SEASONAL AND SPATIAL VARIATION OF EXTRACTABLE TRACE-METAL FRACTIONS IN TOPSOILS UNDER MIXED FOREST

Metal extractability from soils may be affected by analytical errors, sample preparation, inaccurate sampling as well as by natural variatio n in space and time. During one year, we sampled the Ah horizons of a forested Luvi-Calcaric Cambisol from 43 subplots in a visually homogen eous area of 0.5 ha six times to determine the spatial and seasonal va riation of pH, OC, and Fe, Mn, Cr, Cu, Ni, Pb and Zn extractable by 0. 1 M BaCl2 and 0.05 M Na(2)ETDA. As indicated by a two-way ANOVA, the s patial and seasonal variations of all parameters were significant at p < 0.01. The metal-specific ranges of seasonal variation varied betwee n 21% and 90% of the minima for Na(2)EDTA, and 15% and 71% for 0.1 M B aCl2. The error due to within-subplot variability and inaccurate sampl ing, tested by sampling twice in April 1992, was small compared to the natural seasonal and spatial variation. The relative differences betw een the means of the two samplings tended to increase with the coeffic ients of spatial variation, suggesting an increasing contribution of w ithin-subplot variability to the sampling error. The study shows that both spatial and seasonal variation of extractable metal fractions may bias the analysis of mobile metal fractions in a rarely predictable w ay.