Determining uptake of 'non-labile' soil cadmium by Thlaspi caerulescens using isotopic dilution techniques

We assessed the ability of several populations of the metal-hyperaccumulator species, Thlaspi caerulescens , to mobilize non-labile cadmium in soils historically contaminated by Pb/Zn mine spoil or sewage sludge. Radio- labile Cd was determined chemically as an ' E -value', Cd _E ], and biologically as an ' L -value', Cd _L ]. For comparison, chloride-extractable Cd, Cd_chlor], was also determined using 1 M CaCl₂ as a single-step soil extractant. Values of Cd _L ] were measured for six populations of T. caerulescens that varied substantially in their ability to assimilate soil Cd, and a non-accumulator species with a similar growth habit, Lepidium heterophyllum . Seeds were sown in soil spiked with ¹⁰⁹Cd and grown for 9–12 wk in a controlled environment room. Values of Cd _L ] were determined from the specific activity of ¹⁰⁹Cd and concentration of Cd in the plant leaves. For the six soils studied, Cd _E ] ranged from 4.9 to 49% of total soil Cd Cd_T]. Values of Cd _L ] were, in general, in close agreement with both Cd _E ] and Cd_chlor] and substantially less than Cd_T]. However, Cd _L ] showed no correlation with the concentration of Cd in plant tissue, Cd_shoot]. This suggests that, in the soils studied, T. caerulescens did not mobilize non-labile soil Cd by producing root exudates or altering rhizosphere pH. The results imply that there may be significant restrictions to metal bioavailability, even to hyperaccumulator species, in heavily contaminated soils in which a large proportion of the metal may be present in 'non-labile' forms.