Pestic. Sci. 1978, 9, 326-332

The Degradation of Methazole in Soil. I. Effects of Soil Type, Soil Temperature and Soil Moisture Content

Allan Walker

National Vegetable Research Station, Wellesbourne, Warwick CV35 9EF

(Manuscript received 7 December 1977)

[14C]-Labelled methazole was incubated in six soils at 25°C and with soil moisture at field capacity. Under these conditions, methazole was unstable, the concentration declined following first-order kinetics with half-life values in the soils ranging from 2.3 to 5.0 days. The main degradation product was 1 -(3,4-dichlorophenyl)-3-methylurea (DCPMU) which was more stable than the parent compound. After about 160 days, DCPMU accounted for 30 to 45 % of the initial methazole concentration. Degradation of methazole and DCPMU was affected by soil temperature and moisture content. With methazole, half-lives in one soil at field capacity moisture content and temperatures of 25, 15 and 5°C were 3.5, 8.7 and 31.1 days respectively. The half-life at 25°C was in- creased to 5.0 days at 50 % of field capacity and 9.6 days at 25 % of field capacity. A proportion of the initial radioactivity added to the soil could not be extracted and this proportion increased with time. After 160 days this unextractable radioactivity ac- counted for up to 70% of the amount applied.

1. Introduction

The herbicide methazole, 2-(3,4-dichlorophenyl)-4-methyl-1,2,4-oxadiazolidine-3,5-dione, is used in the United Kingdom as a post-emergence treatment for selective weed control in onions and leeks. Bond and Roberts1 reported that methazole activity can be highly persistent in soil, particularly when the soil is dry or when temperatures are low. It has been shown that the herbicide is rapidly degraded in plants to 1-(3,4-dichlorophenyl)-3-methylurea (DCPMU) which is subsequently de- graded to 1-(3,4-dichlorophenyl)urea (DCPU).2-5 The structures of methazole and its principal metabolites are shown in Figure 1. It has been suggested that the primary toxicant is DCPMU,e and that the relative rates of transformation of methazole to DCPMU and DCPU can be important in determining selectivity.2. Studies of degradation in soil have suggested a similar pathway although the amounts of DCPU detected have generally been sma11.6y7 The present experiments were made to determine the effects of soil type, soil temperature and soil moisture content on the degrada- tion of methazole in soil.

/ O \

0-c

I ,N- 6" CI c1

0

HN-C-NHR I 1

I

Cl

(1) (IN Figure 1. Structure of methazole (I) and its principal degradation products, DCPMU (11, R=CHs) and DCPU

(TI, R=H). 0031-613X/78/0800-0326 502.00 0 1978 Society of Chemical Industry

326

Degradation of methazole. I 321

2. Experimental

2.1. Materials Soils from the surface 10 cm of six fields at the National Vegetable Research Station were used and their properties are shown in Table 1. The herbicide was a wettable powder formulation of metha- zole (75 % a.i.) and the sample of [W]-methazole was labelled in the phenyl ring (New England Nuclear Corp., Boston, Massachusetts, USA. Radiochemical purity > 99 % ; specific activity, 83 mCi g-1). Small amounts of unlabelled pure methazole, DCPMU and DCPU were also used.

Table 1. Soil properties

Organic Water content at carbon Clay 100 cm suction

Soil Soil texture (%I (%) PH (% w/w)

Sheep Pens Sandy loam 1.20 18 6 .2 14.2 Big Cherry Sandy clay loam 1.33 20 7 . 0 17.0 Soakwaters Clay loam 1.38 29 6.1 16.8

Gallas Leys Clay loam 2.90 34 6 .9 23.8 Water Meadows Clay 5.32 41 7.2 23.4

Little Cherry Sandy loam 1.38 13 6 .8 12.0

2.2. Preparation of samples for incubation Separate quantities of air-dried soil (1 kg) were treated with labelled methazole by pipetting a solution in methanol (10 ml) over the surface to give a concentration of 2 pCi kg-1 dry soil. The solvent was allowed to evaporate and sufficient formulated methazole in water (20 ml) was added to give a final herbicide concentration of 4 pg g-1 dry soil. The soil-herbicide treatments were thoroughly mixed by passing several times through a 2 mm mesh sieve. After mixing, the soils were stored in polythene bags for 24 h at 4°C during which time a subsample was dried at 110°C to determine the soil moisture content. After this 24 h period, a quantity of each soil was weighed into a 1 litre wide-mouthed polyethylene bottle and water was added to give the required soil moisture content. The bottles were stoppered with foil caps and incubated at the appropriate temperature. The treatments prepared in this way are listed in Table 2. At weekly intervals sufficient water was added to the soils, followed by thorough mixing to maintain the soil moisture content. Immediately after preparation of the samples for incubation and at intervals during the subsequent 160 days, duplicate 30 g amounts of soil were removed from each treatment and the amounts of herbicide and degradation products present were determined.

Table 2. Incubation treatments

Soil moisture content Treatment Temperature

no. Soil (“C) % w/w % of field capacity

1 2 3 4 5 6 7 8 9

10

Water Meadows Gallas Leys Little Cherry Soakwaters Big Cherry Sheep Pens Sheep Pens Sheep Pens Sheep Pens Sheep Pens

25 25 25 25 25 25 15 5

25 25

23.4 23.8 12.0 16.8 17.0 14.2 14.2 14.2 7 . 0 3.5

100 100 100 100 100 100 100 100 50 25

22

328 A. Walker

2.3. Extraction and determination of herbicide and degradation products The soils were extracted with methanol (50 ml) by shaking on a wrist-action shaker for 1 h. The samples were filtered and duplicate 2 ml subsamples of the filtrate were placed in counting vials together with dioxane-based scintillation fluid (5 m1)8 and counted on a Tracerlab Coru-Matic 200 liquid scintillation counter. Quench corrections were determined using a channels ratio calibration curve. The filtrates remaining after preparation of samples for counting were combined for each treatment, evaporated to a small volume on a vacuum rotary evaporator and after the addition of water (50 ml), were extracted three times with ethyl acetate (50 ml). The combined ethyl acetate extracts were evaporated to about 20 ml and dried with anhydrous sodium sulphate. Tests made at intervals during the experiments showed that over 97 % of the methanol-extractable radioactivity partitioned into the ethyl acetate phase. The ethyl acetate solutions were evaporated to near dryness on a vacuum rotary evaporator and the final traces of solvent were removed by leaving the evapora- tion flasks open to the air overnight. The residues were dissolved in a few drops (about 0.1 ml) of acetone and about 10 pl of each sample was applied separately to a silica gel F254 precoated thin- layer plate. Each spot was overspotted with a solution of methazole, DCPMU and DCPU in acetone (each at a concentration of about 1 %). The plates were run for 10 cm above the baseline in

BG k 60

40

2c

0

80 ; 60

40

20

3 40 80 I20 I60 Davs

Figure 2. Degradation of methazole in different soils. 0, Methazole; A, DCPMU. (a) Water Meadows; (b) Gallas Leys; (c) Little Cherry.

Degradation of methazole. I 329

benzene+ acetone (7 + 3, by volume) and then dried. The plates were examined under ultraviolet light and the areas corresponding to the analytical reference standard were scraped off, placed in counting vials with scintillation fluid (5 ml) and counted as before. The RF values for methazole, DCPMU and DCPU in this solvent system were 0.90, 0.50 and 0.30, respectively.

3. Results and discussion

The patterns of degradation of methazole in the six soils (treatments 1-6, Table 2) are shown in Figures 2 and 3. In all of these soils, the parent compound was degraded rapidly and was not detected in significant amounts after 30-40 days. Previous studies have also shown methazole to be unstable in soil. Carringer et aL7 reported a half-life of 2 days in a sandy loam at 30°C and 80% of field capacity. The present data for each soil gave an approximate fit to the first-order rate equation and half-lives for methazole derived from the results were 5.0, 2.3, 3.7, 2.4, 3.0 and 3.5 days respectively for treatments 1 to 6. There is no apparent relationship between these rates of loss and any of the soil properties shown in Table 1.

The results in Figures 2 and 3 show that as methazole was degraded, there was a steady rise in the amount of DCPMU extracted from the soil, which reached a maximum concentration between 10

0

l o o - ( c i

ao L tr

0

l o o - ( c i

ao L

20

I I 1 I 0 40 80 I20 I60

-0-0

Days

Figure 3. Degradation of methazole in different soils. 0, Methazole; A. DCPMU. (a) Soakwaters; (b) Big Cherry; (c) Sheep Pens.

330 A. Walker

and 40 days after the start of the experiment. The results also show that DCPMU was much more stable than the parent herbicide in these soils and under the incubation conditions of 25°C and field capacity soil moisture. After 160 days, the concentration of DCPMU varied from 30% of the initial methazole concentration in Gallas Leys and Soakwaters soil (treatments 2 and 4) to 40% of the initial methazole concentration in soil from Sheep Pens (treatment 6). This suggests that the persistence of methazole activity in the field’ results mainly from the stability of DCPMU in the soil rather than from residues of the parent herbicide.

DCPMU was the only degradation product found in the six soils in quantities greater than 1 % of the applied herbicide. The demethylated derivative, DCPU, never accounted for more than 1 % of the initial herbicide concentration. This suggests that DCPU was either produced slowly, or was itself rapidly degraded. This result is consistent with that of Brockman and Duke,6 who showed that only 1 % of the methazole applied to columns of soil was present as DCPU after 44 days.

In the experiments of Brockman and Duke,G methazole was applied at 2.2 kg ha-1 to the surface of columns of soil in a growth room and 1.3 cm simulated rainfall was applied every fourth day. The soil was exposed to a 15 h day at 22°C and a 9 h night at 14°C for 44 days. Under these conditions, the time for 50% disappearance of methazole was approximately 25 days, which is much greater than the half-lives derived from the results in Figures 1 and 2 of between 2.3 and 5.0 days, and the

Figure 4. Effects of temperature on methazole degradation in Sheep Pens soil. 0, Methazole; A, DCPMU. (a) 15°C; (b) 5°C.

Degradation of methazole. I 331

half-life of 2 days reported by Carringer et al.' One possible reason for this difference is that after application as a wettable powder formulation to the surface of columns of soil,s the low water solu- bility of methazole (1.5 mg litre-l at 25°C) would restrict its rate of solution and hence availability for degradation. The data, however, also suggest a marked effect from the fluctuating moisture and temperature regimes on the rate of methazole degradation. The effects of temperature on methazole degradation in Sheep Pens soil (Table 1) are shown in Figure 3 and the effects of soil moisture con- tent are shown in Figure 4. The data in Figure 4 show that methazole degradation is strongly de- pendent on temperature and at 5"C, 5 % of the methazole added to the soil initially could be ex- tracted after 160 days. Half-lives calculated from these results are 8.7 days at 15°C and 31.1 days at 5°C. A reduction in soil moisture content to 50% and 25 % of field capacity (Figure 5 ) also resulted in slower rates of methazole degradation and the calculated half-lives are 5.0 and 9.6 days respec- tively. These can be compared with the half-life of 3.5 days at the same temperature and field capa- city moisture content. The results in Figures 4 and 5 also show an effect of reduced temperature and soil moisture content on the rate of degradation of DCPMU. In treatments 7,8,9 and 10 (Table 2), even after 160 days, the concentration of DCPMU in the soil was equivalent to over 50% of the initial methazole concentration. This is further conha t ion that the long, residual herbicidal

\ A h h P Y Y .1

I -p i 0 40 80 I20 I60

Days

Figure 5. Effects of soil moisture content on methazole degradation in Sheep Pens soil. 0, Methazole; DCPMU. (a) 50% of field capacity; (b) 25% of field capacity.

A,

332 A. Walker

activity of methazole in the field, particularly when the soil is dry or temperatures are low,l could result mainly from the persistence of DCPMU in the soil.

As in the experiments involving different soils (Figures 2 and 3), DCPU was not extracted in measureable amounts when methazole was incubated in the same soil at different temperatures and moisture contents (Figures 4 and 5) , again suggesting that this compound is not an important degradation product of methazole in soil. The results in Figures 2 to 5 show that during the 160-day period of these experiments, a large proportion of the radioactivity added to the soils initially, ulti- mately became unextractable. This varied from about 40% of the initial amount in treatments 8 and 10 to about 70% in treatment 4. The possible nature of this ‘bound’ material will be discussed in a subsequent paper.9

4. Conclusion

Methazole is unstable in soil and is degraded rapidly to produce DCPMU as the main degradation product. DCPMU is relatively persistent and is probably the compound responsible for the long residual phytotoxicity following methazole application in the field. The rate of methazole degradation is reduced when the soil is dry or when temperatures are low.

Acknowledgements Sincere thanks are expressed to Velsicol Chemical Ltd for a gift of [14C]-labelled methazole and for the samples of analytical standard methazole, DCPMU and DCPU.

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

Bond, W.; Roberts, H. A. Weed Res. 1976, 16, 23. Dorough, H. W. Bull. Envir. Contam. Toxicol. 1974, 12, 493. Dorough, H. W.; Whitacre, D. M. ; Cardona, R. A. J. agric. Fd Chem. 1973, 21, 797. Butts, E. R.; Foy, C. L. Pestic. Biochem. Physiol. 1974, 4, 44. Jones, D. W.; Foy, C. L. Pestic. Biochem. Physiol. 1972, 2, 8 . Brockman, F. E.; Duke, W. B. Weed Sci. 1977, 25, 304. Carringer, R. D.; Rieck, C. E.; Harger, T. R. Proc. 28th A. Meet. Sth Weed Sci. SOC. 1975, 292. Bray, G. A. Analyt. Biochem. 1960, 1, 279. Walker, A.; Robert, M. G. Pestic. Sci. 1978, 9, 333.