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 with soil organic matter (SOM) was investigated in three Japanese soils. Organically bound metals were assayed by elemental analysis of a fraction extracted with acidified hydrogen peroxide (H2O2) and the humic acid extracted with alkaline reagent, from soil sampled at various depths of solums. A Dystric Cambisol showed higher extractability with acidified H2O2 for most of the metals than a Humic Andosol and an Orthic Acrisol. A Humic Andosol had more metals associated with humic acid than the other two soils. Cu showed high extractability with acidified H2O2 and also significant association with humic acid, while Pb showed high extractability with acidified H2O2 but its association with humic acid was relatively low among the metals investigated. 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 the accumulation 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.