Hexamethylcyclotrisiloxane

Toxicity to soil microorganisms

Link to relevant study record(s)

Reference

Reference 1

Endpoint:

toxicity to soil microorganisms

Type of information:

experimental study

Adequacy of study:

supporting study

Study period:

21st of October 1996 to 23rd of January 1997

Reliability:

1 (reliable without restriction)

Rationale for reliability incl. deficiencies:

guideline study

Qualifier:

according to guideline

Guideline:

BBA Part VI, 1-1

Version / remarks:

1990

Deviations:

yes

Remarks:

the moisture content of the supplied soil sample was 10.3 % not 9 % < ± 1 %) .

Qualifier:

equivalent or similar to guideline

Guideline:

OECD Guideline 216 (Soil Microorganisms: Nitrogen Transformation Test)

Qualifier:

equivalent or similar to guideline

Guideline:

OECD Guideline 217 (Soil Microorganisms: Carbon Transformation Test)

GLP compliance:

yes (incl. QA statement)

Analytical monitoring:

yes

Remarks:

short term respiration experiments and monitoring of nitrogen transformations.

Details on sampling:

- Concentrations: 100 & 300 ppm, equivalent to 100 and 300 mg/kg dry weight
- Sampling method: 2 batches of soil were received (due to an error in remoisturising the loam soil for the respiratory study). The soils used were collected from grassland sites that had not received pesticide or fertiliser treatments within the previous 12 months. Soils were collected and prepared according to the draft international standards ISO/DlS 10381-6, Soil Quality - Sampling Part 6. All soils were air dried sufficiently to allow sieving through a 2 mm sieve on the same day. The soils were then thoroughly mixed and re-wetted to 9 ± 1 % moisture content, sub-sampled and delivered to the laboratory.
- Sample storage conditions before analysis: on receipt, the soils were stored at 21 ± 2 °C for 4 days at a moisture content of 9.29 % (sandy loam soil), 9.61 % (loam soil) and 11.34 % (loam soil). Respiration replicates with a dry weight equivalent to 100 g were placed in 250 ml glass jars, whilst nitrogen replicates with a dry weight equivalent to 50 g were weighed into 150 ml glass jars. Each replicate container was sealed with a plastic lid containing two small holes to allow air exchange but minimise moisture loss. These jars were stored at 21 ± 2 °C for 4 days (sandy loam and loam) and 3 days (loam) prior to treatment and adjustment to 60 ± 5 % of their water holding capacity.

Vehicle:

no

Details on preparation and application of test substrate:

AMENDMENT OF SOIL
- Type of organic substrate: Natural soil: The soils used for the test were obtained from Levington Agriculture Limited and collected from the field on the 29th of October 1996 and 1st of November 1996 (due to an error in re-moisturising the loam soil, a second batch was obtained). Further soils were collected on 14th of October 1996 and 29th of October 1996 respectively. All soils were air dried sufficiently to allow sieving through a 2 mm sieve and then mixed and re-wetted to 9 ± 1 % moisture content, sub-sampled and delivered to Chemex International plc on 18th of October 1996 and 1st of November 1996.

APPLICATION OF TEST SUBSTANCE TO SOIL
-Method: The test substance with the concentration 100 or 300 ppm was evenly distributed over the soil surface and thoroughly mixed for the replicate batches of conditioned soils. Each replicate was adjusted to the moisture content whereas the control replicates had water added only. All treatments had lids replaced and were incubated under aerobic conditions at 21 ± 2 °C.

At day zero (within 3 hours of dosing with test substance) and after 14, 28, 55 (sandy loam soil), 63 (loam soil) and 90 days (sandy loam soil) 100 gram samples (dry weight equivalent) were removed for determination of microbial respiration using the glucose amendment method. The optimum glucose amendment level was determined by amending replicate soil samples with differing levels of glucose e.g. 0.0, 0.1, 0.2 and 0.4 gram and by measuring CO2 evolution overnight. The glucose amendment level giving the maximum respiratory response was then chosen for subsequent respiration studies on the soils tested. Carbon dioxide evolution was measured using an infra-red gas analyser (ADC 225 MK3 model) at a flow rate through the detection cells of 2 L/h. The raw data was captured using a data acquisition board (Amplicon LiveLine PC26AT) and ADC gas logging software (PRD 1028 Version 1.07). Data was also captured to a chart recorder as a back up to the data logger. The minimum respiration rate was calculated by converting ppm CO2 by volume for each replicate to ml CO2/h/100 gram soil by the use of the following equation:
mL CO2/h/100 gram soil = ((ppm carbon dioxide-background ppm)*flow rate (L/h))/1000

VEHICLE: N/a

Test organisms (inoculum):

soil

Total exposure duration:

90 d

Test temperature:

19-23 °C

Moisture:

60 ± 5 % of water holding capacity

Nitrogen content (% dry weight):

2.3

Details on test conditions:

TEST SYSTEM
- Testing facility: Chemex International plc
- Test container (type, material, size): Respiration replicates with a dry weight equivalent to 100 g were placed in 250 ml glass jars, whilst nitrogen replicates with a dry weight equivalent to 50 g were weighed into 150 ml glass jars. Each replicate container was sealed with a plastic lid containing two small holes to allow air exchange but minimise moisture loss.
- Amount of soil: 50 g (dry weight) soil
- No. of replicates per concentration: 3
- No. of replicates per control: 3
- No. of replicates per vehicle control: N/a

SOIL INCUBATION
- Method: At the 300 ppm dosing level, the nitrogen transformation study required that each soil replicate was treated with 6.38 mL of the 2350 ppm dosing solution to achieve the final application level. Therefore, the soils were required to have a sufficiently low moisture content that the addition of 6.38 mL of DMSD solution would not cause the moisture content to exceed 60 % of the water holding capacity for each soil. Consequently, it was determined that the soils should be received at 9 ± 1 % in order to fulfil these requirements. Thus, to ensure that DMSD did not polymerise to PDMS, the soils were incubated at 60 % WHC.

SOURCE AND PROPERTIES OF SUBSTRATE
- Geographical reference of sampling site (latitude, longitude): Collected by Levington Agriculture Limited.
- History of site: not reported
- Vegetation cover: The soils used were collected from grassland sites; the vegetation was removed and soil was immediately bagged.
- Treatments with pesticides or fertilizers: No pesticides or fertilisers applied within 12 months.
- Accidental contamination: None reported
- Other: It was critical that the concentration of DMSD in the soil water never exceed 2500 ppm since polymerisation from DMSD to PDMS otherwise would occur. It was only possible to treat soil samples by addition of DMSD in water when adjusting the moisture content of the soils to the final moisture content (60 ± 5 % of water holding capacity).
Organic matter content: sandy loamy silt (both samples) 4.3 %; silty sand 1.7 %

- Depth of sampling: None reported
- Soil texture
- % sand: 38.0 (loam); 64.9 (sandy loam); 39.1 (loam)
- % silt: 50.3 (loam); 27.3 (sandy loam); 49.0 (loam)
- % clay: 11.7 (loam); 7.8 (sandy loam); 11.8 (loam)
- Soil taxonomic classification: Sandy loamy silt and silty sand.
- Soil classification system: USDA Textural Class
- pH (in water): 6.7-6.8
- Initial nitrate concentration for nitrogen transformation test (mg nitrate/kg dry weight):
- Maximum water holding capacity (in % dry weight): 60 ± 5 %
- Cation exchange capacity (mmol/kg): N/a
- Pretreatment of soil: Not reported
- Storage (condition, duration): Each replicate was stored at 21 ± 2 °C for 4 days (for the sandy loam and one of the loam replicates) and for 3 days (for the other loam replicate). Adjustment occurred to 60 ± 5 % of the water holding capacity for each of the replicates.
- Initial microbial biomass as % of total organic C:

DETAILS OF PREINCUBATION OF SOIL: Not reported

EFFECT PARAMETERS MEASURED: Nitrogen transformations, ammonification and nitrification were investigated by determining ammonium-N, nitrate-N and nitrite-N concentrations in soil amended with ground lucerne grass. The effects were measured by periodically determining the rates of CO2 evolution in glucose amended soil treatments and determining the ammonium, nitrate and nitrite-nitrogen in 2 M KCl soil extracts.

VEHICLE CONTROL PERFORMED: N/a

RANGE-FINDING STUDY: None reported. The used concentrations in the study do not represent typical environmental levels - it rather represents levels that exceed those observed in the environment by a factor of at least six. Extremely high levels of the polymerised substance PDMS can be found in a surface sludge disposal area.

Nominal and measured concentrations:

Nominal concentrations: 100 and 300 ppm, equivalent to 300 mg/kg dry weight.

Reference substance (positive control):

yes

Remarks:

Aretit Flussig, containing 46.4 % w/w Dinoseb acetate.

Key result

Duration:

90 d

Dose descriptor:

NOEC

Effect conc.:

>= 300 mg/kg soil ww

Nominal / measured:

nominal

Conc. based on:

test mat.

Basis for effect:

nitrate formation rate

Remarks on result:

other: in a sandy loam soil

Key result

Duration:

55 d

Dose descriptor:

NOEC

Effect conc.:

>= 300 mg/kg soil ww

Nominal / measured:

nominal

Conc. based on:

test mat.

Basis for effect:

nitrate formation rate

Remarks on result:

other: in a loam soil

Key result

Duration:

90 d

Dose descriptor:

NOEC

Effect conc.:

>= 300 mg/kg soil dw

Nominal / measured:

nominal

Conc. based on:

test mat.

Basis for effect:

respiration rate

Remarks on result:

other: in a sandy loam soil

Key result

Duration:

63 d

Dose descriptor:

NOEC

Effect conc.:

>= 300 mg/kg soil dw

Nominal / measured:

nominal

Conc. based on:

test mat.

Basis for effect:

respiration rate

Remarks on result:

other: in a loam soil

Details on results:

Soil microflora respiration:
A statistically significant effect on the microbial respiration occurred at day 0 when 100 or 300 ppm of the test substance was applied to a loam soil. The deviation was 19.42 % and 13.59 % for the 100 and 300 ppm treatment, respectively. After 14 days, the statistically significant effect was 28.00 % at the 100 ppm treatment level. For the 300 ppm treated soil, a deviation of 13.00 % was observed. After 28 days, the deviation was 4.00 % and 7.00 % for 100 and 300 ppm, respectively. The percentage of 7.00 % was not statistically significant. At day 63, no statistically significant effects were observed at either application rate (0.93 % and 2.78 % for 100 and 300 ppm, respectively). No dose related effects were observed in the loam soil during the course of the respiration study.

No statistically significant effects greater than ± 15 % on the soil respiration was observed when the test substance was applied to a sandy loam soil. Percentage deviations in excess of ± 15 % were observed, although, these were not statistically significant.
At day 0, percentage deviations of 0.00 % and -10.53 % for 100 and 300 ppm, respectively were observed. After 14 days, no statistically significant effects were observed at either application rate. Although, a percentage deviation of -17.07 % was observed at the 100 ppm application level. A percentage of -4.88 % at 300 ppm was observed with respect to control values. After 28 days incubation, a percentage deviation of +14.29 % was observed at the 100 ppm level whilst at the 300 ppm level, a non-significant deviation of +19.05 % was observed. After 55 days, percentage deviations of -17.39 % and -21.74 % for 100 and 300 ppm respectively were observed. These deviations were not found to be statistically significant at the 5 % level. Respiration rates were found to be lower for both day 28 and day 55 than for the remainder of the study. This was stated that it may depends on a reduction in oxygen availability in the moist soils. The study continued for 90 days at which time no statistically significant effects in excess of ± 15 % were observed at any of the application rates. Percentage deviations of +14.71 % and +2.94 % for the 100 and 300 ppm treatment levels, respectively, were observed.

No dose related effects were observed in the sandy loam soil during the course of the respiration study.

At the end of the study, there were no observed effects on the soil pH in either soil types at either treatment level.


Soil nitrogen transformations
When the test substance was applied at 100 ppm to a loam soil at day 0, a statistically significant effect was observed on both the ammonium and nitrite concentrations. Following 300 ppm application concentration, a statistically significant effect was observed for nitrite concentrations only. The percentage deviations at 100 ppm were -18.27 % (ammonium), 7.28 % (nitrate), -39.76 % (nitrite) and -9.45 % (total mineral nitrogen, N-min). At 300 ppm, percentage deviations of -5.10 % (ammonium), 12.93 % (nitrate), -30.12 % (nitrite) and -0.06 % (N-min) were observed. After 14 days incubation, ammonium and nitrite concentrations were found to be below the detection limit of 0.27 mg/kg in all the treatments. Percentage deviations of 58.02 % were observed for both nitrate and N-min concentrations at 100 ppm. These deviations were found to be non-significant at the 5 % level, indicaing some variability in the data. At 300 ppm, deviations of 1.35 % were observed for both nitrate and N-min concentrations. At day 28, a statistically significant effect of -22.28 % was observed on nitrate concentrations at the 100 ppm application level. A percentage deviation of -22.28 % was also observed for N-min concentrations at this level. At 300 ppm, no statistically significant effects in excess of ± 15 % were observed, with percentage deviations of 7.17 % for both nitrate and N-min concentrations. At day 55, no statistically significant effects in excess of ± 15 % were observed at either application rate. Percentage deviations were observed to be -6.20 % and 3.61 % for 100 and 300 ppm levels, respectively, for both nitrate and N-min concentrations.
No dose related effects were observed in the loam soil during the study period.

When the test substance was applied at 100 or 300 ppm to a sandy loam soil at day 0, statistically significant effects were observed on both the ammonium and nitrite concentrations. The percentage deviations at 100 ppm were -18.55 % (ammonium), -14.59 % (nitrate), -36.23 % (nitrite) and -17.82 % (N-min). At 300 ppm, percentage deviations of -19.12 % (ammonium), -15.95 % (nitrate), -31.88 % (nitrite) and -18.44 % (N-min) were observed. These results indicate a temporary inhibitory effect of DMSD on the nitrogen transformation processes. After 14 days incubation, ammonium and nitrite levels were below the detection limit of 0.26 mg/kg in all treatment groups. At 100 ppm, a percentage deviation of 7.14 % was observed for both nitrate and N-min levels. At 300 ppm, a non-statistically significant deviation of 33.93 % was observed for neither nitrate nor N-min at both concentration levels. The rate at which nitrogen mineralisation occurred was reduced at day 14 due to a reduction in oxygen availability in the moist soils. After 28 days, a statistically significant stimulatory effect of 34.90 % was observed on nitrate concentrations at the 100 ppm application level. At 300 ppm, a non-statistically significant deviation of 26.98 % was observed for both nitrate and N-min levels. After 55 days, non-statistically significant deviations in excess of ± 15 % were observed for both nitrate and N-min concentrations at 100 and 300 ppm. Specifically, a percentage of 35.26 % at 100 ppm and 29.40 % at 300 ppm were observed for nitrate and N-min concentrations. After 90 days, non-statistically significant effects in excess of ± 15 % were observed at both 100 and 300 ppm concentration levels. A percentage deviation of 22.47 % at 100 ppm and 29.32 % for 300 ppm were observed for both nitrate and N-min levels.
At the end of the study, there were no observed effects on the soil pH in either soil types at 100 or 300 ppm.

Results with reference substance (positive control):

The reference substance produced a significant inhibitory effect on the soil microflora respiration in both soil types.

See result tables in attached document.

Validity criteria fulfilled:

yes

Conclusions:

In a toxicity to soil microorganisms study, conducted according to BBA, Part VI, 1-1 (equivalent to OECD guidelines 216 and 217) and in compliance with GLP, it was concluded that the substance dimethylsilanediol did not cause any significant long-term effects on the soil microflora respiration or the nitrogen transformations in sandy loam or loam soils. Thereby, the NOECs were concluded to be equal to or greater than 300 ppm in studies up to 90-days, equivalent to 300 mg/kg dry weight.

Executive summary:

Soil microflora respiration

A statistically significant stimulatory effect on the soil microbial respiration was observed for 28 days when 100 ppm of dimethylsilanediol was applied to a loam soil. However, after 28 days, no statistically significant effects in excess of ± 15 % were observed for neither nitrate nor N-min. Following application of 300 ppm of dimethylsilanediol, no treatment-related effects were observed throughout the study.

In the sandy loam soil, no statistically significant effects were observed at neither 100 nor 300 ppm levels. However, some percentage deviations were found to be in excess of ± 15 % with respect to control values although, after 90 days, no effects in excess of ± 15 % were observed. The relatively high variability within these samples is believed to depends on the low rates of respiration encountered in moist soil. The soil microflora respiration study did not produce any evidence of a dose response resulting from the application of the test substance.

Soil Nitrogen transformations

When the test substance was applied to a loam soil at day 0, some transient effects on the ammonium and nitrite levels occurred, regardless of the application levels. Additionally, a statistically significant effect after 28 days was observed on the nitrate concentrations at the 100 ppm treatment level. However, after 55 days, no statistically significant effects were observed at none of the treatment levels.

When the test substance was applied to a sandy loam soil at day 0, statistically significant effects were observed at both 100 and 300 ppm on nitrogen levels. Additionally, a slight temporary stimulation of nitrification was observed during the study. However, this was not considered as concentration related and returned to normal after 90 days incubation. Generally, greater variability within the data occurred for this type of soil. It is considered to be due to the required high moisture content to prevent the test substance to polymerise to polydimethylsiloxane (PDMS). This variability caused a number of percentage deviations in excess of ± 15 %, however, these were not significant at the 5 % level. Consequently, it was concluded that the substance did not cause any significant long-term effects on the soil microflora respiration or the nitrogen transformation in soil.

Reference 2

Endpoint:

toxicity to soil microorganisms

Data waiving:

study scientifically not necessary / other information available

Justification for data waiving:

other:

Description of key information

In a toxicity to soil microorganisms study, conducted according to BBA, Part VI, 1-1 and in compliance with GLP, it was concluded that the substance dimethylsilanediol did not cause any significant long-term effects on the soil microflora respiration or the nitrogen transformation in soil. Thereby, the NOEC was concluded to be equal to or greater than 300 ppm, equivalent to 300 mg/kg dry weight (SEHSC, 1997).

Key value for chemical safety assessment

Long-term EC10 or NOEC for soil microorganisms:

300 mg/kg soil dw

Additional information

No data are available for toxicity of the registered substance to soil microorganisms. The data requirement is currently waived. Please refer to the data waiver endpoint study record for further discussion.

Data are available with the ultimate hydrolysis product dimethylsilanediol (DMSD, CAS 1066-42-8; EC No. 213-915-7).

In a toxicity to soil microorganisms study, conducted according to BBA, Part VI, 1-1 (equivalent to OECD guidelines 216 and 217) and in compliance with GLP, it was concluded that the substance dimethylsilanediol did not cause any significant long-term effects on the soil microflora respiration or the nitrogen transformations in sandy loam or loam soils. Thereby, the NOECs were concluded to be equal to or greater than 300 ppm in studies up to 90-days, equivalent to 300 mg/kg dry weight.

Soil microflora respiration

A statistically significant stimulatory effect on the soil microbial respiration was observed for 28 days when 100 ppm of dimethylsilanediol was applied to a loam soil. However, after 28 days, no statistically significant effects in excess of ±15% were observed for neither nitrate nor N-min. Following application of 300 ppm of dimethylsilanediol, no treatment-related effects were observed throughout the study.

In the sandy loam soil, no statistically significant effects were observed at either 100 or 300 ppm levels. However, some percentage deviations were found to be in excess of ±15% with respect to control values although, after 90 days, no effects in excess of ±15% were observed. The relatively high variability within these samples is believed to depend on the low rates of respiration encountered in moist soil. The soil microflora respiration study did not produce any evidence of a dose response resulting from the application of the test substance.

Soil Nitrogen transformations

When the test substance was applied to a loam soil at day 0, some transient effects on the ammonium and nitrite levels occurred, regardless of the application levels. Additionally, a statistically significant effect after 28 days was observed on the nitrate concentrations at the 100 ppm treatment level. However, after 55 days, no statistically significant effects were observed at any of the treatment levels.

When the test substance was applied to a sandy loam soil at day 0, statistically significant effects were observed at both 100 and 300 ppm on nitrogen levels. Additionally, a slight temporary stimulation of nitrification was observed during the study. However, this was not considered as concentration related and returned to normal after 90 days incubation. Generally, greater variability within the data occurred for this type of soil. It is considered to be due to the required high moisture content to prevent the test substance polymerising to polydimethylsiloxane (PDMS). This variability caused a number of percentage deviations in excess of ±15, however, these were not significant at the 5% level. Consequently, it was concluded that the substance did not cause any significant long-term effects on the soil microflora respiration or the nitrogen transformation in soil.