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Toxicity to soil microorganisms

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Description of key information

Toxicity to soil microorganisms was investigated with the soluble Lanthanum trichloride hexahydrate salt.
Respiration rate: NOEC (28d) = 295.3 mg La/kg soil dw
Nitrate formation rate: NOEC (28d) = 281 mg La/kg soil dw

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Additional information

No data on toxicity to soil microorganisms are available for Lanthanum oxide itself, but studies performed with the soluble lanthanum trichloride hexahydrate are used as the toxicity will be mediated by the lanthum3+ ion.

The long term effect of Lanthanum trichloride hexahydrate on carbon transformation activity of soil microorganisms was investigated according to OECD 217 under GLP- conditions. Glucose-induced respiration rates were measured after 0, 7, 14 and 28 days incubation for 12 consecutive hours. The test item caused a maximum inhibition of 19.6% at 1514.2 mg/kg soil dry weight (highest concentration tested) 28 days after application. This is equivalent to 590 mg La/kg soil dw. Only for this concentration a significant difference compared to the control was stated. The EC50 could not be calculated, but it can be concluded that the EC50 is higher than 1514.2 mg test item/kg soil dry weight (590 mg La/kg soil dw), the highest concentration tested. The NOEC of the test item was calculated to be 757.10 mg/kg soil dry weight (295.3 mg La/kg soil dw) after 28 days of exposure.

Furthermore, the long term effect of Lanthanum trichloride hexahydrate on nitrogen transformation activity of soil microorganisms was tested according to OECD 216 under GLP-conditions. After 0, 7, 14 and 28 days of incubation of soil, which was amended with powdered plant meal at the beginning of the test, samples of treated and control soils are extracted and quantities of nitrate in the extracts is determined. After 28 days the treatment with 1441 mg LaCl3*6H20 /kg soil dry weight (562 mg La /kg soil dw) showed an inhibition of 18.5 %. Only for this concentration a significant difference compared to the control was stated. The EC50 could not be calculated, but it can be concluded that the EC50 is higher than 1441 mg LaCl3*6H20/kg soil dw (562 mg La/kg soil dw), the highest concentration tested. The NOEC of the test item derived from the variance analysis was 500 mg LaCl3/kg soil dw (720 mg LaCl3*6H20)/kg soil dw, or 281 mg La/kg soil dw after 28 days of exposure.

As these studies were performed with soluble Lanthanum salts they can be considered to represent a worst case assumption for Lanthanum oxide, as it is likely that the bioavailability of lanthanum oxide to soil microorganisms will be lower due to its low water solubility.This is corroborated by the soil microorganism stuy with Ceriumdioxide showing no effect at a limit concentration of 1000 mg/kg soil dw.