Calcium oxide

Toxicity to soil microorganisms

Link to relevant study record(s)

Reference

Reference 1

Endpoint:

toxicity to soil microorganisms

Type of information:

migrated information: read-across from supporting substance (structural analogue or surrogate)

Adequacy of study:

key study

Study period:

May 23, 2007 - August 28, 2007

Reliability:

1 (reliable without restriction)

Rationale for reliability incl. deficiencies:

other: see 'Remark'

Remarks:

GLP study, conducted according to German guidelines for testing of plant protection products (BBA VI, 1-1, 1990). Methods and results well documented. Rationale for read-across: in the environment, lime substances rapidly dissociate or react with water. These reactions, together with the equivalent amount of hydroxyl ions set free when considering 100mg of the lime compound (hypothetic example), are illustrated below: Ca(OH)2 <-> Ca2+ + 2OH- 100 mg Ca(OH)2 or 1.35 mmol sets free 2.70 mmol OH- CaO + H2O <-> Ca2+ + 2OH- 100 mg CaO or 1.78 mmol sets free 3.56 mmol OH- From these reactions it is clear that the effect of calcium oxide will be caused either by calcium or hydroxyl ions. Since calcium is abundantly present in the environment and since the effect concentrations are within the same order of magnitude of its natural concentration, it can be assumed that the adverse effects are mainly caused by the pH increase caused by the hydroxyl ions. Furthermore, the above mentioned calculations show that the base equivalents are within a factor 2 for calcium oxide and calcium hydroxide. As such, it can be reasonably expected that the effect on pH of calcium oxide is comparable to calcium hydroxide for a same application on a weight basis. Consequently, read-across from calcium hydroxide to calcium oxide is justified.

Qualifier:

according to guideline

Guideline:

BBA Part VI, 1-1

Deviations:

no

Principles of method if other than guideline:

Guideline: BBA VI, 1-1 (1990) under consideration of OECD 216 (2000) and OECD 217 (2000).

GLP compliance:

yes (incl. QA statement)

Analytical monitoring:

yes

Vehicle:

no

Test organisms (inoculum):

soil

Total exposure duration:

96 d

Moisture:

Water content soil: 10.12 g/100 g soil d.w.
Water holding capacity: 39.70 g/100 g soil d.w.

Duration:

96 d

Dose descriptor:

NOEC

Effect conc.:

4 g/kg soil dw

Nominal / measured:

nominal

Conc. based on:

test mat.

Basis for effect:

other: dehydrogenase activity

Duration:

28 d

Dose descriptor:

EC50

Effect conc.:

> 12 g/kg soil dw

Nominal / measured:

nominal

Conc. based on:

test mat.

Basis for effect:

other: dehydrogenase activity

Duration:

48 d

Dose descriptor:

EC50

Effect conc.:

8.1 g/kg soil dw

Nominal / measured:

nominal

Conc. based on:

test mat.

Basis for effect:

other: dehydrogenase activity

Duration:

96 d

Dose descriptor:

EC50

Effect conc.:

8.7 g/kg soil dw

Nominal / measured:

nominal

Conc. based on:

test mat.

Basis for effect:

other: dehydrogenase activity

Validity criteria fulfilled:

yes

Remarks:

Reference item must have an effect of at least 15% on day 28 after treatment. In the most recent study the reference item Dinoterb caused an inhibition of dehydrogenase activity of -69.4, -73.4 and -84.4% at 6.80, 16.00 and 27.00 mg per kg soil d.w.

Conclusions:

The pH of the soil increased with increasing concentrations of calcium dihydroxide. The high pH value of the soil is considered to be the toxic effect. At the highest concentration tested, the maximum pH level was 11.9 which decreased to 8.5 during the course of the study. At low test item concentration the dehydrogenase activity was stimulated.

Reference 2

Endpoint:

toxicity to soil microorganisms

Type of information:

migrated information: read-across from supporting substance (structural analogue or surrogate)

Adequacy of study:

supporting study

Study period:

April 18, 2007 - July 25, 2007

Reliability:

1 (reliable without restriction)

Rationale for reliability incl. deficiencies:

other: see 'Remark'

Remarks:

According to OECD 216. Well documented study. Validity criteria fulfilled. Rationale for read-across: in the environment, lime substances rapidly dissociate or react with water. These reactions, together with the equivalent amount of hydroxyl ions set free when considering 100mg of the lime compound (hypothetic example), are illustrated below: Ca(OH)2 <-> Ca2+ + 2OH- 100 mg Ca(OH)2 or 1.35 mmol sets free 2.70 mmol OH- CaO + H2O <-> Ca2+ + 2OH- 100 mg CaO or 1.78 mmol sets free 3.56 mmol OH- From these reactions it is clear that the effect of calcium oxide will be caused either by calcium or hydroxyl ions. Since calcium is abundantly present in the environment and since the effect concentrations are within the same order of magnitude of its natural concentration, it can be assumed that the adverse effects are mainly caused by the pH increase caused by the hydroxyl ions. Furthermore, the above mentioned calculations show that the base equivalents are within a factor 2 for calcium oxide and calcium hydroxide. As such, it can be reasonably expected that the effect on pH of calcium oxide is comparable to calcium hydroxide for a same application on a weight basis. Consequently, read-across from calcium hydroxide to calcium oxide is justified.

Qualifier:

according to guideline

Guideline:

OECD Guideline 216 (Soil Microorganisms: Nitrogen Transformation Test)

Deviations:

no

GLP compliance:

yes (incl. QA statement)

Analytical monitoring:

no

Details on sampling:

SOIL
- on days 0 (3 hours), 14, 28, 48 and 96 after application, soil samples were taken for the determination of the mineral nitrogen content of the soil.

Vehicle:

yes

Details on preparation and application of test substrate:

COLLECTION
- soil removal: to a depth of 20 cm as mixed sample
- dried at room temperature
- passed through a 2 mm mesh sieve
- characterization: biologically active agricultural loamy sand soil (1.42% organic C, 9.9% clay, 39% silt, 51.2% sand), no fertilisation since 2003, last application of plant protection products in 1990
- origin: wassergut Canitz, Germany (12.694435960 degrees East, 51.403774567 degrees North)

STORAGE
- at a temperature of 4 °C
- in aerobic conditions in the dark
- before applicatin soil was adapted to test conditions

PREPARATION
1)
- 300 g soil (dry weight) (= one sub-sample) weighed per test vessel
- soil mixed with 0.5 % lucerne meal (i.e. 1.5 g/300 g soil d.w.) by means of a hand-stirrer
- vehicle: quartz meal
- water was added to the soil to achieve a water content of approx. 55 % of WHC
- incubation in wide-mouth glass flasks (500 mL) the below described test conditions
2)
- an additional soil sample (without lucerne meal) used for determination of initial NH4-N- and NO3-N-content
- NO3-N-content was 0.96 mg/100 g soil d.w.

Test organisms (inoculum):

soil

Total exposure duration:

96 d

Test temperature:

19.9 - 21.9°C in a climatic room

Moisture:

18.66-19.70 g/100 g soil d.w.

Details on test conditions:

- Illumination: darkness
- 3 replicates per concentrations (i.e. 3 subsamples of soil per concentration)
- humus content of soil: 2.44%
- initial pH 7.1
- microbial biomass 23.55 mg C/100 g soil d.w. (i.e. 1.66% compared to organic C content)

Nominal and measured concentrations:

nominal concentrations: 0, 1, 2, 4, 8, 10 and 12 g test item/kg soil dry weight

Reference substance (positive control):

yes

Remarks:

Dinoterb

Duration:

28 d

Dose descriptor:

NOEC

Effect conc.:

8 g/kg soil dw

Nominal / measured:

nominal

Conc. based on:

test mat.

Basis for effect:

other: nitrogen transformation

Duration:

48 d

Dose descriptor:

NOEC

Effect conc.:

10 g/kg soil dw

Nominal / measured:

nominal

Conc. based on:

test mat.

Basis for effect:

other: nitrogen transformation

Duration:

96 d

Dose descriptor:

NOEC

Effect conc.:

>= 12 g/kg soil dw

Nominal / measured:

nominal

Conc. based on:

test mat.

Basis for effect:

other: nitrogen transformation

Duration:

28 d

Dose descriptor:

EC50

Effect conc.:

9.7 g/kg soil dw

Nominal / measured:

nominal

Conc. based on:

test mat.

Basis for effect:

other: nitrogen transformation

Duration:

48 d

Dose descriptor:

EC50

Effect conc.:

> 10 g/kg soil dw

Nominal / measured:

nominal

Conc. based on:

test mat.

Basis for effect:

other: nitrogen transformation

Duration:

96 d

Dose descriptor:

EC50

Effect conc.:

> 12 g/kg soil dw

Nominal / measured:

nominal

Conc. based on:

test mat.

Basis for effect:

other: nitrogen transformation

Details on results:

not applicable

Results with reference substance (positive control):

Dinoterb caused a stimulation of nitrogen transformation of 44.8% and 58.2% at 6.80 and 16.00 mg Dinoterb per kg soil dry weight, respectively, 28 days after application.

Reported statistics and error estimates:

Not applicale

Validity criteria fulfilled:

yes

Remarks:

The coefficients of variation in the control group of the nitrogen transformation test were maximum 9.3% and thus fulfilled the demanded range (≤ 15%).

Conclusions:

The 96d-NOEC was 12 g/kg soil dry weight, the highest concentration tested. The pH of the soil increased with increasing concentrations of calcium dihydroxide. The high pH value of the soil is considered to be the toxic effect. At the highest concentration tested (12 g/kg soil), the maximum pH level was 11.9 which decreased to 8.5 during the course of the study. At high test concentrations, the nitrogen transformation activity of the soil microflora was shown to recover within 100 days of exposure to the test item. At low test item concentrations the nitrogen transformation was stimulated.

Description of key information

2 chronic studies which are both reliable without restrictions (Klimisch 1), describing the effect of calcium dihydroxide on the nitrogen transformation and dehydrogenase activity in an agricultural loamy sand soil, are available.  (Schulz, 2007a and 2007b).
Rationale for read-across: in the environment, lime substances rapidly dissociate or react with water. These reactions, together with the equivalent amount of hydroxyl ions set free when considering 100mg of the lime compound (hypothetic example), are illustrated below:
Ca(OH)2 <-> Ca2+ + 2OH-
100 mg Ca(OH)2 or 1.35 mmol sets free 2.70 mmol OH-
CaO + H2O <-> Ca2+ + 2OH-
100 mg CaO or 1.78 mmol sets free 3.56 mmol OH-
From these reactions it is clear that the effect of calcium oxide will be caused either by calcium or hydroxyl ions. Since calcium is abundantly present in the environment and since the effect concentrations are within the same order of magnitude of its natural concentration, it can be assumed that the adverse effects are mainly caused by the pH increase caused by the hydroxyl ions. Furthermore, the above mentioned calculations show that the base equivalents are within a factor 2 for calcium oxide and calcium hydroxide. As such, it can be reasonably expected that the effect on pH of calcium oxide is comparable to calcium hydroxide for a same application on a weight basis. Consequently, read-across from calcium hydroxide to calcium oxide is justified.

Key value for chemical safety assessment

Long-term EC10 or NOEC for soil microorganisms:

4 000 mg/kg soil dw

Additional information

The chronic study on the effect of calcium dihydroxide on the dehydrogenase activity in an agricultural loamy sand soil (Schulz, 2007a) was conducted according to German guidelines for testing of plant protection products (BBA VI, 1-1, 1990). The methods and results are well documented. As such a Klimisch 1 score was assigned to the study. The pH of the soil increased with increasing concentrations of calcium dihydroxide. The high pH value of the soil is considered to be the toxic effect. At the highest concentration tested, the maximum pH level was 11.9 which decreased to 8.5 during the course of the study. At low test item concentration the dehydrogenase activity was stimulated.

The chronic study on the effect of calcium dihydroxide on the nitrogen transformation in an agricultural loamy sand soil (Schulz, 2007b) was carried out according to OECD 216. The study is well documented, all validity criteria are fulfilled. As such a Klimisch 1 score was assigned to the study. The 96d-NOEC was 12 g/kg soil dry weight, the highest concentration tested. The pH of the soil increased with increasing concentrations of calcium dihydroxide. The high pH value of the soil is considered to be the toxic effect. At the highest concentration tested (12 g/kg soil), the maximum pH level was 11.9 which decreased to 8.5 during the course of the study. At high test concentrations, the nitrogen transformation activity of the soil microflora was shown to recover within 100 days of exposure to the test item. At low test item concentrations the nitrogen transformation was stimulated.

The chronic effects of calcium carbonate (nano) at a nominal concentration of 1000 mg/kg soil dw on the nitrogen transformation activity of soil microorganisms were assessed in a study performed to OECD TG 216 under GLP (Clarke, 2010). No adverse effects on the nitrogen transformation rate were exhibited at the concentration tested. Hence, the 28 day EC50 for calcium carbonate (nano) was found to be >1000 mg/kg soil dw and the NOEC was 1000mg/kg soil dw. Calcium carbonate is therefore not toxic to soil microorganisms.

In both studies performed with calcium dihydroxide, effects due to pH were seen at high concentrations. Over time, the pH dropped, presumably as the calcium was converted to calcium carbonate and its availability decreased. Therefore it may be concluded that calcium dihydroxide of high purity represents a worse-case for all grades of the substance, although in any case toxicity to soil microorganisms is low. These conclusions may be read across to calcium oxide.