Small-scale variability of metal concentrations in soil leachates

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, Ph, Zn, dissolved org anic C (DOC), and pH in soil leachates were examined as an index of small-s cale soil heterogeneity. Ten undisturbed soil cores (0-4 cm, 100 cm(3)) fro m a 1-m(2) area of a Lithic Haplumbrept (pH 5.2, 3 g CaCO3 kg(-1)) and a Ty pic 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 fir st 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 con centrations 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 litt le in the first 50-mL fractions; Cr, Cu, and Pb concentrations were correla ted with DOC concentrations (9.6-36.3 mg L-1). The variability in metal con centrations of the first 50-mL fractions (coefficients of variation, CV = 2 5-91%) was comparable in both soils and did not change with increasing leac hate volume (mock soil solution) except for Zn in the Haplumbrept (CV up to 174%), In all leachate fractions, variability was markedly higher than tho se reported for salt extracts of composite soil samples (CV = 1-18%), Thus, the analysis of composite samples may be insufficient to address metal bio availability in soils.