Vertical Distribution of Trace Metals in Natural Soil Horizons from Japan Part 2: Effects of Organic Components in Soil

The association of Al, Mn, Fe, Ba, Zn, Cr, Ni, Co and Pb withsoil organic matter (SOM) was investigated in three Japanesesoils. Organically bound metals were assayed by elementalanalysis of a fraction extracted with acidified hydrogenperoxide (H₂O₂) and the humic acid extracted withalkaline reagent, from soil sampled at various depths of solums.A Dystric Cambisol showed higher extractability with acidifiedH₂O₂ for most of the metals than a Humic Andosol and an Orthic Acrisol. A Humic Andosol had more metals associated with humicacid than the other two soils.Cu showed high extractability with acidified H₂O₂ andalso significant association with humic acid, while Pb showedhigh extractability with acidified H₂O₂ but itsassociation with humic acid was relatively low among the metalsinvestigated. As humic acid is highly resistant to weathering,retention of Cu with SOM may last longer than that of Pb.The binding of metals with SOM probably has contributed to theaccumulation of some metals in organic-rich shallow horizons of soil. Such metals were Zn, Cu, Ni and Pb in a Dystric Cambisol,Cu in a Humic Andosol, and Pb and Cu in an Orthic Arisol.