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Environmental fate & pathways

Biodegradation in soil

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Description of key information

Isotianil and its transformation products are not expected to have a potential for accumulation in the environment. The half-lives of Isotianil weare 0.1 to 1.2 days. Formation of degradation products and non-extractable residues indicates a complete degradation of isotianil in soil after shift to anaerobic conditions and consequently isotianil does not have a potential for accumulation in the environment.

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Additional information

Since the test item sowed to be not readily biodegradable biodegradation studies in soil were conducted under aerobic and anaerobic conditions.

In an aerobic metabolism/degradation study the biotransformation of [Isothiazol-3-14C, carboxamide-14C]Isotianil was studied in four soils under laboratory aerobic conditions in the dark for 120 days at 20 ± 2 °C and 55 ± 5% maximum water holding capacity (Stupp & Junge, 2013). The study followed the OECD Guideline for the Testing of Chemicals No. 307 with additional EU requirements (Regulation (EC) No 1107/2009 and Draft SANCO 11802/2010/rev 7), the US EPA OCSPP Fate Transport and Transformation Test Guidelines OPPTS 835.4100 and OPPTS 835.4200 and the Japanese MAFF New Test Guidelines for Supporting Registration of Chemical Pesticides 12 Nousan 8147, Annex No. 2-5-2. The intended maximum single field use rate of Isotianil is 200 g/ha. Based on the conversion according to the OECD guideline (soil depth of 2.5 cm and a bulk density of 1.5 g cm/3), the concentration in soil corresponds to a weight amount of 533 µg/kg soil (DM). The test was performed in static systems consisting of 100 g soil dry weight each filled into Erlenmeyer flasks equipped with traps for the collection of carbon dioxide and volatile organic compounds. Duplicate samples were analyzed after 0, 1, 3, 8, 13, 28, 59, 91 and 120 days of incubation. At each sampling interval, the soil was extracted three times at ambient temperature and twice by microwave-accelerated extraction at 70 °C using ACN/water 1/1 (v/v) for the first extraction and methanol/water 1/1 (v/v) for the second extraction. The amounts of test item and transformation products in soil extracts were determined by liquid scintillation counting (LSC) and by HPLC/radiodetection analysis. The amount of volatiles was determined by LSC. Overall mean material balances were 101.3, 97.6, 96.2 and 96.5% of applied radioactivity [% AR] for soils Laacher Hof AXXa, Dollendorf II, Laacher Hof Wurmwiese and Hoefchen am Hohenseh, respectively. The maximum amount of carbon dioxide was 88.9, 83.3, 81.0 and 77.9% AR at study end (DAT-120) in soil Laacher Hof AXXa, Dollendorf II, Laacher Hof Wurmwiese and Hoefchen am Hohenseh, respectively. Formation of other volatile organic compounds was insignificant as demonstrated by values of ≤ 0.1% AR at all sampling intervals for all soils. Extractable radioactivity decreased from DAT-0 to DAT-120 from 100.3 to 3.3% AR in soil Laacher Hof AXXa, from 98.5 to 2.5% AR in soil Dollendorf II, from 97.4 to 1.6% AR in soil Laacher Hof Wurmwiese and from 97.7 to 3.0% AR in soil Hoefchen am Hohenseh. Non-extractable radioactivity increased from DAT-0 to DAT-120 from 0.9 to 15.6% AR in soil Laacher Hof AXXa, from 1.9 to 13.2% AR in soil Dollendorf II, from 1.1 to 12.4% AR in soil Laacher Hof Wurmwiese and from 1.0 to 14.6% AR in soil Hoefchen am Hohenseh. The amount of Isotianil in the soil extracts decreased from DAT-0 to DAT-120 from 99.4 to 0.8% AR in soil Laacher Hof AXXa, from 94.5 to 0.8% AR in soil Dollendorf II, from 89.2% to < 0.5% AR in soil Laacher Hof Wurmwiese and from 93.4 to 0.6% AR in soil Hoefchen am Hohenseh. Besides the formation of carbon dioxide, one major transformation product, DCIT-acid was identified with the following maximum amounts: 72.6% AR at DAT-8 in soil Laacher Hof AXXa, 66.5% AR at DAT-8 in soil Dollendorf II, 77.7% AR at DAT-3 in soil Laacher Hof Wurmwiese, 81.0% AR at DAT-8 in soil Hoefchen am Hohenseh, decreasing to 1.6%, < 0.5%, 0.8% and 2.3% of AR at study end for soils Laacher Hof AXXa, Dollendorf II, Laacher Hof Wurmwiese and Hoefchen am Hohenseh, respectively. One unknown peak (u2) was detected. The maximum amount was 0.9% AR at DAT-120 in soil Laacher Hof AXXa, 0.8% AR at DAT-120 in soil Dollendorf II, 0.9% AR at DAT-91 in soil Laacher Hof Wurmwiese and < 0.5% AR at DAT-91 in soil Hoefchen am Hohenseh. The experimental data are well described by a double first order in parallel (DFOP) kinetic model. The half-lives of Isotianil under laboratory aerobic conditions were 1.2, 0.1, 0.6 and 0.9 days in soils Laacher Hof AXXa; Dollendorf II; Laacher Hof Wurmwiese and Hoefchen am Hohenseh, respectively. Therefore, Isotianil and its transformation products are not expected to have a potential for accumulation in the environment.

In a second study the route and rate of degradation of isotianil was studied in three European soils under flooded anaerobic conditions following an aerobic incubation phase at 20.0 °C (Stupp & Junge, 2014). The study followed the OECD Guideline for the Testing of Chemicals No. 307. A study application rate of 516 and 543 µg per kg soil dry weight, for the first and repeated application, respectively, was applied based on the intended maximum single field use rate of isotianil of 200 g/ha. The test systems consisted of 300 mL glass Erlenmeyer flasks, each containing 100 g of soil (dry weight equivalent). During the aerobic study phase, air-permeable traps were attached for the collection of CO2 and volatile organics (static test systems). At start of the anaerobic study phase, the traps for volatile components were replaced by sealable two-valve glass stoppers connected with plastic gas sampling bags. Following application of [isothiazol-3-14C, carboxamide-14C] labeled test item to soil the samples were incubated under aerobic conditions in the dark at about 20 °C and 55% of maximum water holding capacity. After 1 day of incubation the soil samples were flooded with oxygen-depleted, de-ionized water (ca. 3 cm layer above soil level) and set under an atmosphere of argon. The water-logged samples were maintained under anaerobic conditions at approximately 20 °C in the dark for a maximum period of 125 days. Duplicate test systems were analyzed after 0 and 1 day of aerobic incubation. Further samples were taken directly after water logging (day 1) and 5, 7, 15, 28, 63, 90 and 121 days after treatment (DAT), corresponding to 0, 4, 6, 14, 27, 62, 89 and 120 days after soil flooding (DASF) for soils Dollendorf II and Laacher Hof Wurmwiese. Test systems of soil Hoefchen am Hohenseh 4a were analyzed after water logging (day 1) and 5, 7, 15, 28, 63, 91 and 126 days after treatment (DAT), corresponding to 0, 4, 6, 14, 27, 62, 90 and 125 days after soil flooding (DASF). Soil and water were separated by decantation and centrifugation to allow for separate analysis of the phases with the water being analyzed directly. Afterwards the soil was extracted three times at ambient temperature using acetonitrile/water 4/1 (v/v). Furthermore, two microwave-accelerated extraction steps were performed using acetonitrile/water 1/1 (v/v) at 70°C and methanol/water 1/1 (v/v). The amounts of test item and degradation products in water soil extracts were determined by liquid scintillation counting (LSC) and by HPLC/radiodetection analysis. The amounts of volatiles and non-extractable residues were determined by LSC and combustion/LSC, respectively. Test item and

degradation products were identified by HPLC-MS(/MS) including accurate mass determination and by co-chromatography with reference items. Mean material balances were 97.1% AR (range from 91.6 to 102.6% AR) for soil Hoefchen am Hohenseh 4a, 98.0% AR (range from 96.1 to 101.8% AR) for soil Dollendorf II and 97.4% AR (range from 94.3 to 102.6%) for soil Laacher Hof

Wurmwiese. Before soil flooding, the amount of carbon dioxide was 0.3%, 0.5% and 0.4% in soil Hoefchen am Hohenseh 4a, Dollendorf II and Laacher Hof Wurmwiese, respectively. Formation of volatile organic compounds (VOC) was insignificant with < 0.1% AR at all samplings intervals in the aerobic and anaerobic incubation phase. In the aerobic incubation phase, non-extractable residues (NER) increased from 1.4 to 2.4% AR in soil Hoefchen am Hohenseh 4a, from 2.4 to 5.0% AR in soil Dollendorf II and from 0.9 to 2.4% AR in soil Laacher Hof Wurmwiese (mean values). During the following anaerobic incubation period NER increased to 8.1% AR in soil Hoefchen am Hohenseh 4a, to 6.2% AR in soil Dollendorf II and to 6.0%AR in soil Laacher Hof

Wurmwiese at study end. Within the aerobic phase of the study, the amount of the test item isotianil in the entire test systems decreased rapidly from 100.4 to 54.5% AR in soil Hoefchen am Hohenseh 4a, from 97.6 to 17.8% AR in soil Dollendorf II and from 100.0 to 41.5% AR in soil Laacher Hof Wurmwiese (mean values). During the following anaerobic incubation period (i.e. flooded state) the amount of isotianil decreased to 1.3% AR in soil Hoefchen am Hohenseh 4a, to not detectable in soil Dollendorf II from DASF-27 onwards and to < LOD in soil Laacher Hof Wurmwiese from DASF-62 onwards. Degradation of isotianil was accompanied by the formation of four degradation products with the following maximum amounts: BYF 01047-DCIT-acid with 93.2% AR at DAT-7 (corresponding to DASF-6) in soil Laacher Hof Wurmwiese, BYF 01047-3-CIT-acid with 9.6% AR at DAT-90 (corresponding to DASF-89) in soil Laacher Hof Wurmwiese, BYF 01047-4-CIT-acid with 77.0% AR at DAT-121 (corresponding to DASF-120) in soil Dollendorf II and BYF 01047-isothiazol acid with 4.8% AR at DAT-126 (corresponding to DASF-125) in soil Hoefchen am Hohenseh 4a. The total sum of unidentified residues amounted to a maximum of 10.8% AR in the entire system and no single component exceeded 4.8% AR at any sampling interval for all soils. The experimental data of the anaerobic degradation of isotianil could be well described by a double first order in parallel (DFOP) kinetic model. The half-life of isotianil under anaerobic conditions was 1.7, 0.7 and 1.3 days in soil Hoefchen am Hohenseh 4a, Dollendorf II and Laacher Hof Wurmwiese. Thus, formation of degradation products and NER indicates a complete degradation of isotianil in soil after shift to anaerobic conditions and consequently isotianil does not have a potential for accumulation in the environment.