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Toxicity to soil microorganisms

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Reference
Endpoint:
toxicity to soil microorganisms
Type of information:
experimental study
Adequacy of study:
key study
Study period:
The test incubation period was conducted between 26 March and 23 April 2019
Reliability:
1 (reliable without restriction)
Rationale for reliability incl. deficiencies:
guideline study
Qualifier:
according to
Guideline:
OECD Guideline 216 (Soil Microorganisms: Nitrogen Transformation Test)
Deviations:
no
GLP compliance:
yes
Analytical monitoring:
no
Details on sampling:
The test substance was not analysed
Vehicle:
no
Details on preparation and application of test substrate:
AMENDMENT OF SOIL
- Type of organic substrate: Speyer 2.3 soil
- Other: N/A

APPLICATION OF TEST SUBSTANCE TO SOIL
- Method: Since the test substance was received as a liquid, the test substance was applied directly to the soil and a carrier was not used. Approximately half of the soil batch was transferred to a 4-L jar. An appropriate amount of the test substance was dosed dropwise across the soil surface using a pipette. The remaining half of the soil batch was then added to the 4-L jar. The soil was mixed by rolling the capped jar for approximately 3 hours at approximately 15 rpm on a rolling mill. The fortified soil was transferred to the bowl of a food mixer and mechanically mixed for 10 minutes at low speed. During this mechanical mixing, the soil was amended with 5.0 g of lucerne meal (i.e. alfalfa). Also during this mechanical mixing a volume of purified reagent water was added to the soil in order to bring the moisture content to 45 ± 5% of the soil’s maximum water holding capacity. After dosing and mixing, each batch of soil was divided equally into three replicate test samples in 1-L glass bottle test vessels.

VEHICLE:
Not used
Test organisms (inoculum):
soil
Total exposure duration:
28 d
Test temperature:
20 ± 2 °C
Moisture:
45 ± 5% of the soil’s maximum water holding capacity
Details on test conditions:
TEST SYSTEM
- Testing facility: Smithers Viscient, 790 Main Street, Wareham, Massachusetts 02571-1037
- Test container (type, material, size): The 1-L glass bottle test vessels, covered with perforated parafilm to allow for air exchange while reducing water loss.
- Amount of soil: not reported
- No. of replicates per concentration: 3
- No. of replicates per control: 3
- No. of replicates per vehicle control: N/a

SOIL INCUBATION
- Method: bulk / series of individual subsamples

SOURCE AND PROPERTIES OF SUBSTRATE (if soil)
- Geographical reference of sampling site (latitude, longitude): Offenbach, Rheinland-Pfalz, Germany “rechts der Landauer Str.”, No. 826/7
- History of site: Over the past 4 years, no pesticides or organic fertilizers have been applied to the collection site.
- Vegetation cover: None reported
- Treatments with pesticides or fertilizers: See above
- Accidental contamination: None reported
- Other:
- Depth of sampling: 0-20 cm
- Soil texture
- % sand: 68%
- % silt: 26%
- % clay: 6%
- Soil taxonomic classification: Not reported
- Soil classification system: Not reported
- pH (in water): 6.5
- Initial nitrate concentration for nitrogen transformation test (mg nitrate/kg dry weight): 79.4-82.5 mg/kg not reported whether dry or wet weight
- Maximum water holding capacity (in % dry weigth): 26.4
- Cation exchange capacity (mmol/kg): 5.7 meq/100 g
- Pretreatment of soil: The soil was collected from the top 20 centimeters of the field and passed through a 2-mm sieve.
- Storage (condition, duration): refrigerated (approximately 4 °C) and in the dark
- Initial microbial biomass as % of total organic C: 2.2% of total organic carbon

DETAILS OF PREINCUBATION OF SOIL (if any): not reported

EFFECT PARAMETERS MEASURED (with observation intervals if applicable) : Nitrate formation (nitrogen transformation) at 0 and 28 d

VEHICLE CONTROL PERFORMED: no, not required

RANGE-FINDING STUDY
- Test concentrations: not conducted
- Results used to determine the conditions for the definitive study: N/A
Nominal and measured concentrations:
Nominal: Untreated control, 1.2, 3.7, 11, 33, and 100 mg/kg dry weight soil
Reference substance (positive control):
no
Key result
Duration:
28 d
Dose descriptor:
EC10
Effect conc.:
> 100 mg/kg soil dw
Nominal / measured:
nominal
Conc. based on:
act. ingr.
Basis for effect:
nitrate formation rate
Duration:
28 d
Dose descriptor:
EC25
Effect conc.:
> 100 mg/kg soil dw
Nominal / measured:
nominal
Conc. based on:
act. ingr.
Basis for effect:
nitrate formation rate
Duration:
28 d
Dose descriptor:
EC50
Effect conc.:
> 100 mg/kg soil dw
Nominal / measured:
nominal
Conc. based on:
act. ingr.
Basis for effect:
nitrate formation rate

 

Table 1 - Nitrate concentration and formation rate

 

Test System

Mean Day 0

Nitrate

Concentration

(mg/kg)

Mean Day 28

Nitrate

Concentration

(mg/kg)

 

Mean Nitrate

Formation Rate

(mg/kg/day)

 

Rate Difference

from the

Untreated Control

(%)

 

Untreated Control

80.3

276

6.98

NAa

 

1.2 mg/kg

81.3

270

 

6.75

-3.33

3.7 mg/kg

81.0

 

264

6.53

-6.35

11 mg/kg

80.3

 

255

6.24

-10.6

33 mg/kg

81.0

 

260

6.40

-8.22

100 mg/kg

82.1

 

266

6.57

-5.80

 

aNA = Not Applicable

 

Table 2 - Test System Result

 

Test System

Result

1.2 mg/kg

Non-Significant Effect

 

3.7 mg/kg

Non-Significant Effect

 

11 mg/kg

Significant Effectb

33 mg/kg

Significant Effectb

100 mg/kg

Non-Significant Effect

 

 

bSignificantly reduced compared to the untreated control, based on Dunnett’s Multiple Comparison Test. However, the guidance indicates that differences in the rate of nitrate formation between control and treated soils at values ≤25% are not expected to have a long-term influence on nitrogen transformation rates. Additionally, there is no significant inhibition at the highest concentration, indicating any statistically significant effects may just be experimental variation.

Validity criteria fulfilled:
yes
Remarks:
Relative standard deviation in nitrate concentration and formation rate for the untreated control samples ranged from 0.1 to 0.6%, meeting the 15% validity criteria.
Conclusions:
No effects were observed in a 28-day exposure of the test substance to soil microorganisms, determining EC10, EC25 and EC50 values of >100 mg/kg.

Description of key information

Key value for chemical safety assessment

Additional information

A 28 day toxicity to soil microorganisms study for effects of L4 on nitrate formation rate of soil microflora has been conducted in accordance with OECD TG 216 (Soil Microorganisms: Nitrogen Transformation Test) at concentrations up to 100 mg/kg soil dry weight. No effects on nitrate formation rate were observed.

A 28 day EC50 value of >100 mg/kg soil dw and an EC10 value of >100 mg/kg soil dw (highest concentration tested) have been determined for the effects of decamethyltetrasiloxane (L4, CAS 141-62-8) on nitrate formation rate of soil microflora based on nominal concentrations.

Analytical verification of test substance concentrations during the earthworm toxicity test indicate that the test substance is unstable in the soil, achieving a 44% arithmetic mean concentration after 56 days. The stability test conducted for the earthworm study indicates that degradation products were not seen in the chromatograms, which together with the steady loss of 14C activity, shows that the primary mechanism of test article loss was volatilization of 14C-L4 from the simulated OECD 222 set-up. The soil was prepared using a similar method in both the earthworm and microorganisms tests, therefore it is likely that the soil microorganisms were exposed to the test substance throughout the test.