Metal Concentrations in Aggregate Interiors, Exteriors, Whole Aggregates, and Bulk of Costa Rican Soils

Abstract

In many temperate soils the preferential weathering and leaching of aggregate surfaces and the nonaggregated material between aggregates depletes geogenic metals. It also shifts metals from strongly to more weakly bound metal forms. Deposited metals are sorbed preferentially on aggregate surfaces and between aggregates. We examined whether preferential desilication under tropical climate causes an enrichment in the aggregate exteriors in oxidic forms of metals. We also studied where deposited metals are bound in these soils. Aggregates (2–20 mm) were selected manually from the A horizons of eight Oxisols, six Andisols, two Mollisols, and two Inceptisols in Costa Rica. All samples were fractionated into interior and exterior portions and treated with a seven‐step sequence to extract Al, Cd, Cu, Fe, Mn, Pb, and Zn. Total concentrations of all metals except Zn were higher in the aggregate exteriors than in the interiors (average differences in % of the interior concentrations: Al, 9.0; Cd, 2.2; Cu, 8.7; Fe, 4.9; Mn, 4.2; Pb, 64.7; Zn, 0.0). The average Cd and Pb concentrations in easily extractable fractions were significantly higher in the aggregate exteriors. There were no significant differences in metal partitioning between interiors and exteriors except for Pb, which had higher proportions in extractable forms with NH2OH · HCl > NH4 − acetate, pH 6.0 > EDTA in the exteriors. There were few significant differences in metal concentrations and partitioning between bulk soil and whole aggregates. The results may be explained by (i) preferential desilication of the aggregate exteriors and (ii) preferential sorption of deposited heavy metals mainly in easily extractable forms.