Assessment of relationships between total and reactive concentrations of cadmium, copper, lead and zinc in Hungarian and Slovakian soils

To account for the difference between the total metal pool and the biologically available pool, a wide range of mild extracts have been proposed to estimate plant uptake. Despite numerous sequential extraction schemes, specific information on the availability of those pools in view of its relation with dissolved concentrations, readily available for plant uptake or leaching to groundwater is lacking. This stresses the need for a single extraction that is able to evaluate the reactive (potential available) metal content in different soils, that can be related to the dissolved metal concentration. In this study a single, dilute acid extraction by 0.43 mol.l-1 HNO3 was applied on 72 soil samples that are representative for Hungary (33 plots) and Slovakia (39 plots). Sampling was limited to the plough layer where most effects on solute uptake (and ecology) are to be expected, i.e. within 0-20 cm. The soil samples were also extracted with EDTA to compare the results obtained with the mild HNO3 extraction. Furthermore, the so-called total metal content was measured by using an aqua regia extraction. The samples included a large range in soil properties and degree of contamination, the latter by including 12 plots from a long-term field experiment with heavy metals in Hungary. The main purpose of this study was to investigate the possibility to derive reactive metal contents from so-called total metal contents, the latter being readily available for a large number of plots in many countries. Results showed a considerable agreement between reactive metal concentrations measured with a mild (0.43 mol/l) HNO3 extraction and with an EDTA extraction. Furthermore, a considerable part of the metals are present in a rather unavailable form in soils. On average, Zn has the lowest reactive to total metal ratio (0.17), followed by Pb (0.30), Cu (0.38) and Cd (0.56). For each metal considered there is a large range in ratios of reactive to total metal content. On average, a reasonable prediction can be made of reactive metal contents from total metal contents and the organic matter and clay content using regression relations. However, large errors can be made for individual samples specifically in the low concentration range. The derived relations are further completely empirical and can only to be used for the types of soil for which they were derived.