Rare earth element mobilization and migration in a Wisconsin spodosol

Spodosols are a soil order frequently exhibiting a redistribution of Al and Fe because of organic matter complexation, illuviation of these complexes, and immobilization of the Al and Fe as poorly crystalline silicates in deeper soil horizons. Little information is available concerning the organic matter complexation and migration of other elements, particularly the rare earth elements (REE). Four pedons representing the Padus series (coarse-loamy, mixed, superactive, frigid Alfic Haplorthods) from Wisconsin were selected to determine if selected transition metals and the REE are mobilized, illuviated, and then immobilized. Sequential soil extractions consisted of Na-pyrophosphate, ammonium oxalate, ammonium oxalate-ascorbic acid, and aqua-regia partitioned elements into operationally defined chemical environments. Total elemental contents of the whole soil and the whole silt fraction were determined using instrumental neutron activation analysis. Aluminum and Fe in the Na-pyrophosphate extraction, for the majority of the pedons, were discretely more abundant in the upper Bs horizon. The light REE were largely recovered by the aqua-regia extraction, implying that these elements are associated with apatite, whereas the heavy REE were only very slightly recovered, suggesting that the heavy REE reside in different minerals. A Na-pyrophosphate leach extracted only minor quantities of the light REE and undetectable to trace quantities of the heavy REE, suggesting that these elements have not been influenced by podzolization processes.