Are rare earths toxic to plants?

Needs of modern industries, including agriculture, leads to an increase in exploration of mineral resources that contain rare earths. Their wide application in modern industry and everyday life including agriculture has increased the presence of rare earths in the soil and plants.

Rare earths (REE) includes such elements as lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), promethium(Pm), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), and lutecium (Lu). Scandium (Sc) and Yttrium (Y) are additional elements often associated with rare earths.

The main sources of rare earths in agricultural plant tissue are:

•phosphate fertilisers, in which rare earths come from apatite, a mineral that naturally can have elevated level of these elements; and

•direct application of rare earths to soil to increase seed germination, root growth, chlorophyll content, plant resistance and agricultural productivity. Such applications are popular in China.

Whether there are more advantages or disadvantages in direct application of rare earths-enriched products in agriculture is somewhat debatable among scientists.

Rare earths concentration in plants varies depending on the species and content of these elements in the environment. Rare earths elements have different mobility in soils that affects their bioavailability for plants. The light rare earths group (La, Ce, Pr, Nd, Pm, Sm, Eu, and Gd + Sc) is more mobile and therefore more readily accumulated by plants in contrast to heavy rare earths group (Tb, Dy, Ho, Er, Tm, Yb, and Lu + Y). It has been found that citrus plants can accumulate high levels of light rare earths (Turra, et al 2012). Rare earths can regulate plant growth and affect uptake of nutrient elements by plants. Below are some examples of studies carried out for different plants and how the presence of rare earths affected their health.

•Tomatoes plants, in which it was found that the highest concentration was in roots and the lowest in berries (Spalla, et al 2009).

•Soybean. Exposure of low concentration of La to soybean plants increased the level of some nutrients, whereas high doses of La reduced the growth of plants (Oliveira, et al 2015).

•Maize plants. Young maize plants can accumulate significant concentrations of Y, which had toxic effects on plant health (Maksimovic, et al 2014).

•Rice. It was found that small concentrations of La increased root growth, whereas higher concentration had the opposite effect (Liu, et al 2013).

•Wheat. Combination of Eu and Ca showed positive effects on wheat seedlings germination (Shtangeeva, et al 2007).

A comprehensive review had been done by Ramos et al. (2016) comparing a wide range of studies on REE's beneficial and toxic effects on various crops. The author discussed how rare earths affect plants at the following stages of their development:

•plant metabolism, including photosynthesis;

•mineral plant nutrition;

•enzyme production;

•plant development, including its growth, seed germination, phytohormone production; and

•development of internal plant structure.

Toxicity of rare earths is variable depending on the exposed organisms and is still a subject of research.

Analytical analysis of rare earths can be performed via inductively coupled plasma mass spectrometry (ICP-MS), which is the most used technique to determine rare earths. Microwave dissolution with HNO3 + H2O2 has proved to be the most suitable digestion procedure for rare earths elements in tomato plants (Spalla, et al 2009).

Reference materials for a full suite of rare earths in plant tissue are not abundant. Swan Leaf Eucalyptus Leaf Reference Material can be used for method validation and quality control of rare earths analysis.