DIFFERENCES IN CONCENTRATIONS AND FRACTIONS OF ALUMINUM AND HEAVY-METALS BETWEEN AGGREGATE INTERIOR AND EXTERIOR

Previous research has indicated that Al and heavy metals are distribut ed unevenly within soil aggregates. This paper will examine statistica lly the differences in metal concentrations in aggregate interior and exterior by including more samples. Concentrations and fractions of Al , Cd, Cr, Cu, Fe, Mn, Ni, Pb, Zn, and the major metal sorbents (cation exchange sites, organic matter, oxides) in mechanically separated soi l aggregate interior and exterior were measured. Aggregates were colle cted from 26 A and B horizons of 13 soils under forest or grassland, M etals were speciated using a 7-step sequential extraction procedure,Th e concentrations of all metal sorbents in the exterior were lower than in the interior,The concentrations of easily extractable fractions of all metals were higher in the exterior than in the interior, whereas the reverse was true for concentrations of metals extractable with str ong acids only (residual). All differences were small but systematic a nd generally significant. We propose the following hypotheses to expla in the results: (i) Preferential weathering of aggregate surfaces caus es a shift from residual to other metal forms and (ii) preferential so rption of deposited metals by the aggregate surfaces leads to a total enrichment of metals in the aggregate exterior, especially in easily e xtractable forms.