Lanthanum oxide

Toxicity to terrestrial plants

Description of key information

NOEC (14 d) = 107 mgLa/kg dw

Additional information

Rare earth elements (REEs) have been widely used as fertilizers in China's agriculture over the last 20 years, whereas Lanthanum is one of the main components of REE-based fertilizers and the application of REE-based fertilizers has therefore caused large amounts of La to enter agricultural ecosystems (Tyler 2004).

The application normally took place via seed dressing or as spray on the foliage of crowing crops. It was believed that REEs could stimulate the growth of roots, improve the quantity and quality of crops, and enhance crop resistance (Hu et al. 2006). In the literature positive and negative effects of rare earth elements on plant growth have been reported by examination the effect of Lanthanum via culture medium. So different investigations with varying plant species and testing procedures using soluble Lanthanum salts, such as Lanthanum trichloride and Lanthanum trinitrate as test substances have been reported in the literature.

Direct application to soil has no or limited effects on plants and is not recommended to be used in agriculture in this way. Only one study that can be used for risk assessment purposes was available, which investigated the effect of Lanthanum spiked in a red loamy soil with Lanthanum trinitrate as test substance(Hu et al. 2006). Zhang et al. (2001) tested a mixture of rare earth chlorides in different plant-soil systems, but these investigations only stated long-term EC50 values and no EC10 or NOEC values, so it was not further regarded.

 

The growth of stem, root and leaf of Nicotiana tobacum showed a maximum increase at 50 mg/L and inhibition effects at concentrations above 100 mg/L within 14 days (Chen et al. 2001). The root growth ofEriobotrya japonicawas investigated in another study within a period of 45 days (Fashui et al. 2005). Inhibition effects were observed at concentrations above 12.26 mg/L, a maximum increase in root growth was stated at 0.12 mg/L. Kinraide et al. (1992) investigated the root elongation forTriticum aestivumand revealed EC50(48h) values of approx. 0.56 mg/L at pH 4.5 and approx. 1.08 mg/L at pH 4.2. The inhibitory effect on germination and tube growth forNicotiana tobacumshowed a EC50(1.5 h) value > 9.81 < 19.61 mg/L. Growth stimulatory effects were observed at lower concentrations with a maximum increase of germination and tube growth at 2.45 mg/L (Sun et al. 2003). The values derived by Kinraide et al. (1992) were based on measured concentrations, whereas the other studies were all based on nominal values. The above mentioned investigations will not be used for the risk assessment of the terrestrial toxicity to plants, since these values are reported in mg/L and can not be conveyed to the unit of a plant soil system (mg/kg dw).

 

However, one further reliable study that is adequate for the risk assessment was available with a plant soil system, using Lanthanum trinitrate as test substance (Hu et al. 2006). Maize seedlings were grown in a red loamy soil, spiked with up to 2.5 g/kg. Primary root elongation and dry weight of roots and shoots were measured after 14 d of application. The NOEC and LOEC for primary root elongation and root dry weight were 250 and 500 mg/kg dw, equivalent to 107 and 214 mg La/kg dw. 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 plants from soil will be lower due to its low water solubility.