Canopy influence on trace metal atmospheric inputs on forest ecosystems: Speciation in throughfall

Atmospheric inputs of selected Trace Metals (TM: Cd, Cu, Ni, Pb, Sb, Zn, as well as Al, Fe and Mn) were studied on six forested sites in France. In order to evaluate canopy interaction with atmospheric inputs, TM were measured in both Open Field Bulk Deposition (BD) and Throughfall (TF). Anthropogenic contribution to BD composition is high for Zn, Cd and Sb, reflecting actual TM emissions trends. Canopy greatly influences precipitation composition, through different processes, including assimilation and leaching by canopy, complexation as well as accumulation/dissolution of dry deposition. TM and Dissolved Organic Carbon (DOC) physical fractionation between colloidal and truly dissolved phases was performed with ultrafiltration. Al, Fe, Pb and Cu are found in the colloidal fraction whereas Cd, Ni, Zn and Sb are mostly in the truly dissolved fraction. Chemical speciation predicted with WHAM-VI shows that in throughfall, Al, Fe, Pb and Cu are almost entirely complexed by DOC, whereas Ni, Cd and Zn are present in average 30% in the free metal ion form. TM present in labile forms (Cd, Ni, Zn) interact with the canopy, are cycled in the ecosystem, and their concentration is either slightly increased or even decreased in throughfall. Sb, Pb and Cu concentration are increased through canopy, as a consequence of dry deposition accumulation.