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Biodegradation in soil

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Endpoint:
biodegradation in soil: simulation testing
Type of information:
experimental study
Adequacy of study:
key study
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
comparable to guideline study with acceptable restrictions
Qualifier:
according to
Guideline:
other: U.S. EP.4 Pesticide Assessment Guidelines, Subdivision N, Environmental Fate, Section 162-1, Aerobic Soil Metabolism
Qualifier:
equivalent or similar to
Guideline:
EPA OPPTS 835.4100 (Aerobic Soil Metabolism)
GLP compliance:
yes
Test type:
laboratory
Specific details on test material used for the study:
The test substance will be MON 13900: oxazolidine, 3-(dichloroacetyl)-2,2-dimethyl-5-(2-furanyl)-]. The 14C isotopic label will be incorporated at the Carbon-4 of the oxazolidine ring. In addition, a 13C label may be incorporated at the same position to facilitate mass spectral analyses. Chemical and radiochemical purities will be greater than 98%.
Radiolabelling:
yes
Oxygen conditions:
anaerobic
Soil classification:
USDA (US Department of Agriculture)
Year:
1985
Soil no.:
#1
Soil type:
silt loam
% Clay:
22
% Silt:
59
% Sand:
19
pH:
6.7
CEC:
55.8 meq/100 g soil d.w.
Bulk density (g/cm³):
1.1
Soil no.:
#2
Soil type:
sandy loam
% Clay:
10
% Silt:
31
% Sand:
59
% Org. C:
0.58
pH:
8
CEC:
10.3 meq/100 g soil d.w.
Bulk density (g/cm³):
1.11
Details on soil characteristics:
Two soils wiIl be used: Sable silt loam and Sarpy sandy loam. These represent soils present in the major soybean growing regions in the United States. The soils were removed from the outdoor soil storage bins at the Monsanto Life Sciences Research Center in Chesterfield, Missouri and allowed to dry sufficiently to allow sieving. After the soil was sifted through a 2-mm sieve, the moisture content of each soil was determined by drying a sample of each soil to a constant weight in an oven at 100 °C. The water content of the soil was adjusted to 85% of the water holding capacity at 0.33 bar.
The incubation periods prior to the initiation of the main experiments were 13 days (main experiment), 33 days (supplemental experiment), and 34 days (large scale experiment).
These soils are representative of the common use areas for MON 13900. Sable silt loam soil is from northern Illinois, in the center of the corn belt and Sarpy sandy loam soil is from Bloomfield, Missouri.
Soil No.:
#1
Duration:
12 mo
Soil No.:
#2
Duration:
12 mo
Soil No.:
#1
Initial conc.:
0.39 mg/kg soil d.w.
Based on:
test mat.
Soil No.:
#2
Initial conc.:
0.39 mg/kg soil d.w.
Based on:
test mat.
Parameter followed for biodegradation estimation:
radiochem. meas.
Soil No.:
#1
Temp.:
25 +- 1°C
Humidity:
65 - 86% of field capacity
Soil No.:
#2
Temp.:
25 +- 1 °C
Humidity:
65 - 86% of field capacity
Details on experimental conditions:
Application of Test Substance
The soils were treated with 110 µL of dosing solution per flask, which corresponds to 0.39 mg/kg (based on soil dry weight). The target treatment rate was 0.4 mg/kg. A Hamilton 800 Series syringe (250-µL capacity) equipped with a Chaney adaptor and a blunt needle was used to apply the test material to the soils. The dosing solution was distributed over the soil surface as evenly as possible. Following treatment, the soil was agitated slightly, and the moisture content adjusted with deionized water to approximately 85% of moisture capacity. A two-piece trapping tower was placed on all flasks (with a Teflon sleeve), and the flasks were placed in the growth chamber and incubated at 25 +-1 °C in darkness.

Study Design
The MON 13900 aerobic soil metabolism studies consisted of three experiments: the main experiment, the supplemental experiment, and the large-scale experiment. Radiolabeled volatiles and radiolabeled CO2 were quantified for all flasks in all experiments.
Main Experiment Study Design Sable silt loam and Sarpy sandy loam soils were treated with 13C/14C-MON 13900 at a rate of 0.4 ppm. For each soil, twentyeight flasks each containing 50 g soil ( dry weight) were treated on April 5, 1988, and incubated for one year at 25 ±1 °C in darkness. Two untreated flasks were maintained as controls for the trapping system. Flasks were harvested in duplicate from each soil at 0, 1, 3, 7, 14, 30, 62, 91, 122, 184, 273, and 365 days after treatment.
The distribution of radioactivity among volatiles, CO2 , soil, and extracts was quantified at all sampling points. Extraction procedures were not fully developed for the Day O through 30 samples, so soil extracts from these sampling points weren't used for metabolite quantification. Data for the first 30 days of soil metabolism were obtained from the supplemental experiment described below. Metabolite fractions were quantified from the the Day 62 through the Day 365 samples by HPLC/RAD analysis.
Supplemental Experiment Study Design This experiment provided additional data for the first month of MON 13900 soil metabolism. Analysis methods were still under development during the 0 to 30-day sampling points from the main experiment, and it was felt that additional data would be useful. Data from this experiment was used for the MON 13900 half-life calculations and metabolite quantification up to Day 30. This experiment was actually the aerobic portion from the Anaerobic Soil Metabolism Studies of MON 13900.1 Soil extracts from this experiment were used for metabolite quantification, since extraction procedures had been fully developed. Higher extractabilities were generally obtained from the supplemental experiment than from the main experiment for the same sampling point.
The treatment rate, soils, and conditions for the supplemental experiment were identical to those for the main experiment. Sable silt loam and Sarpy sandy loam soils were obtained from the same soil storage bins as the soil for the main experiment. For each soil, eighteen fl.asks of 50 g ( dry weight) soil were treated at a rate of 0.4 mg/kg on September 26, 1988, and incubated aerobically for 30 days at 25 ±1 °C in darkness. Flasks were harvested in duplicate from each soil at 0, 1, 3, 7, 14, and 30 days after treatment. Radiolabeled volatiles, CO2 , and metabolite fractions were quantified from these samples.
Large Scale Experiment Study Design Sable silt loam soil was treated with 14C-MON 13900 at a rate of 2.0 mg/kg. Twenty flasks of 50 g soil (dry weight) were treated on June 28, 1988, and incubated for 40 weeks at 25 ±1 °C in darkness. The purpose of this experiment was to generate larger quantities of soil metabolites for characterization experiments. Flasks were harvested individually as needed. Flasks were harvested individually at 24, 56, 57, 127, and 274 days after treatment. Two fl.asks were harvested on Day 127 and three flasks on Day 274. These samples were used to obtain metabolite fractions for characterization experiments.

Maintenance of Soil Flasks
The lower trapping towers were changed at 7 to 10-day intervals in the main and supplemental experiments. The upper trapping towers were changed every 2 to 3 weeks. The moisture content of the soil in each flask was adjusted to 85% of the moisture capacity at 0.33 bar when the lower towers were changed. Deionized water was used for all moisture adjustments. The fl.ask weights were checked to make sure the moisture content of the soil didn't fall below 65% of the water holding capacity.
In the large scale experiment, regular two-piece trapping towers were maintained on two flasks in the experiment. On the remaining flasks, modified trapping towers were used. The modified trapping towers were designed to trap 14C-volatiles and 14CO2 and keep them from entering the growth chamber atmosphere. The towers were changed and the moisture content adjusted as described for the main experiment.

Extraction of Soils
In the main and supplemental experiments, duplicate flasks of each soil type were harvested at each sampling point. For each flask, the soil was transferred quantitatively to a tared 250-mL polyallomer centrifuge bottle.
Typically, 100-mL volumes of solvent were used in each extraction step. Soil and extract mixtures were shaken on a wrist-action shaker for the indicated amount of time and then centrifuged at 12,000 rpm (23,000 x g) for 10 minutes (aqueous acetonitrile extracts) or 20 minutes (other solvents). The extracts were decanted from the extraction bottles into tared amber glass bottles. The total weight of each extract was determined, and aliquots were removed by weight for analysis by LSC. The total extractability of a sample was determined by summing the percent of applied radioactivity present in the extracts from that sample.
The soils were extracted with 60% aqueous acetonitrile at least three times. Many samples were subjected to additional extractions as described below. For each extract, the soil/solution mixtures were shaken on a wrist-action shaker for varying
lengths of time. These shaking times are listed in the table on page 29, along with any solvents used in addition to the aqueous acetonitrile. In an effort to enhance extractabilities, the extraction procedures were modified during the study.
Additional extraction experiments were conducted in an effort to characterize the unextractable radioactivity (bound residues).

Characterization of Bound Residues
Following the standard extraction procedure, the soils were combusted to quantify the unextractable residues. The soils were then extracted with 0.5 N sodium hydroxide for 60 hours with continuous shaking. The sodium hydroxide extracts were analyzed by LSC. The remaining soil, which contained the humin fraction, was lyophilized and analyzed by combustion. The sodium hydroxide extracts were acidified to pH 1 with
concentrated hydrochloric acid. rpon acidification, a precipitate formed, which contained the humic acid fraction. The supernatant contained the fulvic acid fraction. The supernatant was analyzed by LSC, and extracted with ethyl acetate. The resulting aqueous and ethyl acetate fractions were analyzed by LSC.
Soil No.:
#1
% Recovery:
94
St. dev.:
3.8
Soil No.:
#2
% Recovery:
95.4
St. dev.:
4.1
Soil No.:
#1
% Degr.:
83.3
Parameter:
test mat. analysis
Remarks:
by HPLC/RAD
Sampling time:
1 yr
Soil No.:
#2
% Degr.:
90
Parameter:
test mat. analysis
Remarks:
by HPLC/RAD
Sampling time:
1 yr
Soil No.:
#1
DT50:
32.8 d
Type:
(pseudo-)first order (= half-life)
Temp.:
>= 24 - <= 26 °C
Soil No.:
#2
DT50:
52.5 d
Type:
(pseudo-)first order (= half-life)
Temp.:
>= 24 - <= 26 °C
Transformation products:
yes
No.:
#1
No.:
#2
No.:
#3
No.:
#4
Evaporation of parent compound:
no
Volatile metabolites:
yes
Remarks:
Radiolabeled volatile materials were produced in very low quantity (less than 1% for the 1-year study).
Residues:
yes
Remarks:
Several different extraction and characterization procedures were used to characterize the unextractable (“bound”) residues in soil.
Conclusions:
The estimated half-life of MON 13900 using a first-order kinetic model was 32.8 and 52.5 days in Sable and Sarpy soils, respectively. The results of this study indicate that the aerobic soil metabolism of MON 13900 will be a significant route of MON 13900 degradation in the environment.
Executive summary:

A study was conducted according to Pesticide Assessment Guidelines, Subdivision N, Environmental Fati, Section 162-1 which is similar to EPA OPPTS 835.4100 (Aerobic Soil Metabolism) to assess the aerobic metabolism of MON 13900 in different soils. The aerobic soil metabolism of MON 13900, 3-(dichloroacetyl)-2,2-dimethyl-5-(2-furanyl)-oxazolidine, was studied in Sable silt loam and Sarpy sandy loam soils.

The test material was labeled at the carbon-4 position of the oxazolidine ring with 13C and 14C, and was applied to the soil at a rate of 0.39 mg/kg. The soils were incubated for one year at 25 ± 1 ° C in darkness. MON 13900 was extensively metabolized in soil to carbon dioxide and several water-soluble metabolites. The estimated half-life of MON 13900 using a first-order kinetic model was 32.8 and 52.5 days in Sable and Sarpy soils, respectively. The predominant metabolite produced during the study was radiolabeled carbon dioxide. At the end of the study, radiolabeled CO2 accounted for 37.2% and 40.5% of applied radioactivity in Sable and Sarpy soils, respectively. Of the radioactivity remaining in the soil, only one metabolite fraction was significant. This fraction was identified as the MON 13900 oxamic acid, and accounted for up to 5.9% of applied radioactivity in Sarpy soil at Day 30. After Day 30, the quantity of the oxamic acid decreased steadily, until it represented approximately 1.5% of applied dpm at the end of the study. Unextractable radioactivity in the soil increased during the study, and accounted for approximately 30% of applied dpm at Day 365. Total accountabilities during the study averaged 94.0 ± 3.8 % and 95.4 ± 4.1 (average± standard deviation) for Sable and Sarpy soils, respectively.

The results of this study indicate that MON 13900 is readily degraded by soil microorganisms to carbon dioxide and water-soluble metabolites. Aerobic soil metabolism will be a significant route of degradation of MON 13900 in the environment.

Endpoint:
biodegradation in soil: simulation testing
Type of information:
experimental study
Adequacy of study:
key study
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
comparable to guideline study with acceptable restrictions
Qualifier:
according to
Guideline:
other: U.S. EP.4 Pesticide Assessment Guidelines, Subdivision N, Environmental Fate, Section 162-2, Anaerobic Soil Metabolism
Qualifier:
equivalent or similar to
Guideline:
EPA OPPTS 835.4200 (Anaerobic Soil Metabolism)
GLP compliance:
yes
Test type:
laboratory
Radiolabelling:
yes
Oxygen conditions:
anaerobic
Soil classification:
USDA (US Department of Agriculture)
Year:
1985
Soil no.:
#1
Soil type:
silt loam
% Clay:
22
% Silt:
59
% Sand:
19
pH:
6.7
CEC:
55.8 meq/100 g soil d.w.
Bulk density (g/cm³):
1.1
Soil no.:
#2
Soil type:
sandy loam
% Clay:
10
% Silt:
31
% Sand:
59
% Org. C:
0.58
pH:
8
CEC:
10.3 meq/100 g soil d.w.
Bulk density (g/cm³):
1.11
Details on soil characteristics:
Two soils wiIl be used: Sable silt loam and Sarpy sandy loam. These represent soils present in the major soybean growing regions in the United States. The soils will be taken from the outdoor soil storage bins at the Monsanto Life Sciences Research Center in Chesterfield, Missouri. Soil will be sifted to a uniform size through a 2-mm sieve, and then air-dried to approximately 75% of the moisture level at 0.33 bar. Soil moisture will be determined by oven drying a small amount to constant weight. The soils will be incubated for at least two weeks prior to the start of the study.
These soils are representative of the common use areas for MON 13900. Sable silt loam soil is from northern Illinois, in the center of the corn belt and Sarpy sandy loam soil is from Bloomfield, Missouri.
Soil No.:
#1
Duration:
93 d
Soil No.:
#2
Duration:
93 d
Soil No.:
#1
Initial conc.:
0.42 mg/kg soil d.w.
Based on:
test mat.
Soil No.:
#2
Initial conc.:
0.42 mg/kg soil d.w.
Based on:
test mat.
Parameter followed for biodegradation estimation:
radiochem. meas.
Soil No.:
#1
Temp.:
25 +- 1°C
Humidity:
65 - 86% of field capacity
Soil No.:
#2
Temp.:
25 +- 1 °C
Humidity:
65 - 86% of field capacity
Details on experimental conditions:
Application of Test Substance
The soils were treated with 95 µL of dosing solution per flask, which corresponds to 20.87 µg MON 13900 per flask, or 0.42 mg/kg (based on soil dry weight). The target treatment rate was 0.4 mg/kg. A Hamilton 800 Series syringe (250-µL capacity) equipped with a Chaney adaptor and a blunt needle was used to apply the test material to the soils. The dosing solution was distributed over the soil surface as evenly as possible. Following treatment, the soil was agitated slightly, and the moisture content adjusted with deionized water to approximately 85% of moisture capacity. A two-piece trapping tower was placed on all flasks (with a Teflon sleeve), and the flasks were placed in the growth chamber and incubated at 25 +-1 °C in darkness.

Study Design
The MON 13900 anaerobic soil metabolism studies were conducted using Sable silt loam soil and Sarpy sandy loam soil. These soils were the same soils that were used in the aerobic soil metabolism studies. Radiolabeled volatiles and CO2 were quantified using trapping towers.
Eighteen flasks were prepared for each soil that contained 50 grams of soil (dry weight). Preparation included sieving and adjustment of the soil moisture content to 85% of field capacity. Prior to application of the test substance, the prepared soils incubated for 33 days at 25 + 1 °C in darkness. The moisture content was maintained between 65 and 86 % of field capacity during the incubation period.
On September 26, 1988, each flask was treated with 14C-MON 13900 at a rate of 0.42 mg/kg (based on soil dry weight). The flasks were then. incubated aerobically for 30 days at 25 +1 °C in darkness. During the aerobic incubation, the flasks were maintained as described below. After 30 days of aerobic incubation, the flasks were converted to anaerobic conditions. This involved flooding the flasks with deionized water (100 mL) and purging the soil/solution mixture with nitrogen. Glucose (500 mg) was added to each flask prior to flooding as a carbon source. The flasks were purged for 10 minutes with nitrogen and sealed from the atmosphere. The flasks were incubated anaerobically for 63 days, and were maintained.
Flasks were harvested in duplicate for each soil at 0, 1, 3, 7, 14, 30, 63 and 93 days after treatment. The redox potentials of the solutions in the soil flasks were measured at the final sampling point to document the anaerobicity of the test system.
The soil samples were exhaustively extracted. The distribution of radioactivity between the soil extract, and extracted soil, and CO2 and volatiles was determined by liquid scintillation counting of soil extracts and foam plugs, combustion of extracted soil. and CO2 release experiments. The soil extracts were concentrated and characterized by HPLC/RAD. Soil bound residues were characterized by further fractionation procedures. Metabolize fractions were isolated by prep-HPLC and subjected to mass spectral experiments.

Maintenance of Soil Flasks
The lower trapping towers were changed and analyzed at 7, 14, 21, and 30 days after treatment during the aerobic portion of the experiment. The upper trapping towers were analyzed at 7 and 30 days after treatment. During the aerobic aging period, the moisture content of the soil in each flask was adjusted to 85% of the moisture capacity at 0.33 bar when the lower towers were changed. Deionized water was used for all moisture adjustments. The flask weights were checked prior to moisture adjustments to make sure the moisture content of the soil remained between 65 and 85% of field capacity.
The flasks were converted to anaerobic conditions after 30 days of aerobic incubation. During the anaerobic portion of the study, the flasks were purged with nitrogen for 10 minutes at 47, 63, 74, and 93 days after treatment. A “syringe tower” was used to trap volatiles and CO2 during the purging period.

Extraction of Soils
At each sampling point, duplicate flasks of each soil type were harvested. For the flasks harvested during the aerobic portion, the soil was transferred quantitatively to a tared 250-mL polyallomer centrifuge bottle. For the flasks harvested during the anaerobic portion (Day 63 and Day 93), the water layer was separated from the soil by quantitative transfer of the soil/water mixtures to centrifuge bottles, followed by centrifugation for 10 minutes at 12,000 rpm. The supernatant (the water layer) was decanted into a tared bottle. The soil was then extracted as described below.
Typically, 100-mL volumes of solvent were used in each extraction step. Soil and extract mixtures were shaken on a wrist-action shaker for the time indicated below and then centrifuged at 12,000 rpm (23,000 x g) for 10 minutes (aqueous acetonitrile extracts) or 20 minutes (ammonium hydroxide extracts). The extracts were decanted from the extraction bottles into ta~ed amber glass bottles. The total weight of each extract was determined, and aliquots were removed by weight for analysis by LSC. The total extractability of a sample was determined by summing the percent of applied radioactivity present in the extracts from that sample.
The soils were extracted with 60% aqueous acetonitrile three times. The Day 7 samples were extracted a fourth time, and the Day 63 and 93 samples were extracted with basic and/or acidic solvents. For each extract, the soil/solution mixtures were shaken on a wrist-action shaker for varying times. Additional extractions were conducted on several samples. In an effort to enhance extract abilities, the extraction procedures were modified during the study.

Characterization of Bound Residues
Several different extraction and characterization procedures were used to characterize the unextractable (“bound”) residues in soil. All experiments indicated that the 14C-radioactivity was tightly bound to the soil.
The Day 93 soil samples were subjected to further experiments to characterize the bound residues. Following the multiple extractions described above 53.6 and 38.0% of the applied radioactivity remained in the Sable and Sarpy samples, respectively.
The soils were extracted for 48 hours using a soxhlet apparatus using DMF with 1% oxalic acid. The DMF extract contained 6.8 and 5.4% of applied dpm for Sable and Sarpy soils, respectively. The soil was transferred from the cellulose thimble to a 250-mL centrifuge bottle. The transfer was not quantitative, since some of the soil clung to the thimble and could not be removed. The soil was extracted with 0.5 N sodium hydroxide for 60 hours with continuous shaking, The sodium hydroxide contained 8.9 and 7.1% of applied dpm for Sable and Sarpy soils,
respectively. The remaining soil, containing the humin fraction, contained 19.7 and 12.2% of applied dpm in Sable and Sarpy soils, respectively.
Acidification of the sodium hydroxide extract caused a precipitate to form. The precipitate contained the humic acid fraction, while the supernatant contained the fulvic acid fraction. The supernatant contained 7.8 and 3.9% of applied dpm in Sable and Sarpy soils, respectively. The precipitate was not analyzed, and the quantity present in this fraction could not be accurately estimated since some soil was lost and not extracted following the soxhlet extraction described above.
The results of the above experiments indicate that of the unextractable radioactivity in Sable and Sarpy soils, a large portion is associated with the insoluble humic fraction of the soil.
Soil No.:
#1
% Recovery:
93.1
St. dev.:
5.7
Soil No.:
#2
% Recovery:
97
St. dev.:
3.7
Soil No.:
#1
% Degr.:
97.3
Parameter:
test mat. analysis
Remarks:
by HPLC/RAD
Sampling time:
93 d
Soil No.:
#2
% Degr.:
97.7
Parameter:
test mat. analysis
Remarks:
by HPLC/RAD
Sampling time:
93 d
Soil No.:
#1
DT50:
14.7 d
Type:
(pseudo-)first order (= half-life)
Temp.:
>= 24 - <= 26 °C
Soil No.:
#2
DT50:
13 d
Type:
(pseudo-)first order (= half-life)
Temp.:
>= 24 - <= 26 °C
Transformation products:
yes
No.:
#1
No.:
#2
No.:
#3
No.:
#4
No.:
#5
Evaporation of parent compound:
no
Volatile metabolites:
yes
Remarks:
Radiolabeled volatiles and CO2 were quantified using trapping towers
Residues:
yes
Remarks:
Several different extraction and characterization procedures were used to characterize the unextractable (“bound”) residues in soil.
Conclusions:
The half-life of MON 13900 in soil under anaerobic conditions was estimated using a first-order kinetic model to be 14.7 and 13.0 days in Sable and Sarpy soils, respectively. The results of this study indicate that MON 13900 is readily degraded by soil microorganisms under anaerobic conditions to a mixture of water-soluble metabolizes. Anaerobic soil metabolism will be a significant route of degradation of MON 13900 in the environment.
Executive summary:

A study was conducted according to Pesticide Assessment Guidelines, Subdivision N, Environmental Fati, Section 162-2 which is similar to EPA OPPTS 835.4200 (Anaerobic Soil Metabolism) to assess the anaerobic metabolism of MON 13900 in different soils. The anaerobic soil metabolism of MON 13900, 3-(dichloroacetyl)-2,2-dimethyl-5-(2-furanyl)-oxazolidine, was studied in Sable silt loam and Sarpy sandy loam soils. The test material was labeled at the carbon-4 position of the oxazolidine ring with 13C and 14C, and was applied to the soil at a rate of 0.42 mg/kg. Treated soils were aged under aerobic conditions for 30 days in darkness at 25 °C . After the aerobic aging period, the soil flasks were converted to anaerobic conditions by flooding the soils with water and purging with nitrogen. The soils were incubated anaerobically for 63 days in darkess at 25 °C. Redox potential measurements were used to document the anaerobic conditions. Samples were harvested at 0, 1, 3, 7, 14, 30, 63 and 93 days after treatment. Soil samples were extracted and analyzed by high performance liquid chromatography with radioactive flow detection.

MON 13900 was extensively metabolized in both soils to a mixture of water-soluble metabolizes. Carbon dioxide was a significant metabolite produced during the aerobic aging period. The half-life of MON 13900 in soil under anaerobic conditions was estimated using a first-order kinetic model to be 14.7 and 13.0 days in Sable and Sarpy soils, respectively.

The predominant metabolite produced during the anaerobic portion of the study was the MON 13900 oxamic acid. Other soil-contained metabolizes characterized were the MON 13900 alcohol and the MON 13900 methyl sulfide. Radiolabeled carbon dioxide and volatile production was minimal during the anaerobic period. During the aerobic aging period, radiolabeled CO2 production was significant, accounting for approximately 9% of the applied radioactivity. The MON 13900 oxamic acid accounted for up to 8.1 and 13.3% of the applied dpm in Sable and Sarpy soils, respectively, at Day 63. The MON 13900 alcohol accounted for up to 11.9% of the applied dpm in Sarpy soil at Day 63. Unextractable radioactivity (bound residues) in the soil increased during the study, and accounted for approximately 50 and 40% of the applied dpm at Day 93 in Sable and Sarpy soils, respectively. The extracted soils were subjected to a fractionation procedure to characterize the bound residues. The results of the fractionation experiments indicate that of the unextractable radioactivity in Sable and Sarpy soils, a large portion is associated with the insoluble humic fraction of the soil. Total accountabilities during the study averaged 93.1 +- 5.7% and 97.0 +- 3.7% of applied dpm (average +- standard deviation) for Sable and Sarpy soils, respectively.

The results of this study indicate that MON 13900 is readily degraded by soil microorganisms under anaerobic conditions to a mixture of water-soluble metabolizes. Anaerobic soil metabolism will be a significant route of degradation of MON 13900 in the environment.

Description of key information

The aerobic and anaerobic soil metabolism of MON 13900, 3-(dichloroacetyl)-2,2-dimethyl-5-(2-furanyl)-oxazolidine, were studied in Sable silt loam and Sarpy sandy loam soils. The test material was labeled at the carbon-4 position of the oxazolidine ring with 13C and 14C, and was applied to the soil at a rate of 0.39 mg/kg and 0.42 mg/kg in the aerobic and anaerobic study respectively.

In the aerobic study the soils were incubated for one year at 25 ± 1 °C in darkness. MON 13900 was extensively metabolized in soil to carbon dioxide and several water-soluble metabolites. The estimated half-life of MON 13900 using a first-order kinetic model was 32.8 and 52.5 days in Sable and Sarpy soils, respectively. The predominant metabolite produced during the study was radiolabeled carbon dioxide. At the end of the study, radiolabeled CO2 accounted for 37.2% and 40.5% of applied radioactivity in Sable and Sarpy soils, respectively. Of the radioactivity remaining in the soil, only one metabolite fraction was significant. This fraction was identified as the MON 13900 oxamic acid, and accounted for up to 5.9% of applied radioactivity in Sarpy soil at Day 30. After Day 30, the quantity of the oxamic acid decreased steadily, until it represented approximately 1.5% of applied dpm at the end of the study. Unextractable radioactivity in the soil increased during the study, and accounted for approximately 30% of applied dpm at Day 365. Total accountabilities during the study averaged 94.0 ± 3.8 % and 95.4 ± 4.1 (average± standard deviation) for Sable and Sarpy soils, respectively.

For the anaerobic study treated soils were aged under aerobic conditions for 30 days in darkness at 25 °C . After the aerobic aging period, the soil flasks were converted to anaerobic conditions by flooding the soils with water and purging with nitrogen. The soils were incubated anaerobically for 63 days in darkess at 25 °C. Redox potential measurements were used to document the anaerobic conditions.

MON 13900 was extensively metabolized in both soils to a mixture of water-soluble metabolizes. Carbon dioxide was a significant metabolite produced during the aerobic aging period. The half-life of MON 13900 in soil under anaerobic conditions was estimated using a first-order kinetic model to be 14.7 and 13.0 days in Sable and Sarpy soils, respectively.

The predominant metabolite produced during the anaerobic portion of the study was the MON 13900 oxamic acid. Other soil-contained metabolizes characterized were the MON 13900 alcohol and the MON 13900 methyl sulfide. Radiolabeled carbon dioxide and volatile production was minimal during the anaerobic period. During the aerobic aging period, radiolabeled CO2 production was significant, accounting for approximately 9% of the applied radioactivity. The MON 13900 oxamic acid accounted for up to 8.1 and 13.3% of the applied dpm in Sable and Sarpy soils, respectively, at Day 63. The MON 13900 alcohol accounted for up to 11.9% of the applied dpm in Sarpy soil at Day 63. Unextractable radioactivity (bound residues) in the soil increased during the study, and accounted for approximately 50 and 40% of the applied dpm at Day 93 in Sable and Sarpy soils, respectively. The extracted soils were subjected to a fractionation procedure to characterize the bound residues. The results of the fractionation experiments indicate that of the unextractable radioactivity in Sable and Sarpy soils, a large portion is associated with the insoluble humic fraction of the soil. Total accountabilities during the study averaged 93.1 +- 5.7% and 97.0 +- 3.7% of applied dpm (average +- standard deviation) for Sable and Sarpy soils, respectively.

The results of these studies indicate that MON 13900 is readily degraded by soil microorganisms under aerobic conditions to carbon dioxide and water-soluble metabolites and under anaerobic conditions to a mixture of water-soluble metabolizes. Thus, aerobic and anaerobic soil metabolism will be significant routes of degradation of MON 13900 in the environment.

Key value for chemical safety assessment

Half-life in soil:
52.5 d
at the temperature of:
25 °C

Additional information

A study was conducted to determine the residue levels of MON 13900 in soil after one application of MON 69447, a herbicide formulation containing 840 g/L acetochlor and 28 g/L MON 13900.The samples were analyzed using a company-internal validated method. The test substance was applied once on bare soil at the rate of 2.38 L/ha (equivalent to 0.067 kg MON 13900/ha), with a spray volume of 200 L/ha. Soil cores of 30 cm depth were taken at 9 occasions: at 0, 1, 3, 7,14, 30, 90, 180 and 270-300 d after application. MON 13900 residues were analysed in soil samples corresponding to the 0-10 cm and 10-20 cm depth layers. The analytical method consisted of a soil extraction, a filtration and direct injection into a gas chromatograph equipped with a mass specific detector. The method was initially validated at the limit of quantification (LOQ) of 0.005 mg/kg dry weight (dw) and later on at the LOQ of 0.001 mg/kg dw. Residue levels of MON 13900 in the 0-10 cm layer were up to 0.01 mg/kg dw just after application and reached a maximum of 0.001 mg/kg dw after 14 d. In the 10-20 cm soil layer, the concentration of MON 13900 was always below the LOQ of 0.001mg/kg dw, except once at 0.003 mg/kg dw on the day of application. DT50 and DT90 values were calculated using the non-linear Excel Solver model and ranged from 0.5 to 1.5 d and 1.6 to 4.9 d, respectively.