Pestic. Sci. 1976, I, 50-58

Simulation of Herbicide Persistence in Soil II. Simazine and Linuron in Long-term Experiments

Allan Walker

National Vegetable Research Station, Wellesbourne, Warwick C V35 9EF

(Manuscript received 19 September 1975)

The rates of degradation of simazine and linuron were measured in soil from plots not treated previously with these herbicides. Degradation of both compounds followed first-order kinetics and soil temperature and soil moisture content had a marked effect on the rate of loss. With linuron, half-lives increased from 36 to 106 days with a reduction in temperature from 30" to 5°C at 4% soil moisture, and from 29 to 83 days at 12% soil moisture. Similar temperature changes increased the half- life of simazine from 29 to 209 days and from 16 to 125 days at soil moisture contents of 4 and 12% respectively. A computer program which has been developed for simulation of herbicide persistence was used in conjunction with the laboratory data and the relevant meteorological records for the years 1964 to 1968 in order to test the model against previously published field persistence data for the two herbicides. The results with simazine showed a close correspondence between observed and predicted residue levels but those for linuron, particularly in uncropped plots, were satisfactory for limited periods only.

1. Introduction

Experiments were begun in 1963 at the Agricultural Research Council Weed Research Organiza- tion, Oxford to examine possible long-term effects following repeated application of the same herbicides to the same soil. Crop yield and herbicide persistence results for the years 1963 to 1968 were reported by Fryer and Kirkland.' The most extensive persistence data were presented for the herbicides simazine and linuron for the period May 1964 to November 1968. Simazine was applied pre-emergence to maize at 1.68 kg/ha once each year, and was also applied at 3.36 kg/ha at approxi- mately 6 month intervals to plots which were maintained vegetation-free. Linuron was applied to carrots, usually at 0.84 kg/ha pre-emergence, and on three occasions also post-emergence at the same rate about 8 weeks after the first treatment. As with simazine, linuron was also sprayed on bare plots. The rate used was usually I .68 kg/ha and this was applied three times each year. The herbicide persistence data from these experiments provided an opportunity to test the simulation model for herbicide persistence described by Walker2 against data for 5 years in both cropped and uncropped situations. In order to do this, degradation rates of the two herbicides in the Oxford soil were measured at different temperatures and soil moisture contents.

2. Experimental

The soil used was from the unsprayed control plot paired with the vegetation-free simazine treat- ment in the long-term experiment at Oxford. The herbicides used were a commercial wettable powder formulation of simazine (50 % a.i.) and samples of [carbonyl-1*C]-linuron (specific activity, 6.85 mCi/g) and unlabelled linuron.

2.1. Simazine treatments A fresh sample of soil taken from the surface 5 cm of the plot was passed through a 2 mm mesh sieve and allowed to air-dry for 24 h. After this period, simazine was mixed into a subsample of

50

Long-term persistence of simazine and linuron in soil 51

the soil (2.5 kg) by the method previously described3 to give a final concentration of 4.0 pg/g dry soil. Separate 25 g amounts of the treated soil were weighed into wide-mouthed 100 ml conical flasks and the appropriate amount of water was added to each to give the required soil moisture content. Six samples were prepared for incubation at both4and 12% soil moisture and temperatures of 5, 10, 15, 20, 25 and 30°C. Further samples were incubated at 25°C with soil moisture contents of 8 and 16%. The initial weights of all the flasks were recorded and they were returned to these original weights by adding water at 7 day intervals.

Duplicate flasks from each treatment were removed for analysis on three occasions during the subsequent 56 days. The analytical method used for simazine has been described previously.3

2.2. Linuron treatments A methanol solution containing50 pg [14C]-linuron/ml and 750 pg unlabelled linuron/mlwas pipetted over the surface of a sample of the air-dry soil (2.5 kg) to give a final concentration of 4.0 pg linuron/g air-dry soil. The solvent was allowed to evaporate and sufficient water was added to the soil to give a final soil moisture content of approximately 4%. Mixing was achieved by passing the treated soil several times through a 2 mm mesh sieve. Treatments were prepared in 100 ml conical flasks for incubation under the same conditions as those used for simazine (section 2.1).

Duplicate samples were removed for analysis at intervals during the subsequent 56 days. The herbicide was extracted by shaking with 50 ml redistilled methanol on a wrist-action shaker for 1 h. The extracts were filtered and duplicate 2 ml subsamples were transferred to counting vials with 5 ml dioxan-based ~cintillator.~ Activities were determined using a Tracerlab Corumatic 200 liquid scintillation counter. Quench corrections were determined using a sample channels ratio calibration curve.

The extracts remaining after preparation of samples for counting were concentrated to about 5 ml and stored in glass vials at 4°C. Several of these extracts were examined by the thin-layer chromatographic method described by Katz5 to separate linuron from its possible breakdown products. A thin streak of extract was applied to freshly prepared 250 pm layers of silica gel G on 5 x 20 cm glass plates which had been activated for 30 min at 110°C. The plates were developed for 10 cm above the baseline in chloroform-methanol-pyridine (100 : 5 : 1 by volume).

After development the plates were dried and the peaks of activity determined by scraping succes- sive 0.5 cm bands from the plates, dispersing in methanol on 5 cm aluminium planchets and count- ing on a Tracerlab, low-background, end-window Geiger counter.

A further sample (100 g) of the original linuron-treated soil was weighed into a 250 ml conical flask, and 10 ml water was added to give a moisture content of about 14%. Carbon dioxide-free air, saturated with water vapour, was passed through the flask at a flow rate of about 20-30 ml/min. [14C]-Carbon dioxide evolved from the soil was absorbed in 40 ml of a solution of 15 % ethanol- amine in methanol in each of two Drechsel bottles connected in series. At intervals during a period of 70 days, the contents of the bottles were replaced and the solutions removed were made up to 50 ml with methanol. The activity in triplicate 2 ml subsamples was determined by the liquid scintillation counting procedure described above. After the final sample was removed at 70 days, 100 ml redistilled methanol was added to the soil in the incubation flask and the linuron remaining in the soil extracted and determined as before. The extract was also examined by thin-layer chroma- tography to determine the nature of the extracted radioactivity.

3. Results and discussion 3.1. Laboratory experiments The effects of temperature on simazine degradation at 4 and 12% soil moisture are shown in Figure 1 and on linuron degradation in Figure 2. The data are plotted as log concentration against time and the series of straight-line relationships obtained indicates that degradation of both herbicides under these conditions follows first-order kinetics, as shown in earlier experiment^.^^ 8-8

The degradation rates of simazine and linuron under the different conditions can therefore be characterised in terms of half-lives and these are shown for both compounds in Table 1. The results

52 A. Walker

4

2

I

c

0 , 0 Y.

0 4 0

- c - C " 0 V

2

I

0.5

4% Soil moisture

12% Soil moisture

I I I 1 I 0 , 20 40 60

Time (days)

Figure 1. Effects of temperature on simazine degradation at 4 and 12% soil moisture. A, 5"; A, 10"; 0 , 15"; 0, 20"; m, 25"; 0, ~OT.

Table 1. The effects of temperature and soil moisture content on simazine and linuron degradation

Soil moisture content

Half-life (days) at temperature ("C)

( %) 5 10 15 20 25 30

Linuron 4 106 84 67 57 45 36 8 42 12 83 63 53 43 31 29 16 34

Simazine 4 209 140 92 60 40 29 8 28 12 125 80 53 34 26 16 16 23

show that the effects of soil moisture content on the rate of degradation are similar at the different temperatures for any one herbicide. With linuron a reduction from 12% soil moisture (about 75 % of field capacity) to 4% soil moisture (25 % of field capacity) increased the half-life by about 30 % and a similar change with simazine increased the half-life by about 80 %. These increases are similar to those reported by Usoroh and Hance,' and Walker3 for linuron and simazine respectively.

Long-term persistence of simazine and linuron in soil 53

4.0

2.0

- 4% Soil moisture

1 1.0

8 4.0 0

z

0 , 0

1

+

c C

0 V

2.0

1.0 12% Soil moisture

I I 1 I I I

Time ( d o v s )

20 40 60

Figure 2. Effects of temperature on linuron degradation at 4 and 12% soil moisture. A, 5" ; A, 10"; 0, 15"; 0, 20"; ., 25"; 0, 30°C.

200

IOC

- Y) > 0 V - c - 56 I

'c_

I

25

12.5

1 1 1 I .5 5 2c 10

Linuron

1.5 5 10 20 Soil moisture content (%)

Figure 3. Effects of soil moisture content on simazine and linuron degradation. A, 5 " ; A, 10"; 0, 15'; 0.20'; m, 25"; n, 30"c.

54 A. Walker

A feature of the results in Table 1 is the greater effect of both soil temperature and soil moisture content on the rate of degradation of simazine compared with linuron. The half-life of linuron in soil at 30°C and 12% soil moisture is almost twice that of simazine, whereas at 4% soil moisture and 5"C, the half-life of simazine is almost twice that of linuron. The relationships between half- lives and soil moisture content are shown for both simazine and linuron in Figure 3. The straight lines drawn through the data at 25°C show an approximate fit to the empirical equation relating the half-life (H) to soil moisture content (M):

H = A

in which A and B are constants. This equation has previously been shown to fit similar data with propyzamideg, napropamide,2 simazine3 and prometryne.3 The fact that straight lines with similar slope to that at 25°C can be drawn through the two points at the other temperatures is a further illustration that the relative effects of moisture content on degradation rates are similar at different temperatures .

The relationship between the rates of degradation of both herbicides and temperature can be expressed in terms of the Arrhenius equation:

in which H I and Hz are the half-lives at temperatures TI and TZ and AE is the activation energy. The relationships between the half-lives of simazine and linuron and temperature at 4 and 12 % soil moisture are shown in Figure 4, in which the half-life on a logarithmic scale is plotted against

I /Absolute temperature ( K - ' )

Figure 4. Test for the fit of the Arrhenius equation to data for simazine and linuron degradation. 0, 4 % Soil moisture; 0 , 12 % soil moisture.

the inverse of the absolute temperature. From the slopes of the lines, the activation energy for linuron degradation is calculated as 30.2 kJ/mol and for simazine as 57.4 kJ/mol.

The data for linuron degradation in Figure 2 and Table 1 were based on the amounts of radio- activity extracted from the soil expressed as concentrations of linuron. The thin-layer chromato- grams of the extracts from the samples incubated at 25°C and all the extracts at 56 days showed that over 95% of the activity on the plates was located at the RF-value of the parent herbicide (between 0.90 and 0.95), therefore demonstrating that in experiments with [carbonyl-1*C]-linuron changes in extractability of radioactivity give a valid measure of linuron degradation. This con-

Long-term persistence of simazine and linuron in soil 55

clusion is further supported by the results from collection of [14C]-carbon dioxide which are shown in Figure 5. The amount of linuron remaining in the soil (calculated by subtraction of the cumulative loss of [14C]-carbon dioxide from the radioactivity present initially, expressed as linuron), is plotted on a log scale against time of incubation. Also shown is the value for extractable activity, again expressed as linuron, on termination of the experiment at 70 days. The data show a close corre- spondence with first-order kinetics giving a half-life of about 30 days. There was good agreement

Figure 5. Loss of linuron from soil calculated from cumulative amounts o f 14C02 collected (e) and amount o f linuron extracted from the soil after 70 days ( 0).

14 28 42 56 70 84 Time (days)

between the calculated amount remaining after 70 days and that determined. Thin-layer chromato- graphic analysis of the methanol extract at the end of the experiment again showed that over 95 % of the extractable radioactivity was in the form of linuron. These data are therefore consistent with those of Borner,lo which showed that loss of linuron from soil is closely related with loss of the carbonyl group and not associated with buildup of the demethylated or demethoxylated derivatives.

3.2. Simulation of persistence in the field The meteorological records of rainfall (mm/day), evaporation from an open water surface (mm/day) and 10 cm soil temperature ("C) for the period January 1964 to December 1968 were combined in the computer program2 with the respective constants relating half-lives of simazine or linuron with soil temperature and soil moisture content. The weather data used were those recorded at the National Vegetable Research Station since evaporation measurements were not available from the Weed Research Organization. The two stations are approximately 35 miles apart, and the average weather records are very similar. There would have been differences, particularly in the rainfall pattern, on a day-to-day basis, but such differences were probably of little importance when the simulation program was run for long periods. The disappearance curves obtained from use of the simulation program are compared with the published results of Fryer and Kirkland' for the cropped simazine plots in Figure 6 and for the uncropped plots in Figure 7. Similar results for the cropped linuron plots are shown in Table 2, and for the uncropped plots in Table 3. In previous experiments with the simulation model, the determined degradation curves have been drawn following correction of measured residue levels for initial r e c o ~ e r i e s ~ , ~ ~ ~ but in the present study this was not done. The results from the model, shown in Figures 6 and 7 and Tables 2 and 3 were calculated using the actual amounts repeatedly applied, in order to determine whether residue build-up would be pre- dicted and to give the model a strict test against field results.

The results with simazine are encouraging: the simulated disappearance curves are of similar shape to those observed and the predicted residues are often very similar to those determined. With the uncropped plots, the predicted residue at the end of the fifth year is 0.58 kg/ha compared with the determined residue of 0.63 kg/ha. Fryer and Kirkland' conclude that a residue varying from 0.56 to 1.89 kg/ha was present in the soil at the time of the next application of simazine to these plots, and the model predicts a minimum residue of 0.49 kg/ha following spring application and a

56 A. Walker

2 .o

I .o

I

0 c \ W 1

0

I

.< 2.0

cr"

I .c

9 196415 196516

20 40 60 80 100

196718 9 196617 ? P

I

:o ' n ! O

ko 0

yo<, 0 0

Time (weeks)

Figure 6. Persistence of simazine in cropped plots. 0 , Observed; 0, simulated; time 0=27 May 1964.

Table 2. Simulation of linuron persistence in the cropped plots

Weeks Weeks

Date Dose appli- Date Dose appli- after Residue (kg/ha) after Residue (kg/ha)

sprayed (kg/ha) cation Observed Simulated sprayed (kg/ha) cation Observed Simulated

12.6.64 1.12 0 8

5.8.64 0.56 0 7

14 21

25.5.65 0.84 0 8

22.7.65 0.84 0 6

13 21 28 35

0.70 1.12 0.42 0.52 0.98 1.08 0.70 0.60 0.70 0.41

< O . 14 0.27 0.63 0.84

<O. 14 0.38 0.98 (0.98)O 1.22 0.49 (0.57) 0.71 0.42 (0.35) 0.43 0.21 (0.24) 0.29

<0.14 (0.13) 0.15 0.21 (0.18) 0.22

20.6.66 0.84 0 0.56 (0.56)a0.84 8 0.28 (0.25) 0.37

15.8.66 0.84 0 0.77 (0.77) 1.21 10 0.28 (0.34) 0.54 17 0.21 (0.25) 0.39 27 10 .14 (0.15) 0.24

2.6.67 0.84 0 0.77 0.84 6 0.35 0.49

17 0.28 0.16 26 0.28 0.09 33 0.21 0.07 54 <0.14 0.02

Values in parentheses are simulated residues corrected for observed initial recoveries.

Long-term persistence of simazine and linuron in soil 51

I I I I I I I I 100 I20 140 I60 180 2 00 220 240

Time ( w e e k s )

Figure 7. Persistence of simazine in uncropped plots. 0, Observed; 0, simulated; time 0=27 May 1964.

maximum residue of 1.96 kg/ha following autumn application. With the cropped plots, the model predicts the observed rapid initial decline followed by the long persistence of the remaining residue and a correction of the observed data for initial recoveries would give closer fits with the simulated curves than those shown in Figure 6.

With linuron, the overall results are not as good as those with simazine, although on the cropped plots (Table 2), the predictions show some degree of correspondence with the observed residues. With these data, the agreement between simulated and measured residues can be improved follow- ing correction of the simulated values for the observed initial recoveries. An example of this is shown in Table 2 for the plots sprayed on 22 July 1965, 20 June 1966 and 15 August 1966. The values in parentheses between the respective observed and simulated residues are those obtained following correction of the predicted values for the initial recoveries at each spraying date. The corrected values show much closer correspondence with those observed than do the uncorrected values.

The linuron residues in the uncropped plots reported by Fryer and Kirkland' were subject to variation and sometimes showed increases in the determined amounts of linuron with time after spraying. In Table 3 therefore, only the values for the period preceding the date when this first occurred (plot sprayed on 27 July 1965) are shown. From then onwards, the predictions bear little relationship to the observed residue levels, whether corrected for initial recoveries or not. It is not possible to conclude whether this effect is due to errors associated with the original data, or whether they indicate an effect from repeated applications of relatively high rates of linuron on subsequent degradation rates. The residue in December 1968 predicted by the model is approximately 1 kg/ha compared with the determined residue at this time of about 2 kg/ha.

The results from the simulation model therefore predict that repeated applications of simazine

A. Walker

Table 3. Simulation of linuron persistence in uncropped plots

Residue (kgiha) Date Dose Weeks after

sprayed (kg/ha) application Observed Simulated

12.6.64 2.24 0 8

5.8.64 1.12 0 7

I2 28.10.64 1.68 0

6 12 18 23 29

25.5.65 1.68 0 8

27.7.65 1.68 0 6

12

1.82 2.24 0.91 1.04 2.52 2.16 I .47 1.20 I .26 0.90 2.66 2.58 1.54 I .93 1.26 1.55 0.98 1.25 I .05 0.99 0.77 0.64 I .40 2.32 1.33 1.04 2.94 2.72 3.36" 1.59 3.99 1.03

From this point onwards the observed data become unreliable.

and linuron to this particular soil will not lead to a buildup of residues of either herbicide. They confirm the conclusions of Fryer and Kirkland' that repeated applications of simazine for 5 years did not alter the soil's capacity to degrade the herbicide, but the discrepancies between observed and predicted linuron residues following three applications of relatively high doses in each of five successive years suggest the possibility that some change did occur.

Acknowledgements

Thanks are expressed to Mr H. A. Roberts for his encouragement during this work and to Miss P. A. Brown for technical assistance.

References

1. 2. 3. 4. 5. 6. 7. 8. 9.

10.

Fryer, J. D. ; Kirkland, K. Weed Res. 1970, 10, 133. Walker, A. J. Environ. Qualitv 1974, 3, 396. Walker, A. Pestic. Sci. 1976, 7, 41. Bray, G. A. Analyt. Biochem. 1960, 1, 279. Katz, S. J. Ass. off. unalyt. Chem. 1967, 50, 912. Hance, R. J . Pestic. Sci. 1973, 4, 817. Usoroh, N. J.; Hance, R. J. Weed Re$. 1974, 14, 19. Obien, S. R.; Green, R. E. WeedSci. 1969, 17, 509. Walker, A. In Proceedings European Weed Research Council Svniposium, Herbicides and the Soil Paris, 1973, p. 240. Borner, H. Z. Pflanzenkrunkh Pjlunzenschutz 1965, 72, 51 6.