2%) in the 0- to 0.3-m soil layer and it decreased, in general, with soil depth to 0.7% in the 0.9- to 1.2-m soil layer. In contrast, the clay percentage in this soil was 16.2% in the 0- to 0.1-m soil layer, and it increased, in general, with soil depth to 30% in the 0.9- to 1.2-m layer. Measurements of atrazine retention in the soil after its application on the soil surface, drying, and 50 mm of irrigation water indicated that the highest atrazine retention was in the 0- to 0.1-m soil layer, and that further irrigation and rainfall (>200 mm) caused some leaching of the atrazine into the deeper soil layers.
In the case when the SOM content in the upper soil layer is higher than those in the deeper ones, downward movement of the DOM may increase its concentration in the deeper layers, in which the SOM content is relatively low. This could increase the DOM/SOM ratio in the deeper soil layers, with a possible consequent increase in the herbicide transport in the soil profile. On the other hand, because of the low SOM content in the deep soil layers, the leached DOM could be adsorbed on the soil in these layers (Jardine et al., 1989), and thereby decrease the DOM/SOM ratio. It is hypothesized that changes in the DOM/SOM ratio in the soil could affect the atrazine adsorption, its solubility in the soil solution, and its transport in the soil profile. This is the hypothesis that the present study aimed to test.
The method of pesticide application may also affect the herbicide adsorption on the soil. In the field, herbicides are applied directly to the soil surface or are incorporated into it, and the application is followed, in general, by drying before irrigation or precipitation. In previous studies (e.g., Barriuso et al., 1992; Moorman et al., 2001), to determine the effects of DOM on herbicide adsorption on soil, various concentrations of herbicide were usually added to solutions containing a range of DOM concentrations (including zero) and, after a certain equilibrium period, these solutions were mixed with untreated soil in suspension. In this case, no drying period occurred before the irrigation or precipitation. However, it has been found that drying the soil after herbicide application increased the formation of stable herbicide-DOM complexes (Baskaran et al., 1995; Nelson et al., 1998, 2000).
The objective of the present study was to examine the effects of different DOM concentrations in applied solutions, on atrazine adsorption on soil with various SOM contents. To simulate the herbicide application in the field more accurately, the atrazine was applied directly to the soil, dried, and then extracted from the treated soil in water. This water extract, which contained extracted DOM and atrazine in different concentrations, was used in a batch equilibrium procedure with untreated soil.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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250 mm falls only in the winter. The soil samples were air-dried, visible roots and organic residues were removed, and the soil was then ground and passed through a 2-mm sieve. No significant differences among the soil samples, except for the organic matter contents, were observed. The soil samples contained <0.1% CaCO3, the texture was 9.5% clay, 31.8% silt, and 58.6% sand, the dominant clays were vermiculite and kaolinite, and the soil had a cation exchange capacity of 18 cmol kg-1. To obtain a soil with no organic matter, acid-washed quartz sand (1 mm > r > 0.5 mm) and pure kaolin clay from Acros Chemical Company, Pittsburgh, PA (C.A.S. 1332-58-7) were dry-mixed to form an artificial soil with 9.5% clay. Some properties of the soil samples are presented in Table 1. The soil solution phase was obtained by shaking one part of air-dried soil with two parts of water for 2 h and then extracting by centrifuge.
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Soil solutions with various atrazine and DOM concentrations were prepared as follows. An amount of 0.24 kg of soil was spread as a layer of <1 cm in thickness in an aluminum tray, and 24 mL of atrazine solution was applied in 1-mL aliquots with an automatic pipette. The soil was mixed with a spatula for 2 min after each 5 mL of atrazine application, to ensure uniform distribution of atrazine. Following the total atrazine application (24 mL), the treated soil samples were left in a ventilated hood for 60 h at room temperature (23°C) to dry. A preliminary study found that 60 h was sufficient to evaporate all of the acetone. All combinations of soil samples and atrazine concentrations were used.
After the 60 h of drying, 0.08 kg of treated soil were placed in each of three 250-mL Teflon centrifuge tubes to which 160 mL water was added. The tubes were sealed with Teflon-lined caps and shaken mechanically for 2 h. Following equilibration, the suspensions were centrifuged for 10 min at 6500 rpm. The supernatant was removed and the solutions of each combination of soil and atrazine concentration were combined in one beaker. The pH, EC, DOM, and atrazine concentration of these solutions were determined. Since the solutions were extracted from soil with differing organic matter contents that had been treated previously with several different atrazine concentrations, the extract solutions contained zero (extract solution of the artificial soil), low, medium, and high DOM concentrations, and each of these DOM concentrations was combined with various atrazine concentrations. The atrazine concentration in the soil extracts ranged from 0.25 to 1.91 mg L-1 for extracts from Soil A, from 0.3 to 2.02 mg L-1 for extracts from Soil L, from 0.218 to 1.62 mg L-1 for extracts from Soil M, and from 0.11 to 0.84 mg L-1 for extracts from Soil H. These extracts were used for subsequent adsorption experiments.
Adsorption Studies
The atrazine adsorption measurements were conducted by adding 20 mL of soil extract solution to 10 g of untreated soil in a 25-mL Teflon centrifuge tube, sealing the tube with a Teflon-lined cap, shaking the sample for 2 h, and then centrifuging it. After centrifuging, the supernatant was analyzed for pH, EC, atrazine, and DOM concentrations. A preliminary study indicated that >98% equilibrium was achieved in 2 h. These procedures were applied in triplicate to each soil type with each soil extract, at a temperature of 23°C. The amount of atrazine adsorbed by the soil was determined as the difference between the pesticide content of the applied solution and that of the solution after equilibration. Blank samples revealed that no adsorption occurred on the Teflon tubes.
Analysis
Total organic matter in the soil samples was determined with the Walkley-Black procedure (Nelson and Sommers, 1982). Because in soil, most of the total organic matter exists as solid phase, in the present study the total organic matter content was considered to be SOM. The total dissolved C in the solution was determined with a Dohrmann DC-80 C analyzer (Dohrmann/Xertex Corp., Santa Clara, CA). The DOM concentration was calculated from the dissolved C concentration in the solution by assuming that the DOM contains 60% of the organic C on a mass basis. The concentration of the atrazine solution was determined by measuring the 14C radioactivity in a 1-mL aliquot of solution, by means of liquid scintillation counting techniques, with corrections for quenching and background. Activity was measured with a Beckman LS 5000 liquid scintillation counter. Counting rates were converted to concentrations of atrazine in solution.
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RESULTS AND DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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1. The n values for adsorption isotherms of atrazine from Soil A extract for the three soils were all 1 (Table 2). This indicates that for the applied atrazine concentration (<42.0 mg L-1) and in the absence of DOM from the applied solution, the atrazine isotherms were almost linear, and therefore, Kf and Kd were similar (Table 2).
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The Koc values of Soils H, M, and L on mixing with Extract A were: 64, 74, and 238, respectively. The Koc values of Soils H and M were similar, but both were significantly lower than that of Soil L. The assumption in Koc estimation is that the herbicides are adsorbed mostly on the SOM. This assumption is fairly true when the SOM content in the soil is relatively high, so that it masks the clay surface and minimizes its accessibility to the herbicide, thus rendering the role of clay in herbicides adsorption negligible. In Soil L, however, the SOM content was low, and the clay/SOM ratio (w/w) was 68. In this soil, the masking of the clay by organic matter was probably low; therefore, the clay played an important role in atrazine adsorption (Laird et al., 1992; Barriuso et al., 1994), which led to an overestimation of the Koc. The present values of the Koc are in agreement with the findings of Locke (1992), that indicated that when the clay/soil organic C ratio was >30, the role of the clay in atrazine adsorption becomes significant.
Adsorption isotherms of atrazine for the various soils that were mixed with extract of Soil A (no DOM) and for each soil with its extract that contained DOM are presented in Fig. 2. This mixing of each soil with its own extract simulated the situation of soil solution flow, from a soil surface which had previously been treated with atrazine, into the soil profile, when the SOM contents in the soil surface and in the underlying soil layer were similar. Atrazine adsorption on Soils L and M was, in general, significantly higher from Soil A extract than from the soils' own extracts, but this difference decreased as the atrazine concentration in the equilibrium solution increased (Fig. 2). This decrease in atrazine adsorption when there were high atrazine concentrations in the equilibrium solution is discussed below. In contrast, atrazine adsorption on Soil H from Soil A extract was somewhat higher, but not statistically significantly so, than from the Soil H extract. The DOM concentrations in extracts of Soils L, M, and H were 10.1, 41.1, and 156.5 mg L-1, respectively (Table 1). In contrast, for each soil, no significant differences in the pH or in the EC values of the equilibrium solutions were observed, whether the soil was mixed with Extract A or with its own extract (results are not presented). These results and the results displayed in Fig. 2 indicate that mixing a soil with a solution containing DOM could decrease the atrazine adsorption on the soil. However, this effect of the DOM on atrazine adsorption differed among soils with differing SOM contents.
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1, Table 2), the Kf values of these isotherms for differing atrazine concentrations in the equilibrium solution could not be compared. Thus, the Kd value of each isotherm was determined from its linear segment (Table 2), and the quantitative effect of the DOM in the applied solution on atrazine adsorption in each soil was estimated from the Rd ratio that was calculated by using Eq. [3]:
 | [3] |
The DOM molecules can form complexes with the atrazine in the solution. In the present case, it was expected that mixing this solution with soil would decrease the atrazine adsorption on the soil, and an increase of the concentration of the DOM–atrazine complexes in the solution would enhance the reduction of the atrazine adsorption. The Rd values for Soils L, M and H, each soil mixed with its own extract, were 2.68, 1.93, and 1.07, respectively. These results indicate that the effects of the DOM in the applied solutions on reduction of atrazine adsorption diminished in the order: Soil L > Soil M > Soil H. In contrast, the DOM concentrations in the applied solutions diminished in the order: extract of Soil H > extract of Soil M > extract of Soil L (Table 1). Therefore, it can be concluded that the effect of DOM on the reduction of atrazine adsorption did not necessarily increase with increasing DOM concentration in the applied solution. This was probably because of the effect of competition between the SOM and the DOM in the soil on the atrazine association: an increase of the SOM or the DOM content in the applied solution should increase or decrease, respectively, the atrazine adsorption. Therefore, the Rd should be better correlated with the DOM/SOM (w/w) ratio (Rom), as is shown in Fig. 3, than with the DOM concentration in the solution. In Fig. 3, Rd is presented as functions of Rom for the mixing of the various soils with their extracts. A significant, positive linear regression was observed between the Rd and the Rom values for the various soils (Fig. 3), indicating that an increase of the DOM concentration in the applied solution probably led to an increase in the concentration of DOM-atrazine complex, and that when the SOM content in the soil was low, the increase of DOM concentration (an increase in the Rom value) caused a decrease in the atrazine adsorption (an increase in the Rd value).
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The values of the Freundlich equation constant, n, are presented for all adsorption measurements in Table 2. Values of n are normally expected to equal 1 or less (e.g., Singh et al., 1990; Moreau and Mouvet, 1997). However, n values greater than unity were found for Extracts L and M on Soil L and for Extract M on Soil M. This indicates that atrazine adsorption increased with its increasing equilibrium solution concentration. This result is consistent with the hypothesis that DOM-atrazine complexes form and that these complexes decrease atrazine adsorption, except in the cases where DOM is expected to be adsorbed.
Atrazine concentration was determined by measuring 14C activity. This measuring technique does not differentiate between free atrazine and DOM-atrazine complex, but represents the total atrazine concentration. The ratio of total atrazine concentration to DOM concentration for the various extracts increases as the atrazine concentration increased because the DOM content in the extracts of each soil was constant (Table 1) and independent of atrazine concentration. Thus, it is expected that the ratio of free atrazine to DOM-atrazine complex would also increase with increasing atrazine concentration.
It was previously hypothesized that the presence of DOM-atrazine complex would decrease atrazine adsorption except where DOM was adsorbed to the soil. Therefore, in these cases, the proportional increase in atrazine adsorption should be greater than that of atrazine concentration, because there is proportionately less DOM-atrazine complex. The three cases where the n value was greater than unity in Table 2 are consistent with this pattern.
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SUMMARY AND CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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7%), the atrazine-DOM complexes appeared to play a negligible role in atrazine adsorption in the soil. This was probably because of the high SOM content in this soil, which was a dominant factor for atrazine adsorption in the soil, even if the DOM concentration was quite high. These results have implications for the transport of atrazine through a soil profile. For soils in which the organic matter distribution with depth is uniform, the formation of atrazine-DOM complex enhances atrazine mobility, in general. The atrazine–DOM complex decreases the mobility of atrazine if the lower horizons are lower in organic matter than the upper ones (the more typical field case) because of DOM adsorption on the solids.
Received for publication April 30, 2002.
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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