Degradation of Benthiocarb in Soils As Affected by Soil Conditions*

J. Pesticide Sci. 2, 7-16 (1977) Degradation of Benthiocarb in Soils as Affected by Soil Conditions* Yasuo NAKAMURA, ** Kanji ISHIKAWA** and Shozo KUWATSUKA Laboratory of Soil Science, Faculty of Agriculture, Nagoya University, Chikusa-ku, Nagoya 464, Japan (Received May 22, 1976) Some factors affecting the degradation of 14C-benthiocarb (S-4-chlorobenzylN, N- diethylthiocarbamate) labelled at the benzene-ring in soils were studied. The degradation rates of 14C-benthiocarb in three different soils under upland, oxidative flooded and reductive flooded conditions were compared. 14C-Benthiocarb wasrapidly degraded under oxidative conditions, but slowly under reductive conditions. Very small differences in the degradation rates were observed among different soils. Under oxidative conditions 14C-carbon dioxide was liberated remarkably with the degradation of 14C-benthiocarb. The degradation was remarkably retarded by sterilizing the soils. The repeated application of benthiocarb, or its incorporation into the soil with simetryne, CNP or propanil had no significant effect on the degradation rate. Benthiocarb (Saturn(R),S-4-chlorobenzyl N, N-diethylthiolcarbamate), alone or in combing MATERIALS AND METHODS tion with simetryne, prometryne and CNP, is 1. Chemicals extensively used to control weeds mainly in 14C-Benthiocarb labelled at the benzene-ring paddy fields. was used as in the previous study. U The In the previous paper, ' the persistence of specific activity was 3. 21 mCi/mmole and the benthiocarb and its degradation products in radioactive purity was more than 99%. Non- a soil were reported, and its degradation path- radioactive pure chemicals were also described ways were also proposed. The behavior of the in the previous reports. 1'2) degradation products suggested that benthiocarb itself was comparably persistent but the 2. Soil Samples degradation products were rapidly degraded in Anj o soil, a mineral soil with kaolin clay subsequent steps. It was also reported that mineral, was obtained from paddy fields of benthiocarb was degraded in the soil more Paddy Field Experimental Farm, Aichiken rapidly under upland, than under flooded, Agricultural Research Center in Anj o, Aichi conditions. Prefecture; Nagano soil, a mineral soil with In the present paper, the degradation of montmorillonite clay, from Nagano Pref ectural benthiocarb in soils in laboratory was studied Agricultural Experiment Station in Nagano, as affected by soil properties, moisture content Nagano Prefecture; and Tochigi soil, a humic or redoxy conditions, soil sterilization, repeated volcanic ash soil, from a paddy field near application, and combination with other herbi- Utsunomiya, Tochigi Prefecture. The three cides. soil samples were collected from furrow slices * Studies on the Metabolism of Benthiocarb of paddy fields in winter. The soils were crushed and passed through a 2mm sieve, and (Part IV). See reference'. **Visiting researchers from Life Science Re- stored at 5C. The physicochemical properties search Institute, Kumiai Chem., Ind., Co., of these soil samples are shown in Table 1. Ltd., Kikugawa-cho, Ogasa-gun, Shizu- oka-ken, 439, Japan (Present address). 8 口本農薬学会誌第2巻第1号昭和52年2月 Table 1 Properties of soil samples used. Oven-dry soil basis. 3. Soil Conditioning (Pre-incubation) the glass rod was finally rinsed into the flask (A) Upland conditions : Fifty grams (oven- with a small amount of water. The soil under dried weight basis) of Anjo or Nagano soil oxidative flooded conditions was mixed well by sample, or 25 g of Tochigi soil sample was shaking. The soil under reductive flooded placed in a 100 ml Erlenmeyer flask. Water conditions was mixed with a glass rod, and was added to adjust the soil moisture to 40% then by shaking gently upside down after the of the maximum water holding capacity. The opening was stopped tight. These soil samples mouth of the flask was covered with aluminum treated with 14C-benthiocarb were incubated foil perforated with several small holes. The for 10, 20, 40 and 80 days under the same soil was incubated at 30C in the dark for 2 conditions of pre-incubation as above. weeks. The moisture content was maintained at the initial level by the addition of water 5. Determination of 14C02Liberation once a week to replace the water lost by evapo- For determining 14C02 liberated from the ration. incubated soil, the soil was incubated as follows. (B) Oxidative flooded conditions: Fifty As shown in Fig. 1, a small vessel containing grams (25 g for Tochigi soil) of each soil sample 3 ml (for the upland soil) or 7 ml (for the was placed in a 500 ml Erlenmeyer flask. The oxidative flooded soil) of 12.5 N NaOH aqueous soil was flooded with water up to 0. 5 cm deep. solution was placed in the soil. 14C02liberated The soil layer of each sample was also about from the soil was largely absorbed by the NaOH 0.5 cm deep. The mouth of the flask was solution. For the soil under upland or oxida- covered with aluminum foil. The soil was tive flooded conditions about 51 Jk of C02-free incubated at 30C in the dark for 5 weeks. air was discharged into the flask containing the Water was supplied once a week to maintain soil sample was passed through toluene to the water depth during the experiment. trap volatile organic materials, and then (C) Reductive flooded conditions : Fifty through an alkali scintillator (5 g of PPO and grams (25 g for Tochigi soil) of each sample 0.3g of POPOP in one liter of ethanolamine- was placed in a big test tube (3 cm inner dia- methyl cellosolve-toluene (1:3: 6) solution3') to meter and 20 cm long). The soil was flooded trap 14C02 which had not been absorbed by with water up to 3 cm deep. The soil layer of the NaOH solution in the incubation flask. each sample was about 8 cm deep. The mouth The NaOH solution, toluene and alkali scientil- of the tube was closed with a rubber stopper lator were renewed every time after C02-free covered with paraffin paper. The soil was air was discharged into the flask. The NaOH incubated at 30C in the dark for 5 weeks. solution was diluted to 50 ml with water and 1 ml aliquot of the diluted solution was radio- 4. Application and Incubation of 14C-benthio- assayed with 10 ml of Bray's liquid scintillator carb (PPO 4 g, POPOP 0. 2 g, naphthalene 60 g, '4C-Benthiocarb (25.8 X 105 dpm) dissolved methanol 100 ml, ethyleneglycol 20 ml, and in 0. 5 ml of acetone was applied dropwise on dioxane, to make 1000 ml) using a liquid the soil surface or added to the flooded water scintillation spectrometer. The toluene and at a concentration of 10 ppm on dry soil basis. alkali solutions were also radioassayed with The soil under upland conditions was mixed the alkali scintillator. After incubation, 1 ml well with a glass rod. The soil remaining on of 10 N H2SO4 was added to the soil (40 ml of Journal of Pesticide Science 2 (1), February 1977 9 water was also added to the upland soil), air was neutralized with 20 ml of 2 N NaOH, was bubbled through the mixture, and the extracted by shaking with 100 ml of methanol 14002liberated was trapped in the alkali scintil- two times, and centrifuged by the same pro- lator and determined. cedure as described above. The radioactivity For the soil under the reductive flooded of the methanol extracts was measured using conditions, a small vessel containing 3 ml of liquid scintillation spectrometer. The radio- 12.5 N NaOH was placed on the soil in the big activity in the residual soil was determined by test tube described, which was closed tightly combustion. 1) and incubated. An aliquot of the alkali solution was taken out periodically and radio- 7. Radioassay and Thin Layer Chromatography assayed. After the incubation, the soil was The method were described in the previous transferred into a 500 ml Erlenmeyer flask and paper. 1) 14002 in the soil was determined by the same procedure as above described. 8. Degradation in Sterilized Soil Twenty grams (oven-dried weight basis) of Anjo soil sample was placed in a 200 ml flask. Water was added to adjust the soil moisture to 40% of maximum water holding capacity for upland conditions or to maximum water holding capacity plus 10 ml for oxidative flooded conditions. The soils were pre-incubated at 30C in the dark for 2 weeks, then sterilized at 120C for 30 min in an antoclave. After cooling, 14C-benthiocarb (78. 4 X 104 dpm) dissolved in acetone was added to each soil Fig. 1 Apparatus for the determination of sample (10 ppm of the final concentration on liberated 14C02. dry soil basis) under sterile conditions. The mouth of each container was closed with sterile 6. Extraction and Separation of Radioactive cotton. The soil samples were incubated at Compounds 30C in the dark for designated periods. At After 14C02determination, the soil was trans- the same time, non-sterilized soil was incu- fered into a centrifuge tube, rinsing away with bated by the same procedure. acetone and a small amount of water. A total In this case the radioactive substances were volume of 100 ml acetone was added into the extracted with methanol instead of acetone tube, which was then capped tightly. The to avoid the formation of acetone-complex tube was shaken vigorously for 30 min and during the fractionation process. The incubated centrifuged at 3,000 rpm for 10 min. The soil was flooded, and extracted by shaking supernatant solution was transfered by decanta- twice with 150 ml of methanol and centrifuged tion. The soil residue was again extracted with by the same procedure described above.

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