Pestic. Sci. 1973, 4, 665-675

Availability of Linuron to Plants in Different Soils

Allan Walker

National Vegetable Research Station, Wellesbourne, Warwick (Manuscript received 25 January I973 and accepted 6 March 1973)

Total uptake of linuron by wheat seedlings in nutrient solutions was close to that expected from the product of the amounts of water transpired by the plants and the concentrations of herbicide in solution. Uptake from 19 different soils was less than the amount supplied by mass-flow when the concentrations of linuron in the soil solution were estimated from slurry adsorption measurements. Using a pressure-membrane technique, it was shown that the actual soil solution concen- trations of linuron were less than those estimated, and following rewetting of pressure-membrane samples, the rate of redistribution of linuron between the adsorbed and solution phases was slow. The results suggest that under the con- ditions of the uptake experiments, the systems were not in equilibrium, and show that the rates of adsorption and desorption of linuron may be important in deter- mining its availability to plants.

1. Introduction

Previous experiments have shown that the uptake of atrazine by wheat and turnip seedlings growing in different soils can be related to the amounts supplied to the root surface by mass-flow in response to transpiration.' The initial concentrations of atrazine in solution in the different soils could be predicted from slurry adsorption measurements and the changes in soil solution concentration over a period of time could be taken into account by measuring the changes in aqueous extractable atrazine from the soil. In preliminary experiments with linuron,2 the initial soil solution concentrations of this herbicide could not be predicted from slurry adsorption measurements and the evidence indicated that incomplete reversibility of linuron adsorption may have accounted for the discrepancies. In the present experiments, linuron uptake by wheat seedlings from 19 different soils was measured and a pressure-membrane technique was used to extract water from 10 of these soils at low soil moisture content so that the actual concentrations of linuron in solution in these soils could be measured.

2. Experimental and results 2.1. Materials The soils used were obtained from fields at the National Vegetable Research Station and from outside sites. Their analytical data are shown in Table 1. Organic carbon was determined by the Walkley-Black method and clay contents were determined by pipette

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666 A. Walker

separation. The herbicide used was linuron, ''C-carbonyl-labelled with specific activity 6.85 pCi/mg, or unlabelled.

TABLE 1. Soil properties and herbicide adsorption

Soil no.

1 2 3 4 5 6 I 8 9

10 1 1 12 13 14 15 16 17 18 19

Carbon ( %) - .-

0.61 0.70 0.77 0.85 0.90 0.94 1.01 1.07 1.30 1.30 1.30 1.32 1.32 1.48 1.58 1.71 1.83 2.02 2.50

Clay ( %) -

18 16 23 20 19

5 11 21 9

15 10 14 18 21 34 21 18 11 13

Freundlich constants . Soil moisture K n content (ml/g)

5.39 0.96 0.15 3.65 0.96 0.15 4.38 0.84 0.20 6.82 0.94 0.17 5 . 0 0 0.93 0.20 6.01 0.89 0.30 5.48 0.95 0.15 5.10 0.95 0.35 1.45 0.94 0.25 7.04 0.94 0.25 6.14 0.95 0.17 6.10 0.98 0.15 5.64 0.90 0.20 6.23 0.90 0.25 5.90 0.92 0.35

11.19 0.94 0.20 12.67 0.97 0.20 10.96 0.97 0.25 14.71 0.91 0.20

-

Estimated solution concentration (jigilitre)

20.2 29.9 14.8 14.5 19.1 11.8 19.0 19.8 13.1 13.9 15.3 19.3 14.8 13.2 15.4 8.6 8.8 8.1 1.5

- - ___

2.2. Linuron adsorption Linuron adsorption by the different soils was measured by shaking duplicate 4 g amounts of 2-mm sieved air-dry soil with 20 ml herbicide solution. Five initial solution concentra- tions in the range 50 to 1000 pg/litre were used and each solution contained labelled linuron at 50 pg/litre. After 24 h shaking the suspensions were centrifuged and duplicate 2 ml subsamples of the clear supernatants were transferred to counting vials with 5 ml dioxan-based scintillator3 and their activities were determined using a Tracerlab Corumatic 200 liquid scintillation counter. Quench corrections were determined by the internal standard method. Samples of the original herbicide solutions were counted similarly and from the differences between initial and final count rates, herbicide adsorption by the soils could be calculated. Adsorption isotherms fitted the empirical Freundlich equation :

where x / m is the amount of linuron adsorbed by unit weight of adsorbent at equilibrium solution concentration C, and K and n are constants. By plotting log x / m against log C a straight line is obtained with slope n and intercept K. The values for Kand n forlinuron adsorption by the 19 soils are shown in Table 1.

2.3. Linuron uptake from soil by wheat seedlings Linuron absorption by wheat seedlings growing in the soils was measured using the experimental methods described in detail previously.'.' Radioactive herbicide was

xlm = K C" (1)

Availability of linuron to plants 667

uniformly incorporated into separate samples of the different soils to give a concentra- tion of 0.130 pg/g dry soil. Four 7-cm pots were prepared for each soil containing 250 g soil and 5-pre-germinated seeds of wheat (Triticum aestioum L, cv. Cappelle Desprez). The soil moisture content was raised to approximately 75 % pot capacity for each soil (Table 1) by sub-irrigation, and the surface of the soil in each pot was covered with a I-cm layer of polystyrene granules to reduce surface evaporation. After 12 and 19 days, the plants from duplicate pots from each treatment were harvested and the linuron contents of the shoots and roots determined separately.' The soil moisture content was maintained by watering to constant weight at intervals during the experiment and the

I I I

12 days

I I I

19 days

1

19 days f ..

I

/ I I I

Soil solution concentration (pg/ I i t re )

Figure 1 . Relationships between measured shoot concentrations of linuron and estimated soil solu- tion concentrations.

amounts of water transpired by the plants were calculated from the differences in weight loss each day during the experiment between pots with plants and pots with soil only under the same conditions. Soil solution concentrations of linuron were estimated from the adsorption isotherms and from the concentration of herbicide added to the soils and are shown in Table 1.

The relationships between the measured shoot concentrations of linuron at the two harvests and the estimated soil solution concentrations were approximately linear (Figure I), but the closeness of fit of the lines is not as good as that reported previously for atrazine.' In Figure 2 the total uptake of linuron (shoots plus roots) is plotted as a function of the theoretical supply of herbicide to the root surface by mass-flow in response to transpiration (the volume of water transpired by the plant multiplied by the soil solution concentrations of herbicide). The data from all soils fit the same general relationship but in every case the measured uptake was less than the theoretical supply

668 A. Walker

(shown by the full line in Figure 2). The differences between uptake and supply are in general greater than those reported for atrazine,' a result which agrees with that of preliminary experimenk2

2.00 1 I I I

/ 0

0 0

0 0

0

Y I I I I

0 50 I 00 I .50 2 . 0 0

Transpiration x soluiion concentration (pg)

Figure 2. Relationship between total uptake of linuron from 19 soils and supply by mass-flow. .,I2 days; C, 19 days.

2.4. Uptake from nutrient solutions The observed differences between the estimated supply of linuron to wheat seedlings growing in soil and the amounts of herbicide taken into the plant may have resulted from an exclusion of herbicide from the plant or from a preferential uptake of water. To examine these possibilities, wheat seedlings were grown in nutrient solutions, as described previously,' containing linuron at concentrations of 12,24,36 and 48 pg/litre. Harvests were made after 2,4 ,7 ,9 and 11 days when duplicate plants from each concen- tration were divided into root and shoot and their herbicide content was determined. The relationship between the measured uptake by these solution-grown plants and the amounts of linuron supplied by mass-flow is shown in Figure 3. It can be seen that total plant uptake of linuron from nutrient solutions can be more accurately predicted by a

Availability of linuron to plants 669

simple flow theory than can uptake from soil (Figure 2). When plants were grown in soil, a vigorous washing procedure was required to remove the roots from the soil prior to herbicide extraction and it is possible that loosely associated herbicide may have been washed from the roots during this process. Any such loss would lead to an under- estimate of total uptake by these plants. In Figure 3, the relationship between the amounts of linuron translocated to the shoots in the solution-grown plants and the

I I . . a '7

. / / .

. 0.2 0.4 0.6 0.8 1.0

Transpiration x solution concentrotion ( p g )

, , ,

Figure 3. Relationships between total uptake and translocation of linuron by wheat seedlings in nutrient solutions and supply by mass-flow.

670 A. Walker

amounts supplied to the roots in response to transpiration is also shown, from which it can be seen that on average, about 70% of the herbicide supplied to the root surface was translocated. Even this degree of difference, however, does not explain the dis- crepancies recorded for soil-grown plants in Figure 2. These data therefore suggest that the estimates of supply of linuron to soil-grown plants were not accurate.

2.5. Measurements of soil solution concentrations 14C-labelled linuron was incorporated into separate samples of 10 different soils at a concentration of 0.130 pg/g dry soil. Forty g subsamples of these soils were prepared in extraction units of a pressure-membrane apparatus by standard technique^.^ The mois- ture content of each soil was raised to 75 %pot capacity (Table 1) by adding the required

TABLE 2. Estimated and measured soil solution concentrations of linuron

Pressure' Desorption Adsorption" Desorptionb membrane

Soil distribution C W C W C W no. coefficient (pgilitre) (psilitre) (&litre)

. - _ _ _ _ _ _ _ _ _ - - __ - - _ _ _ _ _ -

1 8.8 20.2 14.5 9.0 2 5.9 29.9 21.5 18.6 3 10.4 14.8 12.3 9.0 4 10.7 14.5 12.0 10.4 6 13.0 11.8 9.8 10.9

10 21.6 13.9 5.9 6.7 11 9.9 15.3 12.9 13.8 13 15.6 14.8 8.2 9.8 14 11.2 13.2 11.4 8.4 16 21.4 8.6 6.0 5.2 17 27.2 8.8 4.7 5.2

Solution concentration estimated from adsorption isotherm. Solution concentration estimated from desorption distribution coefficient. ' Concentration in pressure-membrane extract.

amount of water from a micrometer syringe. The units were sealed and allowed to stand for 5 days to allow equilibration of linuron between soil and water to occur. A pressure of 1500 kN/m2 was then applied and the expelled waterwas collected in specimen tubes. At least 3 ml water were collected within 24 h of applying the pressure. Duplicate 1-ml subsamples of the extracted solution were transferred to counting vials with 5 ml scintillator and counted as previously described. Duplicate 10-g subsamples of the linuron-treated soils were extracted with methanol (to measure the total extractable herbicide) or with 0.02 M-calcium chloride (to measure the aqueous extractable fraction) by shaking with 10 ml extractant in centrifuge tubes. After centrifugation, duplicate 2-ml amounts of the clear supernatants were transferred to counting vials and their activity determined. From the relative distribution of activity in the methanol and aqueous extracts, a desorption distribution coefficient could be calculated, from which a second estimate of the soil solution concentrations was derived. The results (Table 2) illustrate that the measured soil solution concentrations of linuron can be more accu- rately estimated from the desorption distribution coefficients than from the slurry

Availability of linuron to plants 671

adsorption measurements. Previous experiments have shown that soil solution con- centrations of atrazine can be predicted from adsorption measurements1 and therefore for comparative purposes, the above experiment was repeated with atrazine. I4C-ring- labelled atrazine (specific activity, 10.22 pCi/mg) was incorporated into each of the soils in Table 2 at a concentration of 0.085 pg/g dry soil. Samples of these soils were prepared as above in the pressure-membrane apparatus, or were extracted with metha- nol or calcium chloride as before. Atrazine adsorption by these soils was measured by the method previously described.' The adsorption isotherms were found to be effectively linear and therefore adsorption could be expressed in terms of a distribution coefficient or The results shown in Table 3 demonstrate that the desorption distribution

TABLE 3. Estimated and measured soil solution concentrations of atrazine

Pressure' Adsorption Desorption Adsorptiona Desorptionb membrane

Soil distribution distribution C W C W CW no. coefficient coefficient (,Witre) (&litre) (mlli tre) - - - __ __ - - ._ - _ - - - - - - - - - - - .

1 0.98 0.95 14.3 76.4 58.5 2 1.69 1.71 45.7 45.2 50.1 3 1.03 1.26 68.3 57.5 49.3 4 1.75 1.72 43.8 44.4 46.3 6 1.68 1.86 42.4 38.9 36.9

10 2.16 2.09 34.9 35.9 30.4 11 1.41 1.50 51.2 50.3 44.7 13 1.43 1.53 51.5 48.3 39.4 14 1.41 1 .so 48.8 48.0 43.4 16 3.24 3.56 44.4 23.3 26.0 17 3.61 3.92 22.0 20.5 21.8

Solution concentration estimated from adsorption measurement. Solution concentration estimated from desorption measurement. ' Concentration in pressure-membrane extract.

coefficients for atrazine were similar to the adsorption distribution coefficients, and that the measured soil solution concentrations were predicted to within 10 or 15 % by the use of the adsorption measurements.

2.6. Changes in soil solution concentration with time To assess possible changes in soil solution concentrations of linuron during an uptake experiment, the above experiment was repeated using one soil only. Twenty-eight subsamples of soil no. 4 containing 0.130 pg/g linuron were prepared in the pressure- membrane units as before but following remoistening to 75 % pot capacity and sealing of the units, the pressure of 1500 kN/m2 was applied to quadruplicate subsamples after 1,4, 8, 1 1, 15, 18 and 22 days. The water extracted from the soils after 8,15 and 22 days was replaced, using the micrometer syringe and duplicate samples were again extracted 2 and 6 days later. At the same time, linuron uptake from this soil by wheat seedlings was measured after 4, 7, 9, 11, 14, 16, 18, 21 and 23 days. At each harvest, samples of soil from the pots were extracted with methanol or calcium chloride as before, and from

672 A. Walker

the relative amounts of herbicide extracted by the different solvents (corrected for the moisture content of the soil), a desorption distribution coefficient was calculated and the effective soil solution concentrations were estimated. The results are shown in Figures 4 and 5 . There was little change in total extractable herbicide during the experimental

O> 0 10 20

Time (days)

30

Figure 4. Soil solution concentrations of linuron estimated from desorption measurements (o), determined in pressure-membrane extracts (A) and determined in pressure-membrane extracts following rewetting (A).

Total transpirotion (ml)

Figure 5. Totaluptakeoflinuronfromsoil no.4asa functionoftheamount of watertranspired. Line A, solution concentration estimated from adsorption measurement; line B, solution concentration estimated from desorption measurement,

Availability of linuron to plants 673

period of23 days(thischanged from0.126 toO.l04pg/gdrysoil), whereas the amounts of linuron extracted into 0.02 M-calcium chloride decreased during this time. This brought about an increase in the desorption distribution coefficient with a resultant decrease in the calculated soil solution concentration (Figure 4). The measured solution concen- trations increased during the first 8 days and thereafter remained approximately con- stant. Desorption after rewetting of the soils in the pressure-membrane units also showed a slow rate of equilibration. The measured solution concentrations, however, were eventually greater than those estimated from the desorption distribution co- efficients. The data from the plant uptake experiment are shown in Figure 5 . Total plant uptake has been expressed as a function of the amount of water transpired by the plants. Line A represents the expected uptake if the soil solution concentration is estimated from the slurry adsorption measurements and line B shows the expected uptake if the soil solution concentration is estimated from the initial desorption distri- bution coefficient. The results show that neither estimate of the solution concentration gives a good assessment of uptake.

3. Discussion

The results in Figures 1 and 2 show that the absorption and translocation of linuron by wheat seedlings is a passive process related to the concentration of herbicide i n solution and the amount of water transpired by the plant. Similar results have been shown for the absorption of simazine from nutrient solution by cotton and oats,6 by barley' and by parrotfeather,* for atrazine absorption from soil by wheat and turnip seedling^,^ for diuron absorption from nutrient solution and from soil by barley,'O and for atrazine and linuron absorption from nutrient solutions by turnip, lettuce, carrot and parsnip." The data in Figure 3 show that 70% of the linuron supplied to the roots of wheat seedlings in nutrient solution was translocated to the shoots. Moyer, McKercher and H a n d o showed that 50 to 60 :< of the diuron supplied to the roots of barley seedlings was translocated and that this proportion was the same when the plants were grown in nutrient solution and in soil. Where uptake from soil was concerned, these authors used the mean value for the soil solution concentration of diuron determined following centrifugation of soil samples at the beginning and end of the uptake experiments. With one soil, this solution concentration decreased from 60 pg/litre to 27 ,ug/litre during the 7-day experimental period. The present experiments with linuron show that the pre- diction of uptake by soil-grown plants requires an accurate assessment of the soil solution concentration of herbicide, and that measurements of adsorption under slurry conditions overestimate the actual soil solution concentrations to which the plants respond under the particular experimental conditions. In preparation of the soil samples prior to measuring herbicide uptake, the required amount of linuron in acetone was added to samples of air-dry soil, the solvent was allowed to evaporate and the soils were rewetted to 8 "/, soil moisture to allow thorough riddle mixing. The soils were then stored in polyethylene bags for 7 days at 10 "C to allow distribution of herbi- cide between soil and water to occur. The pots were finallypreparedcontainingsoil with a moisture content of 75 % pot capacity. It is therefore probable that desorption rather than adsorption would determine the ultimate solution concentration. The data in

674 A. Walker

Table 2 suggest that this is so, and show that the concentration of linuron in the soil solution extracted by a pressure-membrane technique was lower than that predicted from adsorption measurements but with the majority of soils, was close to that predicted from desorption measurements. These data also show that under the experimental conditions, adsorption of linuron was not fully reversible, a conclusion which agrees with that from previous studies on adsorption and desorption of linuron from l3

The results in Table 3 show that atrazine adsorption is more fully reversible than that of linuron, which again agrees with the results from previous experiments.2* ''9 l4 The rate of redistribution between adsorbed and solution-phase linuron was slow (Figure 4). With the particular soil examined, the solution concentration increased until 8 days after sample preparation. The pressure-membrane concentrations in Table 2 were deter- mined after 5 days had been allowed for equilibration, and in 6 of the 10 soils examined the measured concentrations were lower than those estimated from slurry desorption measurements, which suggests that equilibrium had not been established. The data in Figure 5 show how the kinetics of desorption can affect the availability of linuron to plants. When it is assumed that solution concentrations of linuron remain constant, whether estimated from adsorption measurements (line A) or from desorption measure- ments (line B), overestimates of plant uptake are obtained. Under the conditions of an uptake experiment, where water is added to the soil to replace that lost through evapora- tion and transpiration, solution concentrations are unlikely to remain constant, but will be reduced each time the pots are watered and will increase between waterings. The system is unlikely therefore to be in equilibrium, but will be frequently in a state of flux. The results in Figure 5 suggest that this is so. There is also some evidence that the extent of adsorption increases with time (Figure 4), which would also lead to changes in solution concentration with time.

These data therefore show how the dynamics of sorption processes may affect herbicide availability to plants in different soils. Taken in conjunction with the data previously reported for atrazine' which showed how breakdown of herbicide in the soil may also affect availability, they provide further information on some of the rate processes which may be involved in determining herbicide uptake from the soil.

Acknowledgements Thanks are expressed to Du Pont (United Kingdom) Limited for providing the samples of labelled and unlabelled linuron, to Mr H. A. Roberts and Dr D. J. Greenwood for their interest and advice during the course of this work, and to Mrs E. J. Theodorson for technical assistance. The help of Mr R. L. K. Drew, Irrigation Section, Welles- bourne, with the pressure-membrane determinations is also gratefully acknowledged.

References 1. Walker, A. Pestic. Sci. 1972,3, 139. 2. Walker, A. Proc. 11th BY. Weed Control Con$ 1972,2,800. 3. Bray, G. A. Analyt. Biochem. 1960,1,279. 4. Heining, B. J. agric. Engng Res. 1963,8,48. 5. Talbert, R. E.; Fletchall, 0. H. Weeds 1965, 13,46. 6 . Sheets, T. J. Weeds 1961,9, 1. 7. Shone, M. G . T.; Wood, A, V. J. exp. Bot. 1972,23,141.

Availability of linuron to plants

8. Sutton, D. L.; Bingham, S . W. WeedSci. 1969,17,431. 9. Walker, A. Pestic. Sci. 1971,2, 56.

10. Moyer, J. R.; McKercher, R. B.; Hance, R. J. Can. J . Plant Sci. 1972,52,668. 11. Walker, A.; Featherstone, R. M. J. exp. Bot. 1973,24, in press. 12. Hance, R. J. Weed Res. 1967,7,29. 13. MacNamara, G.; Toth, S. J. Soil Sci. 1970,109,234. 14. Moyer, J. R.; McKercher, R. B. ; Hance. R. J. Can. J. Soil Sci. 1972.52.439.

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