Use and abuse of isotopic exchange data in soil chemistry

Abstract

Isotope dilution techniques have been widely applied in research investigating the fate and availability of different elements in soils. One technique, known as the 'E-value' method, involves tracking the depletion of isotope from solution in a soil suspension to determine the amount of isotopically exchangeable element at a given time following addition of isotope. An assumption underlying this method is that the isotope behaves exactly as the cold element in soil and hence, the amount of depletion of isotope from solution relative to the quantity of cold element at a given time can be used to determine specific elemental compartments in the soil. However, there has been some confusion in the literature with many studies neglecting to consider that an E-value can be composed of either a solid-phase compartment (E_e) or a combination of a solid-phase and solution compartment (E_a) depending on how it is calculated. Selection of the correct E-value formula is crucial for meaningful interpretation of data in soils with low buffering capacities, high concentrations of the element of interest in solution, or methods which use very short incubation times of isotope with soil. Here we discuss the derivation of E-value equations and circumstances where each should be applied.