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 sail Cd, and a non-accumulator species with a similar growth habit, Lepidium heterophullum. Seeds were sown in soil spiked with Cd-109 and grown for 9-12 wk in a controlled environment room. Values of [Cd-L] were determined from the specific activity of Cd-109 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.