Rare earth elements in lichens to trace atmospheric deposition: insights from historical and contemporary samples

Lichens are well known for their ability to accumulate trace elements from the atmosphere, making them effective biosensors for air pollution. Among these elements, rare earth elements (REEs) are particularly valuable chemical tracers for identifying sources and processes. This is especially useful in heterogeneous environments where multiple pollution sources (traffic, industry, agriculture, etc.) and diverse geographical contexts coexist. The aim of this study was to assess the relative contributions of REEs accumulated in lichens (Xanthoria parietina) collected from contrasting urban and rural sites. To this end, we sampled lichens at 132 sites across Wallonia, southern Belgium, covering an urbanization gradient from rural areas to large cities (Namur, Liège, and Charleroi). At 42 of these sites, we analyzed samples from two time periods: a recent collection and historical specimens (1955–1986) from university herbaria. Results revealed regional patterns likely influenced by lithology, although lichens did not always reflect the same geochemical signatures (e.g., Eu and Ce anomalies, La/Yb and La/Sm ratios). Local land cover also appeared to influence dust deposition, with factors such as proximity to urban roads or the extent of permanent plant cover in agricultural landscapes playing a role. Historical samples displayed similar spatial patterns to their modern counterparts, but with a relative enrichment in light REEs over time, suggesting changes in global dust deposition. Overall, REEs show potential for discriminating between environmental sources across landscapes and for improving trace element studies by helping to characterize natural dust backgrounds.