Improving the reliability of using rare earth elements as soil erosion tracers

Rare earth elements (REEs) have been successfully used as tracers in field-scale erosion experiments to provide spatially distributed erosion data for process-based soil erosion models. This study aimed to demonstrate how differences in the erodibility of individual REEs naturally present in the soil affect the evaluation of data from such experiments. A four-year field experiment was carried out at an experimental site used for arable farming to investigate soil erosion. The concentrations of tracer REEs (Pr, Sm, Gd, Ho, Er, and Yb) and reference REEs (all other REEs) were evaluated and compared both in the soil and eroded sediment. The tracer REE concentrations in the eroded sediments were consistently significantly higher than the average background soil concentrations. Even when compared with the maximum values of background soil concentrations, enrichment of Er, Sm, and Ho in the sediment was still evident. However, in some cases, the reference REE concentrations in the sediment were often higher than the maximum soil background concentrations by more than 10%. These enrichment differences indicate the varying erodibility of individual REEs during erosion. The elements chosen as tracers could also be enriched in the sediment from their natural background sources, thus influencing the measured concentration values in the sediment after tagging. This study demonstrated that different REEs naturally present in soil exhibit varying degrees of erodibility, which affects data evaluation in soil erosion tracing experiments using REE oxides as tracers. Therefore, the reliable application of the REE tracer method can only be suggested under certain requirements defined in this study.