Stable Isotope Methods for Estimating the Labile Metal Content of Soils

Estimation of the labile or available metal content of soils relies extensively on the use of chemical-extraction techniques. However, where interest is focused on only one or two elements that have more than one isotope, an alternative approach is to use isotope-dilu-tion analysis and measure E- or L-values. The E-value is an isotopic estimate of the labile metal content of the soil. It can be established by suspending a soil in water, adding an isotope tracer, and then measuring the isotope content of the solution phase after equilibra-tion. The labile metal content of a soil may also be measured by growing plants in soil to which an isotope of the metal of interest has been added. The isotopic signature within the plant tissue provides an estimate of the labile metal content of the soil and is referred to as the L-value. The L-value is a biological estimate of the labile metal pool. The use of isotopes to determine E- and L-values is described here, with an emphasis on the use of stable isotopes.