Rare earth elements in soil profiles of various ecosystems across Germany

Abstract

Abstract Rare earth elements (REEs) are of emerging interest. In the environment, they occur in minerals, however, they are also used in many key technologies and in agriculture. In consequence, REEs are released into the environment and have thus an impact on it. However, our knowledge regarding REEs in soils is incomplete. We selected eight soil profiles across Germany aiming to determine REE contents (aqua regia) and potential mobilization (hydroxylammonium chloride and hot water extractable) and to explore relations between REEs and soil properties. The composition of the parent material strongly influenced by the ice ages during the Pleistocene (especially in northeastern Germany) is a crucial factor differentiating the content of REEs in our studied soils. The silty, clayey soils in the western part of Germany possessed higher REE contents than the sandy and peaty soils in the northeast (average REE content Cambisols: 825\u202fμmol\u202fkg−1, Retisols: 407\u202fμmol\u202fkg−1, Arenosol 180\u202fμmol\u202fkg−1, Histosols 122\u202fμmol\u202fkg−1) because REE can adsorb onto clay minerals. The characteristic soil genetic processes of each soil type were reflected by the vertical distribution of the REE content. The vertical distribution of REEs and the hydroxylammonium chloride-extractable REE contents were mainly controlled by Al, Fe, Mn and their oxides (additional to clay) which provide important sorption sites for REEs. High contents of clay, Al/Fe/and Mn oxides caused high REE contents. The hydroxylammonium chloride-extractable REE contents varied in average from 266 to 8\u202fμmol\u202fkg−1 in the soils. The hot water extractable REE contents (in average 0.94\u202fμmol\u202fkg−1) tended to be higher in horizons which were rich in organic matter. In future, mechanistic experiments concerning the dynamics of REE mobilization should be performed as well as monitoring in plants and wildlife to improve our knowledge about REEs in the environment.