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Toxicity to soil microorganisms

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Reference
Endpoint:
toxicity to soil microorganisms
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 thiram (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:
BBA Part VI, 1-1
Deviations:
no
GLP compliance:
yes
Analytical monitoring:
no
Vehicle:
no
Test organisms (inoculum):
soil
Total exposure duration:
90 d
Remarks:
soil type 1, 28 days (soil type 2)
Test temperature:
20 ± 2 °C
Moisture:
Soil moisture: adjusted to 45 % (soil 1) and 60 % (soil 2) of the maximum soil water holding capacity
Details on test conditions:
Approximately 800 g soil for each study group were filled into 1000 mL glass bottles. The bottles were closed loosely with screw caps and weighed. They were stored at 20 ± 2ºC in the dark. About every 7 days, the moisture loss was determined by reweighing and adding water to adjust vessels to the initial weight.
Two soil types (loamy to clayey sand (soil type 1) and loamy silt to loam (soil type 2)) were tested. Four study groups with 3 replicates were performed:
- untreated soil (control),
- soil treated with test substance 1 × application rate
- soil treated with test substance 10 × application rate
- soil treated with reference substance
For the nitrogen turnover test the soil was enriched with ground Lucerne meal prior to the application. The amount of Lucerne meal was 0.5 % of the soil dry weight.
In the short term respiration the amount of glucose for an optimum respiration rate during the first 12 hours of measurement was determined prior to the application. Finally, 500 mg glucose per 100 g wet weight were added to each sample of soil type 1 and 200 mg to soil type 2 prior to the measurement of respiration.
The test substance was applied at an application rate referring to 3 kg thiram 80 WG (2.4 kg a.s.) and 30 kg thiram 80 WG (24.0 kg a.s.) at a penetration depth of 5 cm and a specific density of 1.5 g/cm3. The water content was adjusted to 45 % (soil 1) and 60 % (soil 2) of its maximum water holding capacity.
The reference substance was applied at a concentration of 20 L/ha (5 × the field use rate).
Immediately after sampling, one aliquot of 10 g was taken for the determination of the water content (weighing the samples before and after drying at 105 ºC for 24 h) and one aliquot of 10 g for the determination of the pH (calibrated pH electrode according to DIN 19684).
Nitrogen turnover:
At each sampling an aliquot was taken from the corresponding vessel, and the following parameters were determined: soil dry weight, pH (not after 14 days), ammonium-N, Nitrate-N and nitrite-N.
Short term respiration:
For the test assays in the respirometer, 100 g of soil, each, were transferred into the reaction vessels and were mixed with the quantity of glucose to achieve an optimum short term respiration rate previously determined. The incubation period was at least 20 h at 20 ± 2 ºC. Taking into consideration the quantity of oxygen consumed during the measurement period the CO2 produced per hour per 100 g dry substance was calculated.
Sampling intervals: Nitrogen turnover: 3 h, 14 and 28, 56 (only soil type 1) and 90 days (only soil type 1)
Short term respiration: 3 h, 14 and 28 days
Nominal and measured concentrations:
Initial: 1 × recommended application rate (3 kg product/ha, i.e. 2.4 kg a.s./ha) and 10 × recommended application rate (30 kg product/ha, i.e. 24 kg a.s./ha).
Reference substance (positive control):
yes
Remarks:
herbogil liquide (dinoterb)
Duration:
90 d
Dose descriptor:
NOEC
Effect conc.:
>= 24 kg/ha
Nominal / measured:
nominal
Conc. based on:
act. ingr.
Basis for effect:
nitrate formation rate
Remarks on result:
other: deviations less than 15 % compared to control
Duration:
28 d
Dose descriptor:
NOEC
Effect conc.:
>= 24 kg/ha
Nominal / measured:
nominal
Conc. based on:
act. ingr.
Basis for effect:
respiration rate
Remarks on result:
other: deviations less than 15 % compared to control

There were no distinct effects of the test substance on the nitrogen turnover activities of soil microorganisms in both soil types in comparison to the control (deviations less than 15 %). Short term respiration rate in the treatment groups of both soil types was not substantially different from the control (deviations less than 15 %).

The deviation of the reference item dinoterb from the untreated control was more than 15 % at the end of the test.

 

Validity criteria fulfilled:
yes
Conclusions:
The impact of Thiram 80 WG on soil nitrogen turnover and short term respiration activities can be assessed as negligible even at the 10 × recommended application rate of 30 kg product/ha (24 kg a.s./ha)
The study is considered to be valid (deviation of the reference substance from the untreated control > 15 %, pH values of the control and the treatment groups did not vary remarkably).
Executive summary:

Materials and methods: The possible effects of Thiram 80 WG on the soil microflora were measured in a test on nitrogen turnover after addition of lucerne meal and on short term respiration after enrichment with glucose prior to the measurement of respiration. The test was performed in accordance with BBA guideline, part VI, 1-1. Two soil types were used, specified as sandy soil (BBA type 2.3) and a sandy-loam soil type.
Thiram 80 WG was incubated over a period of 90 days (only nitrogen turnover soil type 1) or 28 days (all other tests) at 1 time and 10 times dosage rates, referring to 3 kg product/ha (2.4 kg a.s./ha) and 30 kg product/ha (24 kg a.s./ha), respectively.

The control consisted of a treatment group with deionised water. A reference group with Herbogil liquide, a formulation containing 250 g dinoterb/L was also tested to demonstrate the normal sensitivity of the soil microflora against pesticides.

Results and discussionThere were no distinct effects of the test substance on the nitrogen turnover activities of soil microorganisms in both soil types in comparison to the control (deviations less than 15 %).

Short term respiration rate in the treatment groups of both soil types was not substantially different from the control (deviations less than 15 %).

The deviation of the reference item dinoterb from the untreated control was more than 15 % at the end of the test.

Description of key information

No inhibition in soil microflora activity was reported

Key value for chemical safety assessment

Additional information

No data on toxicity of disulfiram to soil microorganisms (CAS No. 97-77-8) were available. Therefore, a conclusion based on analogue approach had to be drawn. The toxicity of tetramethylthiuram disulfide (CAS No. 137-26-8), substance structurally and composition related to tetraethylthiuram disulfide, to soil microorganisms was investigated in one study (Wachter, 2000). This test was conducted according to the BBA-Guideline for the Official Testing of Pesticides, Part VI, 1-1 (1990) under GLP conditions. Tetramethylthiuram disulfide was incubated in two different soil types for 90 days (soil 1) and 28 days (soil 2), and the effects on soil microflora activity recorded. At test termination, no significant effects on nitrogen turnover or short term respiration were detected.