Evaluation of the BCR sequential extraction for trace elements in European reference volcanic soils

Trace metal behaviour in volcanic ash soils displays distinctive features related to the soils' large contents of metal-binding phases and to the rapid release of trace metals from glasses and weatherable minerals. In this work, the BCR (Community Bureau of Reference) sequential extraction scheme (exchangeable + weak acid soluble, reducible, oxidizable, and non-extractable metal fractions) was applied to selected COST-622 European reference volcanic soils to determine partitioning of zinc and copper between various solid-phase constituents, along with the major elements Al, Fe and Mn. The total extracted Al (Sigma Al) was strongly correlated with acid ammonium oxalate extractable Al (Al-o) (Sigma Al = 0.985Al(o) + 0.11, R-2 0.98), while the total extracted Fe clearly underestimated the amorphous fraction. Large values for the non-extractable Al fraction were associated with the presence of gibbsite and phyllosilicates. Although the Zn and Cu contents of the soils were generally large, total amounts extracted (the potentially mobilizable fraction) were small, especially for Zn and for soils with crystalline secondary minerals. The fraction of the total Cu which was potentially mobilizable generally exceeded that of Zn. In the potentially mobilizable Cu the oxidizable fraction was generally dominant. Biocycling appears to play an important role in the surface enrichment of potentially mobilizable Zn and Mn. Although further methodological research seems necessary, the BCR sequential extraction appears to be a valuable tool for studies on metal dynamics in soils with andic properties.