ABSTRACT
No “reference soil materials” (RSMs) are available for use by research laboratories when they are determining the natural abundance of the 13C isotope. Consequently, laboratories have resorted to using other reference materials as either reference or working standards. These laboratories also obtain RSMs of their own or from other laboratories, but these soils have not necessarily been adequately characterized through interlaboratory comparisons or against suitable reference materials (RMs). The 13C isotope concentration for soil and plant material is expressed as a ratio to the 12C isotope concentration and that ratio is, by convention, expressed relative to the same ratio of a CaC03 standard known as PDB from the Cretaceous Pee Dee formation in South Carolina (Boutton, 1991). The sign of the 5 13C value indicates whether the sample has a higher or lower 13C/12C isotope ratio than PDB.
