PROGRESSIVE EXTRACTION METHOD APPLIED TO ISOTOPIC EXCHANGE OF C-14

Isotopic exchange in natural settings is essentially an irreversible p rocess, so that it progresses continuously until there is complete iso topic equilibrium. In soils, this process involves interaction between isotopes in the liquid and solid phases, and complete isotopic equili brium may take a very long time. Measurements after partial isotopic e xchange have been used to characterize the labile fraction of elements in soils. We describe a method to characterize the extent of isotopic exchange, with application here to incorporation of inorganic carbon- 14 (C-14) into mineral carbonates and organic matter in soils. The pro cedure uses a continuous addition of extractant, acid, or H2O2 in the examples presented here, coupled with sequential sampling. The method has been applied to demonstrate the degree of isotopic exchange in soi l. The same strategy could be applied to many other elements, includin g plant nutrients.