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

We assessed the ability of several populations of the metal-hyperaccumulato r species, Thlaspi caerulescens, to mobilize non-labile cadmium in soils hi storically contaminated by Pb/Zn mine spoil or sewage sludge. Radio-labile Cd was determined chemically as an 'E-value', [Cd-E], and biologically as a n 'L-value', [Cd-L]. For comparison, chloride-extractable Cd, [Cd-chlor], w as 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-accumulato r species with a similar growth habit, Lepidium heterophullum. Seeds were s own in soil spiked with Cd-109 and grown for 9-12 wk in a controlled enviro nment 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 studi ed, [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 s ubstantially less than [Cd-T]. However, [Cd-L] showed no correlation with t he 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 p roducing root exudates or altering rhizosphere pH. The results imply that t here may be significant restrictions to metal bioavailability, even to hype raccumulator species, in heavily contaminated soils in which a large propor tion of the metal may be present in 'non-labile' forms.