Pestic. Sci, 1976, I, 41-49

Simulation of Herbicide Persistence in Soil I. Simazine and Prometryne

Allan Walker

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

(Manuscript received 19 September 1975)

The effects of soil temperature and soil moisture content on the rates of degradation of simazine and prometryne were measured under controlled conditions. The time for 50 % disappearance of simazine in a sandy loam soil varied from 37 days at 25°C and 13 % soil moisture to 234 days at 15°C and 7 % soil moisture. With prometryne, changes in soil moisture content had a greater effect on the rate of loss than similar changes with simazine. The time for 50 % disappearance at 25°C was increased from 30 to 590 days with a reduction in soil moisture content from 14 to 5%. With both herbicides, the rate of degradation increased as the initial herbicide concentration decreased and the data suggest that a hyperbolic rate law may be more appropriate than simple first-order kinetics. Degradation curves for three separate field applica- tions of the two herbicides were simulated using the laboratory data and the relevant meteorological records in a computer program. A close fit to the observed pattern of loss of incorporated prometryne was obtained, but prometryne surface-applied was lost rapidly during the first 30-40 days after application. This initial rapid loss could not be predicted by the program. With simazine, the patterns of loss of surface and incorporated treatments were similar, but the simulation model tended to overestimate residue levels. Possible reasons for the discrepancies are discussed.

1. Introduction

Previous experiments have shown that a simple computer model can be used to simulate the per- sistence of propyzamide and napropamide in the The model combines the effects of soil temperature and soil moisture content on the rates of herbicide loss, determined under controlled conditions, with the fluctuations in surface soil temperature and moisture content in the field. The field environment is simulated from standard meteorological records. In its present form, the model only allows for microbial or chemical breakdown of herbicide and does not include terms for volatilisation, photochemical decomposition or leaching. Herbicides of the triazine group are, in general, relatively immobile in the soil,3 and are considered to be lost predominantly through microbial or chemical change.4* 5 Volatilisation and photochemical decomposition can occur with certain members of the group,697 but it is usually considered that these processes are relatively unimportant in determining persistence in the field. Soil temperature and soil moisture content are major factors affecting the persistence of the triazine herbicides in soil;8 warm, moist climatic conditions promote their disappearan~e,~~ l o and persistence is more prolonged in cold, dry cli- mates.l1>l2 Because of this strong influence of climate on persistence and because microbial or chemical change is considered to account for the major loss of the triazine herbicides from soil, the present experiments were made to test the model with the herbicides simazine and prometryne.

2. Experimental 2.1. Materials The soil was a sandy loam with 2 % organic matter, 18 % clay and pH of about 7 and was taken from the surface 5 cm of Gravel Pits field at the National Vegetable Research Station. The same

41

42 A. Walker

soil was used for assessment of persistence in the field. Gravimetric moisture contents at suctions of 0.33, 1.0, 2.0 and 15.0 bar were 11.6, 9.8, 9.0 and 6.1 % respectively. The herbicides used were commercial wettable powder formulations of simazine and prometryne (both 50% a.i.).

2.2. Laboratory experiments A fresh sample of soil was passed through a 2 mm mesh sieve and allowed to air-dry for 24 h. Separate 5 kg quantities of the air-dry soil were treated with simazine or prometryne by adding the required amount of herbicide to the soil in 100 ml water to give a final herbicide concentration of 8 pg/g dry soil. Mixing was achieved by passing the soil several times through a 2 mm mesh sieve. After mixing, the treated soils were stored in polyethylene bags for 24 h at 4"C, whilst duplicate 50 g subsamples from each treatment were dried at 1IO"C. After this 24 h period, separate 600 g amounts from each herbicide treatment were weighed into 1 litre wide-mouthed polyethylene bottles and appropriate amounts of water were added to give the required soil moisture contents. The bottles were loosely stoppered with cotton wool. Samples were prepared for incubation at 25°C with nominal soil moisture contents of 5 , 6, 8, 10, 12 and 14%, and at 15°C with moisture contents of 6 and 12%. Further treatments were prepared with both herbicides for incubation at 12% moisture and 25°C with initial herbicide concentrations of 4 and 2 pg/g air-dry soil, and samples of soil containing 4 pg/g simazine or prometryne were stored at - 10°C for use as recovery checks for the analytical method throughout the experimental period.

At intervals during a period ranging from 10 to 28 weeks, duplicate 20 g subsamples from appropriate treatments were extracted with 50 ml methanol in 100 ml conical flasks by shaking for 1 h on a wrist-action shaker. The extracts were filtered and 25 ml of the filtrate was concentrated on a vacuum rotary evaporator and made up to 5 ml with methanol. A 10 g subsample from each treatment was also dried at 110°C for 24 h to determine the soil moisture content. At weekly intervals through the experiment sufficient water was added to the various treatments to maintain the nominal moisture content of the soil.

The concentration of simazine or prometryne in the methanol extracts was determined using a Pye-Unicam Series 104 gas-liquid chromatograph fitted with a thermionic nitrogen detector (rubidium chloride tip). A glass column (4 mm i.d. x 1.5 m) packed with 2% neopentyl glycol succinate on 80-100 mesh Chromosorb W High Performance was used. The operating temperatures of the injection port, column and detector were 215", 195" and 250°C respectively and gas flow rates were carrier gas (nitrogen) 60 ml/min, air 450-500 ml/min and hydrogen 28 ml/min. The air flow rate was adjusted daily to obtain maximum response. Duplicate 3 pI injections of the unknown solutions were made and the peak heights recorded were compared with those obtained from similar injections of standard solutions of analytical grade simazine or prometryne in methanol to determine the concentration of herbicide. The mean recoveries of the herbicides from the samples stored at - 10°C throughout the experimental period were 98.4+5.07% for simazine and 93.9k 4.16 % for prometryne.

2.3. Field experiments Field plots were prepared on 29 March, 1 May and 7 June 1974. Separate plots (6 x 1.5 m) were sprayed with simazine or prometryne at rates equivalent to 2.0 kg a.i./ha. On each occasion, four plots were prepared for each herbicide and on two of these plots the herbicide was incorporated to a depth of 3-4 cm with a rotary power harrow. On the other two plots the herbicide remained on the surface. Immediately after application, 30 cores (2.5 cm diameter to a depth of 7.5 cm) were taken from each plot at random positions. The cores from each plot were bulked, thoroughly mixed by passing several times through a 2 mm sieve and the total weight of sieved sample per plot recorded. A subsample of 500 g from each treatment was stored at - 10°C until analysis. Further soil samples were taken at intervals during the subsequent 16 to 25 weeks. The herbicide concentra- tion in the soils was determined by the gas-liquid chromatographic method described above.

Persistence of simazine and prometryne in soil 43

3. Results and discussion

3.1. Laboratory experiments 3.1 .I. Evidence for first-order kinetics

The effects of soil moisture content on the degradation of simazine at 25°C are shown in Figure 1 and on the degradation of prometryne in Figure 2. The data are plotted as the concentration

I I I I \2 I

'e

14 28 42 56 70 84 98 Time (days 1

Figure 1. Effects of soil moisture content on simazine degradation at 25°C. Lines: I , 13.2%; 2, 10.7%; 3, 9.7%; 4, 7.9%; 5 , 6.0%; 6, 4.8% soil moisture content.

remaining in the soil on a logarithmic scale against time of incubation in days and the straight lines shown are those of best fit calculated by regression analysis. The straight line relationships obtained indicate that degradation of both compounds followed first-order kinetics. The data for degradation at 15°C (not presented) also showed a close correspondence to the first-order rate law with both compounds. The half-lives calculated from the slopes of the lines together with the correlation coefficients for the lines of best fit are shown in Table I . The first-order rate law has been used to interpret many studies of triazine herbicide degradation in s0ils,l~-~6 and although the present data appear consistent with these kinetics, there is a suggestion that the true picture may be more complex.

3.1.2. Possibility of other kinetics The half-lives at the same temperature and similar soil moisture contents were different for different initial herbicide concentrations (Table 1). This effect was more apparent with prometryne where the half-lives at 25°C and 11-12% soil moisture were 42, 30 and 25 days with initial concentrations of 8, 4 and 2 pg/g respectively. With simazine, under the same conditions, half-lives were 47, 39 and 36 days. Similar effects have been observed for atrazine degradation in a soil perfusion system.17 Hamaker18 has suggested that a hyperbolic rate law may be more appropriate to these data than

44 A. Walker

Table 1. Half-lives for simazine and prometryne degradation

~- Simazine Prometryne

Corre- Half-life Corre- Half-life Tempera- Moisture letion‘ Moisture lationU

ture content coefficient Observedb Calculatedc content coefficient Observedb Calculatedc (“C) ( %) r (days) (days) ( %) r (days) (days)

Initial concentration 8.0 pg/g 25 13.2 -0.978 36.8 38.3 13.7 -0.983 30.3 23.4

10.7 -0.996 46.5 47.0 11.4 -0.995 41.5 39.1 9 .1 -0.994 52.8 51.7 9.8 -0.984 45.5 59.9 7.9 -0.981 63.5 63. I 8.2 -0.986 83.0 100.4 6.0 -0.982 16.6 8 2 . 5 5 . 7 -0.837 239.4 269.9 4.8 -0.991 84.6 101.9 4.8 -0.814 585.3 445.5

15 1 1 . 5 -0.963 112.1 123.9 11.2 -0.982 1 1 1 . 5 108.2 6.6 -0.934 234.1 211.9 6.3 -0.845 513.9 529.8

Initial concentration 4.0 pg/g

Initial concentration 8.0 pgig

25 11.2 -0.974 39.4 45.3 11.7 -0.995 29.6 36.7

25 11.4 -0.980 36.3 44 .5 11.8 -0.991 25.1 35.8

a All values significant at P<0.001. From the slopes of the regression lines. Derived from equations (2) and (3) with appropriate values for the constants.

14 28 42 56 70 84

28 56 84 112 140 168 196 Time (doys)

Figure 2. Effects of soil moisture content on prometryne degradation at 25°C. Lines: 1, 13.1 %; 2, 1 I .4 %; 3,9.8 %; 4, 8.2%; 5 , 5.7%; 6, 4.8% soil moisture content.

Persistence of simazine and prometryne in soil 45

simple first-order kinetics and presented evidence that the initial rates of degradation in these experiments could be represented by the equation :

in which c is the concentration at time t and K1 and K Z are constants. A test for its validity is obtained by plotting I/(dc/dt) against I/c which should give a straight line with slope KZIK1 and intercept l / K i . A test for the fit of equation (1) to the present data is shown in Figure 3 in which the reciprocals of the initial rates of degradation (calculated from the regression equations)

4 0 -

30 -

D -I: : 20-

10 -

0.1 0.2 0.3 0.4 0.5 0.6 0.7 I

lniliol concenlrolion

F i w 3. Test for the fit of a hyperbolic rate equation to data for simazine ( 0) and prometryne ( 0) degradation.

are plotted against the reciprocals of the initial herbicide concentrations. Although only limited data are available, the results suggest that, as in the case of atrazine above, some form of hyperbolic rate law may be more appropriate than first-order kinetics. Hance and McKonels examined the effects of initial concentration on the rates of degradation of atrazine, linuron and picloram in two soil types, and showed that with each compound, the rate was increased as the initial concentration decreased. When the initial rates of herbicide loss are calculated from the mean rate constants over the experimental period of 4 months, the data from these experiments with atrazine and linuron in both soils also fit the hyperbolic rate equation [equation (I)] , although those with pic- loram do not.

3.1.3. Effect of soil moisture and temperature Assuming first-order kinetics, Walker'. 2 has shown that the half-lives for propyzamide and napro- pamide in soil under a range of controlled conditions can be represented by two equations:

Hi = A (2)

and

Equation (2), in which A and B are constants, represents the relationship between the half-life (H) and soil moisture content (M) at constant temperature. Equation (3) is the Arrhenius equation relating the half-life to temperature in which Hi and HZ are the half-lives at temperatures Ti and TZ respectively and A E is the activation energy.

46 A. Walker

- e - r

25

The relationships between the half-lives of simazine and prometryne and soil moisture content are shown in Figure 4 and these demonstrate that the empirical equation (2) gives good approxima- tion to the data. The calculated values for the constants A and B are 461 and 0.96 respectively for simazine and 35 080 and 2.79 for prometryne. Equation (2) was used to calculate half-lives for

Figure 4. Effects of soil moisture content on simazine (0) and prometryne ( 0 ) degradation at 25°C.

simazine at 25°C and soil moisture contents of 11.5 and 6.6 % and for prometryne at moisture contents of 11.2 and 6.3%. These values were substituted in equation (3) with the appropriate half-lives at the same soil moisture contents at 15°C (Table 1). An activation energy of 61.5 kJ/mol was calculated for simazine degradation and 56. I kJ/mol for prometryne degradation. The half- lives calculated by combining equation (2) with equation (3) for the range of conditions used in these experiments are shown for both simazine and prometryne in Table 1. These results show that, as with propyzamide and napropamide, these two equations represent the data reasonably well.

One of the main features of the results in Table 1 is the relatively large effect of soil moisture content on the rate of loss of prometryne. A change from approximately 80 to 25 % of field capacity increased the half-life by a factor of 19.3 compared with only 2.3 with simazine. There are few data in the literature with which to compare these values, but the relative effects of soil moisture content on the rates of degradation of propyzamide’ and napropamidez in the same soil were similar to those reported here with simazine. Usoroh and Hance20 reported that a decrease in soil moisture content from 75 to 25 % of water holding capacity increased the half-life of linuron by a factor of about 1.5.

3.2. Field experiments The results from the field experiments are shown for simazine in Figure 5 and for prometryne in Figure 6. The data are plotted as percentages of the amounts present initially against time in days. Initial recoveries of the herbicides varied considerably bztween plots and ranged from 60 to 107% of the amounts expected. However, even when there were large differences between replicate plots, the same relative differences remained throughout the experimental period. An example of this is shown in Table 2 for the incorporated applications of simazine made on 1 May, the initial recoveries from which showed the greatest difference between replicates (64 and 98%). Correcting for these recoveries improves the agreement between replicates, suggesting that spraying rather than sampling errors were responsible for the initial differences.

At each spraying date, the pattern of loss of surface-applied simazine (Figure 5 ) was similar to that given by the respective incorporated treatment but with prometryne (Figure 6) there were marked differences between treatments : the final residue level which remained following incorpora- tion was between three and four times greater than that from surface treatments. The data suggest an initial relatively rapid loss of prometryne from the soil surface during the first 30-40 days. It

Persistence of simazine and prometryne in soil 41

r r

* C 3

E a

0 40 80 120 160 200 40 80 120 160 200

Time (days)

Figure 5. Persistence of simazine in the field (Time 0, 29 March 1974). Symbols refer to experimental values, dashed lines to computer simulations. (a) Incorporated ; (b) surface.

40 20

E 4

r

Time ( d a y s )

Figure 6. Persistence of prometryne in the field (Time 0, 29 March 1974). Symbols refer to experimental values, dashed lines to computer simulations. (a) Incorporated; (b) surface.

has been suggested that under certain conditions, volatilisation may account for a significant loss of atrazine from soil,6 and since the vapour pressure of prometryne is greater than that of a t r a ~ i n e , ~ such losses might also occur with this herbicide. Talbert et have demonstrated significant injury to cotton seedlings by prometryne vapours in closed containers following application of the herbicide to the surface of soils. Although these effects were, in general, increased with increasing temperature and soil moisture content, significant responses were obtained with air-dry soil when plants were grown for a 12 h day at 31 "C and a 12 h night at 24°C. In the present experiments, surface temperatures in the field within this range would almost certainly have occurred for certain periods. Spencer et aLz2 presented data from which a "potential volatilisation rate" for a surface deposit of a pesticide may be calculated and showed that the values obtained were similar to those observed for certain insecticides. Similar calculations for prometryne give potential rates of volatili- sation of 0.071 kg/ha per day at 30T, and 0.018 kg/ha per day at 20°C. Although these figures must be at best approximate, they suggest that significant losses might occur through volatilisation when there is a period of high temperature after application. In the present experiments, however,

48

Table 2. The agreement between replicate treatments for incorporated simazine; plot prepared 1 May 1974

A. Walker

~ _ _ _ _ _ ~

kg/ha determined % of initial recovery in replicate in replicate

Days 1 2 1 2

0 5 9

14 29 42 61 71 85 98

112 131 142

1.28 1.95 1 . 1 5 1.70 1.17 1.56 1.33 1.78 1.10 1.41 0.83 1.18 0.66 0.93 0.63 0.75 0.46 0.67 0.53 0.66 0.49 0 . 5 5 0.20 0.32 0.26 0.38

-

90 91

104 86 65 52 49 36 41 38 16 20

- 87 80 91 72 61 48 39 34 31 28 16 20

losses of about 1 kg/ha were recorded in the first 3040 days after application, and it seems unlikely that losses of this magnitude could be accounted for by volatilisation alone. Clearly, further infor- mation on this point is required before a conclusion can be reached.

3.3. Simulation of persistence Full details of the computer program, written in the simulation language CSMP, have been pub- lished previously.2 The basis of the model is to combine equation (2) and equation (3) (with appro- priate values for the constants) with the soil moisture and temperature regimes found in the field. The field environment is simulated from meteorological records. Values for the half-life (H2 in equation (3)) are calculated at short time intervals and the rate of change of herbicide concentration (c) is derived from the modified form of the first-order rate equation:

0.6932 c Hz

dcldt = - - (4)

The rates of change calculated from equation (4) are finally integrated over the whole experimental period. The weather data required are rainfall (mm/day), the evaporation from an open water surface (mmlday) and the 10cm soil temperature ("C). The results from use of the simulation program with the constants derived from the laboratory data with simazine and prometryne and the weather data for 1974 are shown by the dotted lines in Figures 5 and 6. A relatively close fit is obtained to the data with prometryne when incorporated, but the present model, which only allows for chemical or microbiological breakdown, will not simulate the loss from surface applica- tions of this herbicide. With simazine, the fit of the model is not as good as that obtained with incorporated prometryne nor as good as that obtained previously with propyzamidel or napro- pamide.2 The reasons for the underestimation of the rates of loss are not clear. The soil used for the laboratory experiments was the same as that used for prometryne, and the field plots for the two herbicides were within the same randomised blocks. Measurements of distribution of herbicide with depth also showed no movement of simazine or prometryne out of the depth sampled (7.5 cm). One reason for the differences may be the very mild extraction method used, which involved mechanical shaking for 1 h. There is evidence that more rigorous extraction procedures give much higher residue recoveries of atrazine from field soils.23 Another possible reason for the discrepancies is the assumption of first-order kinetics. If a hyperbolic-type rate law is more appropriate to the

Persistence of simazine and prometryne in soil 49

data (which is suggested by the results in Figure 3), then this implies, within limits, a more rapid rate of loss as the concentration of herbicide remaining in the soil declines. However, the present data suggest that the kinetics of prometryne degradation may be more at variance with strict first-order kinetics than those for simazine, yet with prometryne a more acceptable simulation of the field situation was obtained. The results therefore confirm that a simple simulation model can be used to predict the persistence of some herbicides in soil with an accuracy probably sufficient for practical purposes. There are certain factors which require further study which may lead to improve- ments in the agreement between observed and predicted residue levels.

Acknowledgements

Thanks are expressed to Mr H. A. Roberts for his interest and advice during this work and the technical assistance of Miss P. A. Brown and Mrs D. Y. McLeman is gratefully acknowledged.

References

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

10. 11. 12. 13. 14. 15. 16. 17. 18.

19. 20. 21. 22. 23.

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