ABSTRACT
Isotopic dilution (ID) has been used to discriminate between “labile” and “nonlabile” pools of various elements in soils for several decades. The earliest applications of the general technique focused on major nutrient availability and dynamics (
P,
K,
Ca), and gave rise to the terminology and methods still widely used today (Larsen 1952; Russell et al. 1954; Larsen and Cooke 1961; Deist and Talibudeen 1967). They also exposed many of the methodological and conceptual problems discussed in contemporary studies. In the 1970s, there was a marked increase in the application of ID methods to determining micronutrient metal availability (Lopez and Graham 1972; Tiller et al. 1972a,b; Graham 1973), using isotopes such as
Co,
Cu,
Mn,
Fe,
Ni, and
Zn. However, in more recent years the emphasis within soil chemistry has shifted in response to greater concern over environmental contamination. Applications of ID to soils over the past 2 decades have increasingly reflected this change, and recent studies have utilized
As,
Cd,
Cd,
Cd,
Hg,
Ni, and
Zn to assess the reactivity and bioavailability of metal and metalloid contaminants (Fujii and Corey 1986; Nakhone 1989; Nakhone and Young 1993; Hamon et al. 1997, 1998; Echevarria et al. 1998; Pandeya et al. 1998; Tandy 1998; Sinaj et al. 1999; Smolders et al. 1999; Gerard et al. 2000; Hutchinson et al. 2000; Stanhope et al. 2000; Sun
et al. 2000; Young et al. 2000, 2001; Ahnstrom and Parker 2001; Collins et al. 2001; Tye et al. 2002).
