Sodium selenite

Toxicity to soil microorganisms

Link to relevant study record(s)

Reference

Endpoint:

toxicity to soil microorganisms

Type of information:

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

Adequacy of study:

key study

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

other: No guideline followed, but study well performed and well documented.

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

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

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)

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

Duration:

21 d

Dose descriptor:

NOEC

Effect conc.:

>= 5.9 mg/kg soil dw

Nominal / measured:

meas. (initial)

Conc. based on:

element

Remarks:

Se

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 ≥7.4 mg Se/kg dw.

Description of key information

The lowest unbounded NOEC of ≥5.9 mg Se/kg dw (added concentration) was observed 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).

Key value for chemical safety assessment

Long-term EC10 or NOEC for soil microorganisms:

5.9 mg/kg soil dw

Additional information

Two studies on the toxicity of selenium compounds to soil microorganisms were judged reliable with restriction (Klimisch 2). The reliable data cover several endpoints: N-mineralisation, nitrification, biomass, respiration and ATP-content. Only unbounded NOEC values are reported in these studies, 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). As a conservative approach, the lowest unbounded NOEC (5.9 mg Se/kg) was selected for the effects assessment of all inorganic selenium compounds in soils. All results are based on nominal added or background corrected measured Se concentrations.