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Biodegradation in soil

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Endpoint:
biodegradation in soil: simulation testing
Type of information:
migrated information: read-across from supporting substance (structural analogue or surrogate)
Adequacy of study:
key study
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
other: see 'Remark'
Remarks:
GLP - Guideline study, tested with the source substance tetramethylthiuram disulfide (CAS No. 137-26-8). In accordance to the ECHA guidance document “Practical guide 6: How to report read-across and categories (March 2010)”, the reliability was changed from RL1 to RL2 to reflect the fact that this study was conducted on an read-across substance
Qualifier:
according to guideline
Guideline:
EU Method C.23 (Aerobic and Anaerobic Transformation in Soil)
Deviations:
no
GLP compliance:
yes
Test type:
laboratory
Radiolabelling:
yes
Oxygen conditions:
aerobic
Soil classification:
not specified
Soil no.:
#1
Soil type:
sandy loam
% Clay:
5.6
% Silt:
10.9
% Sand:
83.5
% Org. C:
1.05
pH:
6.1
CEC:
5.62
Soil no.:
#2
Soil type:
sandy clay loam
% Clay:
26.6
% Silt:
26.2
% Sand:
47.2
% Org. C:
3.8
pH:
6.9
CEC:
24.9
Soil no.:
#3
Soil type:
clay
% Clay:
48
% Silt:
30
% Sand:
22
% Org. C:
1.8
pH:
7.8
CEC:
39.9
Details on soil characteristics:
SOIL COLLECTION AND STORAGE
- Geographic location: The sandy loam (Borstel, RJ-93) soil was collected from the top 30 cm of the surrounding area of a barley field in the Lower Saxony (Niedersachsen) D-23057 Borstel, Germany on June, 1998. The sandy clay loam (UK-1, RJ-94) and clay (IT01, RJ-95) soil were sampled on March, 1998 and October, 1997 from the upper 20 cm of a field 3 miles south of Droitwich, Worcestshire, England and Piacentino, Italy, respectively
- Pesticide use history at the collection site: The barley field from where the sandy soil was collected, was treated with the herbicides diflufenican and isoproturon which are not structurally or by mode of action similar to that of the test substance (thiram). The other two soils were not treated with any pesticide for at least 3 years
- Storage conditions: kept at ca. 4°C
- Soil preparation: prior to incubation, each soil was acclimated for several days at the approximate temperature (i.e., either 20 ± 2 °C or 10 ± 2 °C). The soil (approximately 10 g per tube, dry weight equivalent) was adjusted to 45% of MWC before treatment.
Duration:
120 d
Initial conc.:
24.4 mg/kg soil d.w.
Based on:
act. ingr.
Parameter followed for biodegradation estimation:
radiochem. meas.
Soil No.:
#1
Temp.:
20°C
Humidity:
45% MWC
Microbial biomass:
0.27 g C/kg soil
Soil No.:
#2
Temp.:
20°C
Humidity:
45% MWC
Microbial biomass:
0.43 g C/kg soil
Soil No.:
#3
Temp.:
20°C
Humidity:
45% MWC
Microbial biomass:
0.44 g C/kg soil
Soil No.:
#1
Temp.:
10°C
Humidity:
45% MWC
Microbial biomass:
0.11 g C/kg soil
Details on experimental conditions:
PRELIMINARY EXPERIMENT:
- Prior to initiation of the definitive study, a preliminary investigation on the sandy loam, sandy clay loam and clay soils was conducted for 12 days to estimate the approximate degree of degradation of 14C-thiram in the soils, and to establish method recoveries, material balance, and the efficiency of trapping volatiles using the test apparatus. The definitive study was conducted for 120 days under aerobic conditions at a temperature of 20 ± 2 °C for the three agriculturally relevant soils (sandy loam, sandy clay loam and clay). An additional incubation with one of the soils was perforrned at
10 ± 2 °C.

EXPERIMENTAL DESIGN
- The test apparatus was comprised of individual 50-mL Teflon® incubation tubes equipped with a manifold having fourteen inlet valves connected to the incubation tubes. Each incubation tube contained approximately 10 g (dry soil weight equivalent) sieved (2mm) soil adjusted to a soil moisture content of 45% of maximum water holding capacity (MWC) and was connected to a series of traps consisting of two sodium hydroxide, one methanolic KOH, and one ethylene glycol, in that order. The aerobicity of the test system was maintained by continuously passing, via positive pressure, humidified air (approximately 2 mL/ min/tube) through each incubation tube and its trapping solutions.
- With the exception of two tubes, radiolabeled test substance was added to the soil of each incubation tube. The two incubation tubes containing untreated soil were used for checking soil moisture content periodically. These two incubation tubes having untreated soil were removed from the environmental chamber once a week and were weighed to determine the loss of water during incubation. The calculated amount of deionized water (Milli-Q) was added to eaeh test incubation tube to maintain the soil mojsture content at 45% of MWC.


Test material application
- For the sandy loam (RJ-93) and sandy clay loam(RJ-94) soil, 3.10 mg of 14C-thiram (Lot# 98120A, specific activity 60 mCi/mmol) and
12.10 mg of non-radioactive thiram (Lot# AC-1 315-148, purity 99.80%) were dissolved in 4 mL of acetone (Lot # 960165, purity 99.6%, Fisher Scientific) and sonicated for approximately 3 minutes to be sure all test substance was in solution. For the clay soil (RJ-95) and the sandy loam soil (RJ-93) incubated at 10 ± 2 °C, the dosing solution was prepared by dissolving 3.00 mg of ‘4C-thiram (Lot# 98120A, specific activity 60 mCi/mmol) and 12.50 mg of non-radioactive thiram (Lot# AC-1315-148, purity 99.80%) in 4 mL of acetone (Lot # 960165, purity 99.6%, Fisher Scientific) and sonicating for approximately 3 minutes to be sure all test substance was in solution.

Experimental conditions
- Moisture maintenance method: yes
- Continuous darkness: Yes


Soil No.:
#1
% Degr.:
100
Parameter:
radiochem. meas.
Sampling time:
64 d
Soil No.:
#2
% Degr.:
100
Parameter:
radiochem. meas.
Sampling time:
64 d
Soil No.:
#3
% Degr.:
94.4
Parameter:
radiochem. meas.
Sampling time:
120 d
Soil No.:
#4
% Degr.:
100
Parameter:
radiochem. meas.
Sampling time:
64 d
Soil No.:
#1
DT50:
3.7 d
Type:
(pseudo-)first order (= half-life)
Remarks on result:
other: DT90= 12.4 d
Soil No.:
#2
DT50:
5.5 d
Type:
(pseudo-)first order (= half-life)
Remarks on result:
other: DT90= 18.3 d
Soil No.:
#3
DT50:
6.8 d
Type:
(pseudo-)first order (= half-life)
Remarks on result:
other: DT90= 61.3 d
Soil No.:
#1
DT50:
9.8 d
Type:
(pseudo-)first order (= half-life)
Remarks on result:
other: DT90= 32.7 d (10°C)
Transformation products:
yes
No.:
#1

N,N dimethyl carbamosulfonic acid (DMCS) was a major metabolite in the study, accounting for 12.12% at maximum (day 32). However, the metabolite exceeded 10% AR only in two soils and in each at one sampling event. Another degradate (< 7% AR), tetramethylthiuram monosulfide (TMTMS), was investigated in each soil, however, the compound is also an impurity of the radiolabelled test substance.

CO2as final mineralisation product was formed at amounts between 59.2 to 71.9% until study termination. Soil bound residues increased to maxima between 24.2 and 33.0 %. A fractionation indicated these residues to be mainly distributed among humic and fulvic acids. 

Endpoint:
biodegradation in soil
Type of information:
migrated information: read-across from supporting substance (structural analogue or surrogate)
Adequacy of study:
supporting study
Study period:
15 January 1993- 18 May 1995
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
other: GLP-Guideline study. However, only one soil was tested. Test conducted with the source substance tetramethylthiuram disulfide (CAS No. 137-26-8)
Qualifier:
according to guideline
Guideline:
other: US-EPA 540/9-82-021
Deviations:
no
GLP compliance:
yes
Test type:
laboratory
Radiolabelling:
yes
Oxygen conditions:
aerobic
Soil classification:
not specified
Duration:
205 d
Initial conc.:
2.04 mg/kg soil d.w.
Based on:
test mat.
DT50:
2 d
Type:
(pseudo-)first order (= half-life)
Remarks on result:
other: DT90= 7 days
Transformation products:
yes

Thiram was mineralised to a high extent. The CO2formation accounted for 74.9% of the applied radioactivity (AR) at study termination (65.9% after 84 days). The extractable radioactivity declined from 103.1% at study initiation to 56.3% and 5.7% at days 2 and 14 (1.1% at day 84). Vice versa, non-extractable residues increased up to day 14 (47.6%), thereafter a decrease was observed to 30.9% at day 205 (36.1% at day 84). Soil organic matter fractionation indicated that up to 72.1% of the non-extractable radioactivity was bound to the humin fraction and humin acids after 205 days. Analysis of the extractable radioactivity showed that besides14C-thiram, 11 radioactive fractions were present, however, at any time below 10% AR. Thiram presented the main fraction of extracted radioactivity until day 21. The major transient radioactive fraction was N,N dimethyl carbamosulfonic acid. It was present from day 1 onwards, increasing to a maximum of 8.7% AR on day 4, thereafter quickly decreasing to 0.4% on day 42. All other radioactive breakdown products were ≤ 3.1% AR.

Description of key information

Disulfiram is not expected to be persistent in soil (DT50 < 10 days).

Key value for chemical safety assessment

Additional information

No data on biodegradation of disulfiram in soil (CAS No. 97 -77 -8) were available. Therefore, a conclusion based on data from an analogue substance had to be drawn. Biodegradation of tetramethylthiuram disulfide (CAS No.137-26-8), substance structurally and composition related to tetraethylthiuram disulfide, was investigated in two studies.In the key study (Nag and Regis, 2000), conducted according to Comission Directive 95/36/EC, and EU Method C.23: Aerobic and Anaerobic Transformation in Soil under GLP conditions, the degradation rate of14C-thiram was investigated in three european soils at 20°C plus an additional one at 10°C, for 120 days.14C-thiram degraded rapidly in all three soils, obtaining half-lives between 3.7 and 9.8 days, and being ultimately mineralised to CO2. Similar results were observed in the supporting study, where rapid degradation of 14C-thiram took place in the soil tested (mainly due to mineralisation), with a half-life of 2 days.