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

Trace Metal (TM: Cd, Cu, Ni, Pb, Sb and Zn) atmospheric inputs have been studied on six forested sites. In order to evaluate canopy interaction with atmospheric inputs, TM have been measured in both Open Field Precipitation (OFP) and Throughfall (TF). Anthropogenic contribution to OFP 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 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 complexation by DOC is the dominant speciation for TM in TF. Al, Fe, Pb and Cu are almost entirely complexed, whereas Ni, Cd and Zn are present in average 30% in the free metal ion form. TM speciation in TF provides new insights on processes occurring on the canopy. Elements (Cd, Ni, Zn) present in labile forms interact with canopy and are cycled in the ecosystem. Their concentration is slightly increased or even decreased in throughfall. Sb, Pb and Cu concentration are increased through canopy. Influence of dry deposition accumulation is expected.