Rare Earths in Soils

This chapter aims to review and summarize the scattered existing literature on the presence of the so-called rare earth elements (REEs) in soils by drawing on their basic characteristics and applications, contents, origin and possible sources, basic lanthanide chemistry, the chronology of their discovery, importance in soil genesis and the soil–plant system, and environmental consequences. The ultimate reason for this selection is that REEs accumulation in soil has increased in recent times not so much because of natural processes, but of anthropogenic activities. Thus, after the conceptualization and brief review of its historical identification, the origin and occurrences of REEs in soils are revealed. It is stated that they depend on: their presence in parent material; weathering of minerals; discharges from several sources (mining, industrial waste, P-fertilizers, sewage sludges, among others). Finally, attention is also paid to the potential risks that they entail. As occasional and uncontrolled REEs applications to agricultural soils are known to pose several environmental risks, it is necessary to undertake new studies on their bioavailability and subsequent possible adverse effects on microbial biomass and, therefore, on soil fertility. In this way, future research needs to: (i) include an understanding of REEs contents, their incorporation into soils by various external sources, but also their interactions with other soil constituents; (ii) promote new studies related to their interactions with plant growth; (iii) investigate impacts for the soil environment and human and animal health because of increasing REEs utilization. Therefore, future research lines should emphasize soil content control to identify the negative implications in environmental and agricultural sectors. Finally, the development of recycling processes for REEs is becoming an increasingly relevant, but challenging, issue.