SMALL-SCALE HETEROGENEITY OF ALUMINUM AND HEAVY-METALS IN AGGREGATES ALONG A CLIMATIC TRANSECT

Concentrations and forms of Al and heavy metals in aggregate core and surface fractions of forest soils in Central Europe differ significant ly. The objectives of this study were to (i) test if there is a simila r small-scale metal heterogeneity in topsoils of the native North Amer ican prairie and (ii) investigate the influence of climatic parameters on this heterogeneity. Aggregates of eight A horizons along a climati c transect in the North American prairie were separated into core and 0.5-mm-thick surface fractions. Seven forms of Al, Cd, Cr, Cu, Fe, Mn, Ni, Pb, and Zn were determined in the aggregate fractions using a seq uential extraction procedure. The differences of metal concentrations between aggregate surface and core fractions are 1 to 25% of the total concentrations of the core fractions. The percentages of easily extra ctable metals are up to 120% higher in the aggregate surface than in t he core fractions. The percentages of the mainly silicate-bound metals (extractable only with strong acids) are up to 25% lower than in the core fractions. These results support the hypotheses that (i) preferen tial weathering of aggregate surfaces causes a shift from strongly to more weakly bound metal forms and (ii) atmospheric metal input is sorb ed preferentially at the aggregate surfaces, and (iii) this input is w eakly bound only. Lead is significantly enriched in the surface fracti ons compared with the cores, indicating ubiquitous input of ph to the soils. The depletion of the mainly silicate-bound metals in the aggreg ate surface fractions compared with the core fractions correlates with the mean annual temperature and the mean annual precipitation, which both favor silicate weathering.