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Ecotoxicological information

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:
weight of evidence
Study period:
from 09-JUL-1996 to 09-DEC-1996
Reliability:
1 (reliable without restriction)
Rationale for reliability incl. deficiencies:
guideline study
Qualifier:
equivalent or similar to
Guideline:
OECD Guideline 217 (Soil Microorganisms: Carbon Transformation Test)
Deviations:
yes
Remarks:
the incubations with cerium dioxide were carried out with 400 g (dry weight) pre-incubated soil samples instead of 500 g. This deviation is assumed not to have affected the results of the study.
GLP compliance:
yes
Analytical monitoring:
no
Details on sampling:
no data
Vehicle:
no
Details on preparation and application of test substrate:
AMENDMENT OF SOIL
- Type of organic substrate: A preliminary test was done to determine the amount of glucose needed to elicit a maximum respiratory response. To induce a substrate response from the soil which can be related to the microbial biomass present, the substrate (glucose) must not be limiting. Appropriate amounts of glucose were added to 0.5 g portions of talc powder, and after thoroughly mixing, these mixtures were added to 50 g (dry weight) soil samples. The final nominal concentrations of glucose were 0, 1000, 2000, 4000 and 6000 mg/kg dry soil. As there was no difference in respiration rates in the soil samples amended with different amounts of glucose, this means that the lowest glucose concentration tested was not limiting. The determination of the effects of cerium dioxide on soil microorganisms was thus conducted with 1000 mg glucose/kg dry soil.


APPLICATION OF TEST SUBSTANCE TO SOIL
- Method: Appropriate amounts of the test substance were weighed and added to 4 g portions of quartz sand. After mixing thoroughly, these mixtures were added to 400 g (dry weight) portions of soil.

Test organisms (inoculum):
other: soil micro-organisms
Total exposure duration:
28 d
Test temperature:
20 ± 2°C
Moisture:
The moisture content was adjusted to 40% of the water holding capacity.
Details on test conditions:
TEST SYSTEM
- Testing facility: a specially designed apparatus allowing to determine the glucose induced respiration rate according to the method of Anderson & Domsch
- Test container (type, material, size): glass bottles of about 700 ml covered with aluminium foil containing several small holes.
- Amount of soil: 400 g (dry weight) portions of soil
- No. of replicates per concentration during incubation: 1
- No. of replicates per control during incubation: 1

SOIL INCUBATION
- Method: The thoroughly mixed soil, quartz sand and test substance mixtures were incubated as bulk samples in singlefold at 20 ± 2°C in glass bottles of about 700 ml covered with aluminium foil containing several small holes. The loss of water was checked every fortnight, and water was added to compensate for evaporation.

SOURCE AND PROPERTIES OF SUBSTRATE (if soil)
- Geographical reference of sampling site (latitude, longitude): The sandy loam soil was obtained from an agricultural grassland located at the Maasdijk in Heerewaarden, the Netherlands (51°48'N, 5°22'E) on July 9, 1996.
- History of site:
- Vegetation cover: no data
- Treatments with pesticides or fertilizers: no data
- Accidental contamination: no data
- Depth of sampling: samples were taken from a depth of 0 to 20 cm.
- Soil texture
- % sand: 60.2 % (> 50 µm)
- % silt: 25.7% (2-50 µm)
- % clay: 14.1% (< 2 µm)
- Soil taxonomic classification: no data
- Soil classification system: no data
- pH (in water): 7.3
- Maximum water holding capacity (in % dry weigth): 47.5%
- Cation exchange capacity (mmol/kg): 11.5 cmol/kg dry soil
- Pretreatment of soil: Before the start of the experiment, the soil was partly air-dried to pass through a 2 mm sieve. The moisture content was adjusted to 40% of the water holding capacity.
- Storage (condition, duration): The soil was stored at refrigerator temperature pending use.
- Initial microbial biomass as % of total organic C: 747 mg microbial carbon / kg dry soil

DETAILS OF PREINCUBATION OF SOIL : the soil was pre-incubated for nine days at 20 ± 2°C to restore microbial activity.

EFFECT PARAMETERS MEASURED :
After 14 and 28 days of incubation, triplicates of 50 g (dry soil) were taken in the bulk soil samples. These 50-g sub-samples were amended with the proper amount of the glucose/talc powder mixture. The soil samples were then placed in glass columns, through which CO2-free air was led. The air flow was adjusted with Show-Rate (Brooks) flowmeters to yield a CO2 measurement in the range of 0-1000 µl CO2/l air. The residence time of the gas sample changer (ADC) was set to 5 minutes. CO2 concentrations in the gas streams were measured for 11 to 12 hours with an ADC-225 mk3 type Infrared CO2 meter. The values measured were recorded with an electrical recorder. The heights of the peaks on the output (paper) of the recorder were measured and converted to amounts of CO2 expressed in µl/l air.
The CO2 evolution Y was calculated as follows:
Y = x * v * 2 / 1000
where x = CO2 concentration calculated from the recorder output (µl/l)
v = gas flow (l/h)
2 = factor because 50 g of dry soil was used
1000 = correction factor (µl/ml)

RANGE-FINDING STUDY
no data
Nominal and measured concentrations:
Nominal concentrations: 0, 10, 32, 100, 320 and 1000 mg/kg (dry soil) of cerium dioxide
Reference substance (positive control):
no
Key result
Duration:
28 d
Dose descriptor:
NOEC
Effect conc.:
>= 1 000 mg/kg soil dw
Nominal / measured:
nominal
Conc. based on:
test mat.
Basis for effect:
respiration rate
Details on results:
The statistical evaluation shows two significant deviations from the control:
- after 14 days of incubation, the soil samples with (nominal) 10 mg cerium dioxide/kg dry soil show a stimulation of active microbial biomass (level of significance 99%)
- after 28 days of incubation, the soil samples with (nominal) 32 mg cerium dioxide/kg dry soil show an inhibition of active microbial biomass (level of significance 95%)
The latter result is probably an artefact, because the higher concentrations of cerium dioxide did not affect the soil microorganisms. The background cerium content of the soil used for the study was 27 mg/kg dry soil, this is equivalent to approximately 33 mg cerium oxide/kg dry soil.
It may be concluded that cerium dioxide at a concentration of at least 1033 mg/kg dry soil (that is 841 mg/kg cerium) shows no adverse effects on the carbon mineralisation activity of the microbial biomass in soil.
This concentration is approximately 30 times the natural background of the tested soil. As the difference in respiration rates between soil samples with the highest cerium dioxide concentration and the control soil samples was less than 15%, it could be concluded that cerium dioxide will have no long-term influence on carbon mineralisation in soils.
Results with reference substance (positive control):
none
Reported statistics and error estimates:
The glucose induced respiration rates (expressed as µl CO2/l air) found in the treated samples were compared to those found in the control. The values were analysed using a two-tailed Dunnett test (multiple comparison with a control).

Table 1: Results of the soil carbon mineralisation test with cerium dioxide. Average CO2 evolution (measured for 11 hours) and standard deviation of triplicate 50 g (dry weight) soil samples, incubated for 14 and 28 days with various amounts of cerium dioxide (nominal concentrations)

Incubation period

14 days

 

28 days

 

 

Average CO2 evolution

(µL/L air)

Standard deviation

(µL/L air)

Average CO2 evolution

(µL/L air)

Standard deviation

(µL/L air)

0

10

32

100

320

1000

163

181**

159

156

158

160

9.6

11.8

11.6

12.2

12.3

12.6

159

166

145*

151

153

169

15.6

12.3

10.1

11.8

12.0

13.4

* level of significance: 95%

** level of significance: 99%

Validity criteria fulfilled:
yes
Remarks:
standard deviation between the control replicates after 14 days was ± 6.0%, and after 28 days ± 9.8% (< 10%)
Conclusions:
The NOEC is equal or higher than 1000 mg/kg. Cerium dioxide is therefore not expected to have a long-term influence on carbon mineralisation in soil.
Executive summary:

Cerium dioxide has been tested for its toxicity on the carbon mineralisation activity of soil microorganisms. The study was carried out according to a draft proposal for a new OECD Guideline (June 1996) and in compliance with the principles of Good Laboratory Practices.

 

A non limiting glucose concentration of 1000 mg/kg was added to a sandy loam soil as carbon source. The following nominal concentration of cerium dioxide were tested : 0, 10, 32, 100, 320 and 1000 mg/kg dry soil. After 14 and 28 days of incubation the carbon mineralisation activity was determined by measuring the CO2 evolution.

 

The statistical evaluation of the carbon minaralisation activity shows two significant deviation compared to the control; a stimulation of the activity at 10 mg/kg and an inhibition of the activity at 32 mg/kg. But as this inhibitory effect was not observed at higher concentrations of cerium dioxide, it is considered as an artefact.

 

Cerium dioxide at a concentration of at least 1000 mg/kg dry soil shows no adverse effect on the carbon mineralisation activity of the microbial biomass in soil.The NOEC is equal or higher than 1000 mg/kg. Cerium dioxide is therefore not expected to have a long-term influence on carbon mineralisation in soil.

Description of key information

The 28-day NOEC of cerium dioxide was 100 mg/kg dry soil based on the respiration rate. Hence, Cerium dioxide is not expected to have a long-term influence on carbon mineralisation in soil.

Key value for chemical safety assessment

Additional information

The existing data only concerns the bulk form. One experimental study, scored as Klimisch 1 ( Muttzall & Hanstveit, 1996), was available and selected as a key study.