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Toxicity to soil microorganisms

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Endpoint:
toxicity to soil microorganisms
Type of information:
experimental study
Adequacy of study:
key study
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
study well documented, meets generally accepted scientific principles, acceptable for assessment
Qualifier:
no guideline followed
Principles of method if other than guideline:
Effect of Se on microbial activity in a loamy soil (Haplic Luvisol) with and without organic substrate addition after 28 days of incubation. The endpoint studied was microbial biomass.
GLP compliance:
not specified
Analytical monitoring:
yes
Details on sampling:
Soil samples were taken to measure background Se in the control treatment.
Vehicle:
no
Details on preparation and application of test substrate:
AMENDMENT OF SOIL
- Type of organic substrate: Loamy soil (Haplic Luvisol) with and without organic substrate addition (glucose and maize straw).

APPLICATION OF TEST SUBSTANCE TO SOIL
- Method: Se, maize leaf straw, and glucose were thoroughly mixed with 1 kg of soil in the amendment treatments immediately before incubation
Test organisms (inoculum):
soil
Total exposure duration:
28 d
Test temperature:
25 deg C
Moisture:
50% water holding compacity (111 ml water/kg soil)
Details on test conditions:
TEST SYSTEM
- Testing facility: laboratory incubation experiment
- Test container (type, material, size): 50 mL beakers in a 1000 mL incubation vessel
- Amount of soil: 40 g (oven-dried)
- No. of replicates per concentration: 3
- No. of replicates per control: 3

SOURCE AND PROPERTIES OF SUBSTRATE (if soil)
- Geographical reference of sampling site (latitude, longitude): an arable site close to Goslar
- Soil texture: Loess-derived Haplic Luvisol
- % sand: No data
- % silt: No data
- % clay: 14%
- Soil taxonomic classification: Haplic Luvisol
- pH (in water): 7.6
- Initial microbial biomass: 280 µg C/g soil

EFFECT PARAMETERS MEASURED (with observation intervals if applicable) : Microbial biomass C, microbial biomass N, adenylate energy charge, and metabolic quotient
Nominal and measured concentrations:
Nominal: 0 and 20 mg Se/kg
Selenium background concentration (measured): 1.6 mg Se/kg
Key result
Duration:
28 d
Dose descriptor:
NOEC
Effect conc.:
>= 20 mg/kg soil dw
Nominal / measured:
nominal
Conc. based on:
element
Remarks:
Se
Basis for effect:
respiration rate
Key result
Duration:
28 d
Dose descriptor:
NOEC
Effect conc.:
>= 20 mg/kg soil dw
Nominal / measured:
nominal
Conc. based on:
element
Remarks:
Se
Basis for effect:
other: microbial biomass
Key result
Duration:
28 d
Dose descriptor:
NOEC
Effect conc.:
>= 20 mg/kg soil dw
Nominal / measured:
nominal
Conc. based on:
element
Remarks:
Se
Basis for effect:
other: ATP content
Details on results:
No adverse effects were observed on microbial biomass C, microbial biomass N, ATP, AEC, ATP-to-microbial biomass C, and qCO2.
Reported statistics and error estimates:
One-way ANOVA, Fisher protected least significant difference test.
Conclusions:
In this Klimisch 2 study it was found that the addition of 20 mg Se/kg had about 30% effect on the CO2 evolution in substrate-amended soil, while Se addition did not have a significant adverse effect on any other endpoint studied (microbial biomass C, microbial biomass N, ATP, AEC, ATP to microbial biomass). The NOEC was determined to be greater or equal to 20 mg Se/kg dw.
Endpoint:
toxicity to soil microorganisms
Type of information:
experimental study
Adequacy of study:
key study
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
study well documented, meets generally accepted scientific principles, acceptable for assessment
Remarks:
The original study language is German; this study has not been translated.
Qualifier:
no guideline followed
Principles of method if other than guideline:
Measurement of microbial biomass and dehydrogenase activity in soil after long-term exposure to metals.
GLP compliance:
not specified
Specific details on test material used for the study:
Se added in the form of Na salt
Analytical monitoring:
yes
Details on sampling:
- Sampling method: Four subsamples were taken from each treated plot and half of the controls.
- Sample storage conditions before analysis: Samples were sieved (2 mm) and stored at -18 deg C.
Vehicle:
no
Details on preparation and application of test substrate:
AMENDMENT OF SOIL
- Type of organic substrate: The A horizon (0-25 cm) of a sandy cambisol from a field trial of the Biologische Bundesanstalt was used.

APPLICATION OF TEST SUBSTANCE TO SOIL
- In 1975, the soil was placed in concrete frames of 1x1x1 m. The underlying material was a layer of peat (about 15 cm thick).
- Selenium was added to the soil as a sodium salt and mixed with the A horizon in one to three additions. All treatments were carried out fourfold in two concentrations.
- In the following years tomatoes, potatoes, beans, rye, wheat, barley and rape seed were grown on the plot.
- Weeds were removed occasionally. The soil was dug up twice a year in spring and autumn.
- Samples for mineralisation experiments were obtained by taking four subsamples from the A horizon.
Test organisms (inoculum):
soil
Total exposure duration:
468 wk
Remarks:
9 years
Test temperature:
not reported
Moisture:
not reported
Organic carbon content (% dry weight):
1.2
Nitrogen content (% dry weight):
0.08
Details on test conditions:
Soil characteristics are as follows: 
pH: 6.0
sandy cambisol
silt: 12%
sand: 79% 
clay: 9% 
CEC: 10.3 cmol/kg
Nominal and measured concentrations:
Nominal: control, 20 and 40 mg Se/kg
Measured (after 9 years): 1.5, 5.7 and 7.4 mg Se/kg (neutron activation)
Reference substance (positive control):
no
Key result
Duration:
468 wk
Dose descriptor:
NOEC
Effect conc.:
4.2 mg/kg soil dw
Nominal / measured:
meas. (not specified)
Conc. based on:
element
Remarks:
Se (added concentration)
Basis for effect:
other: total microbial biomass
Remarks on result:
other: 5% stimulation at NOEC
Duration:
468 wk
Dose descriptor:
other: LOEC
Effect conc.:
5.9 mg/kg soil dw
Nominal / measured:
meas. (not specified)
Conc. based on:
element
Remarks:
Se (added concentration)
Basis for effect:
other: total microbial biomass
Remarks on result:
other: 36% inhibition at LOEC
Duration:
468 wk
Dose descriptor:
NOEC
Effect conc.:
>= 5.9 mg/kg soil dw
Nominal / measured:
meas. (not specified)
Conc. based on:
element
Remarks:
Se (added concentration)
Basis for effect:
other: dehydrogenase activity
Remarks on result:
other: 3% stimulation at largest dose tested; total concentration: ≥7.4 mg Se/kg
Duration:
468 wk
Dose descriptor:
NOEC
Effect conc.:
>= 5.9 mg/kg soil dw
Nominal / measured:
meas. (not specified)
Conc. based on:
element
Remarks:
Se (added concentration)
Basis for effect:
other: ATP content
Remarks on result:
other: 16% stimulation at largest dose tested; total concentration: ≥7.4 mg Se/kg
Results with reference substance (positive control):
Not available
Reported statistics and error estimates:
Statistical  differences were evaluated using t-tests (p <0.05)
Validity criteria fulfilled:
not applicable
Conclusions:
The NOEC for effect of selenium on microbial biomass in a sandy Cambisol soil after 9 year exposure is 4.2 mg Se/kg soil (added dose).
Selenium did not inhibit the other indicators for the total microbial biomass (dehydrogenase activity and ATP content) of this soil after 9 year exposure to total soil concentrations up to 7.4 mg Se/kg soil dry weight (corresponding to an added dose of 5.9 mg Se/kg soil).
Endpoint:
toxicity to soil microorganisms
Type of information:
experimental study
Adequacy of study:
key study
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
study well documented, meets generally accepted scientific principles, acceptable for assessment
Qualifier:
no guideline followed
Principles of method if other than guideline:
Effects of selenium on soil nitrogen transformations in a sandy cambisol.
GLP compliance:
not specified
Specific details on test material used for the study:
Se added in form of Na salt
Analytical monitoring:
yes
Details on sampling:
- Sampling method: Four subsamples were taken from each treated plot and half of the controls.
- Sample storage conditions before analysis: Samples were sieved (2 mm) and stored at -18 deg C.
Vehicle:
no
Details on preparation and application of test substrate:
AMENDMENT OF SOIL
- Type of organic substrate: The A horizon (0-25 cm) of a sandy cambisol from a field trial of the Biologische Bundesanstalt was used.

APPLICATION OF TEST SUBSTANCE TO SOIL
- In 1975, the soil was placed in concrete frames of 1x1x1 m. The underlying material was a layer of peat (about 15 cm thick).
- Selenium was added to the soil as a sodium salt and mixed with the A horizon in one to three additions. All treatments were carried out fourfold in two concentrations.
- In the following years tomatoes, potatoes, beans, rye, wheat, barley and rape seed were grown on the plot.
- Weeds were removed occasionally. The soil was dug up twice a year in spring and autumn.
- Samples for mineralisation experiments were obtained by taking four subsamples from the A horizon.
Test organisms (inoculum):
soil
Total exposure duration:
28 d
Test temperature:
25°C
Moisture:
3 mL H2O added to 10-g sample of field-moist soil
Organic carbon content (% dry weight):
1.2
Nitrogen content (% dry weight):
0.08
Details on test conditions:
TEST SYSTEM
- Test container (type, material, size): 100-mL Erlenmeyer flasks containing 10-g samples of field-moist soil and 3 mL of deionized water
- Amount of soil: 10 g
- No. of replicates per concentration: 6
- No. of replicates per control: 6

SOIL INCUBATION
- Method: at 25°C

SOURCE AND PROPERTIES OF SUBSTRATE (if soil)
- Geographical reference of sampling site (latitude, longitude): A horizon (0-25 cm) of a sandy cambisol from a field trial of the Biologische Bundesanstalt (Berlin-West)
- Vegetation cover: several vegetables were grown on the soil between addition of Se and sampling for mineralization experiments
- Depth of sampling: A horizon (0-25 cm)
- % sand: 79%
- % silt: 12%
- % clay: 9%
- Soil taxonomic classification: sandy cambisol
- pH (in water): 6.0
- Cation exchange capacity (mmol/kg): 10.3 (meq/100g)
- % organic carbon: 1.2%
- Initial microbial biomass as % of total organic C: no data

EFFECT PARAMETERS MEASURED (with observation intervals if applicable): N-mineralisation and nitrification

Nominal and measured concentrations:
Nominal: control, 20 and 40 mg Se/kg
Measured (after 9 years): 1.5, 5.7 and 7.4 mg Se/kg (neutron activation)
Key result
Duration:
28 d
Dose descriptor:
NOEC
Effect conc.:
>= 5.9 mg/kg soil dw
Nominal / measured:
meas. (initial)
Conc. based on:
element
Remarks:
Se (added concentration)
Basis for effect:
other: N-Mineralisation
Remarks on result:
other: 0% inhibition at largest dose tested; total concentration: ≥7.4 mg Se/kg
Key result
Duration:
21 d
Dose descriptor:
NOEC
Effect conc.:
>= 5.9 mg/kg soil dw
Nominal / measured:
meas. (initial)
Conc. based on:
element
Remarks:
Se (added concentration)
Basis for effect:
other: nitrification
Remarks on result:
other: 20% stimulation at largest dose tested, total concentration: ≥7.4 mg Se/kg
Details on results:
No effect at all (0%) was observed on N mineralization at the highest concentration tested.
No adverse effect at all (-20%) was observed on nitrification at the highest concentration tested.
Reported statistics and error estimates:
Statistics: t test
Conclusions:
Klimisch 2 study on the effects of selenium (added as sodium salt) on N-mineralization and nitrification in A horizon of a sandy cambisol that had been spiked with selenium 9 years before. N-mineralization by native soil microorganisms was studied during 28 days, whereas nitrification was studied during 21 days. No significant adverse effects were observed on nitrogen transformations at the highest concentration tested, resulting in an unbounded NOEC of ≥5.9 mg Se/kg dw (added concentration).

Description of key information

In total, 8 reliable NOEC values for chronic selenium toxicity to soil microorganisms were retrieved from 3 published study report (Wilke, 1988; Wilke, 1989 and Chander and Joergensen, 2007). All data were derived from soils amended with sodium selenium salts. These data cover effects on microbial nitrification, biomass and respiration. The reliable NOEC values range between 4.2 mg Se/kg soil dw for microbial biomass and ≥20 mg Se/kg dw for microbial respiration. All NOEC values are expressed based on added elemental Se concentrations in soil. The lowest reliable NOEC values for microbial nitrification, biomass and respiration were taken forward to the derivation of the PNEC according to the statistical extrapolation approach.

Key value for chemical safety assessment

Additional information

Three studies on the toxicity of selenium compounds to soil microorganisms are judged reliable with restriction (Klimisch 2). The reliable data cover several endpoints: N-mineralisation, nitrification, biomass, respiration and ATP-content. Only one bounded NOEC value was observed (4.2 mg Se/kg soil for microbial biomass in a sandy Cambisol soil after 9 years exposure; Wilke, 1988). In addition, unbounded NOEC values are reported ranging from ≥5.9 mg Se/kg dw for the effect of selenium on nitrogen transformations 9 years after application of a sodium salt of selenium (not reported whether Na2SeO4 or Na2SeO3) to a field plot in Germany (Wilke, 1989) to ≥20 mg Se/kg for the effect of Na2SeO4 on microbial biomass, respiration or ATP content in a Haplic Luvisol (Chander and Joergensen, 2007). All results are based on nominal added or background corrected measured Se concentrations.