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DIVISION S-2-SOIL CHEMISTRY

Small-Scale Variability of Metal Concentrations in Soil Leachates

^a Institute of Soil Science and Soil Geography, University of Bayreuth, D-95440 Bayreuth, Germany

wolfgang.wilcke{at}uni-bayreuth.de

Soil tests often use composite soil samples to assess metal bioavailability. Composite soil samples cannot address small-scale soil heterogeneity. In this study, the concentrations of Al, Cd, Cu, Mn, Ni, Pb, Zn, dissolved organic C (DOC), and pH in soil leachates were examined as an index of small-scale soil heterogeneity. Ten undisturbed soil cores (0–4 cm, 100 cm³) from a 1-m² area of a Lithic Haplumbrept (pH 5.2, 3 g CaCO₃ kg^-1) and a Typic Hapludoll (pH 4.3) under forest canopy were equilibrated with deionized water. The soil cores were then leached with a mock soil solution (pH 4.0, 6.8–11.4 mg L^-1 DOC, 0.001 M CaCl₂). In the Haplumbrept, the pH of the first 50-mL fraction of the leachates (deionized water extract) was 4.2 to 7.4, DOC concentrations were 11.4 to 38.9 mg L^-1. Aluminum, Cd, Mn, and Ni concentrations were significantly correlated with pH (r = 0.88, 0.93, 0.69, 0.78, respectively; P < 0.05). In the Hapludoll, the pH (4.1–4.6) varied little in the first 50-mL fractions; Cr, Cu, and Pb concentrations were correlated with DOC concentrations (9.6–36.3 mg L^-1). The variability in metal concentrations of the first 50-mL fractions (coefficients of variation, CV = 25–91%) was comparable in both soils and did not change with increasing leachate volume (mock soil solution) except for Zn in the Haplumbrept (CV up to 174%). In all leachate fractions, variability was markedly higher than those reported for salt extracts of composite soil samples (CV = 1–18%). Thus, the analysis of composite samples may be insufficient to address metal bioavailability in soils.

Abbreviations: CV, coefficient of variation • DOC, dissolved organic C • DOM, dissolved organic matter • ECEC, effective cation-exchange capacity • Fe_d, dithionite–citrate-extractable Fe • K_s, saturated water conductivity