Impact of atmospheric deposition, biogeochemical cycling and water–mineral interaction on REE fractionation in acidic surface soils and soil water (the Strengbach case)

Leaf litter contains not only important quantities of elements derived from decomposition of organic matter, but also atmospheric compounds. This is illustrated by Sr and Nd isotope data from soils of the Strengbach catchment, which show that soil litter contains more than 50% of atmospheric Sr and Nd. Downward migrating soil water becomes strongly acidic during flow through litter and humus layers (pH 3.8–4 at 5–10 cm depth), causing not only intense mobilization of elements in the underlying acid brown and podzolic soils at depths between 10 and 50 cm, but also the downward transfer of some litter derived compounds. The Sr and Nd isotope data allow to quantify that soil water from 10 to 70 cm depth contains 45 to 65% of litter derived Nd and Sr. Sequential leaching experiments have been performed on soil samples in order to recover adsorbed trace elements or those fixed in acid soluble mineral phases such as Fe–Mn oxyhydroxides and phosphate minerals. The isotope data of these leachates of the uppermost soil horizons point similarly to the presence of important quantities of litter derived Sr and Nd in the uppermost 30 cm of the soils. With the exception of the litter layer, the soil profiles are substantially depleted in Ca and P down to 50 cm depth, which can be related to low apatite and plagioclase contents. These minerals have probably been preferentially weathered and Ca and P have been removed by percolating soil water and plant uptake. Leaching of primary granite derived apatite controls rare earth element (REE) fractionation only below 50 cm depth. Rhabdophane (REE-PO4) has been observed as secondary replacement mineral of apatite. This replacement is reflected by Ca/P ratios that are low at the surface and which increase regularly with depth, implying that some of the apatite derived P has been integrated into newly formed rhabdophane and some of the P and Ca have been exported, either by soil solutions or plant uptake. During downward transport, litter derived elements are adsorbed on soil particles and/or coprecipitate with authigenic minerals such as Fe–Mn oxyhydroxide or rhabdophane. Alternatively, litter derived elements may remain in solution or, like Ca, be absorbed by vegetation. The soil solutions from the uppermost 30 cm of the soil profiles and the corresponding soil leachates show very similar REE distribution patterns and are, compared to the waters of the Strengbach stream, which drains the forested catchment, strongly enriched in light rare earth elements (LREE). However, soil solutions from 60 to 70 cm depth are, compared to litter, strongly LREE depleted and show stream water-like REE distribution patterns. It is suggested that this evolution of the REE in the soil solutions is due to the following processes taking place above 50 cm depth: (1) precipitation of LREE-rich phosphate minerals like rhabdophane, (2) diminution of the formation of organic dissolved or colloidal phases in association with Fe–Mn and Al oxyhydroxides and (3) preferential LREE uptake by vegetation. Below 50 cm the REE distribution patterns are further modified by dissolution–precipitation reactions and adsorption. The combination of all these fractionation processes finally leads to the REE patterns of the Strengbach stream at the catchment outlet.