Zinc accumulation by Thlaspi caerulescens from soils with different Zn availability: a pot study

The role of Zn bioavailability in soil on Zn hyperaccumulation by Thlaspi caerulescens was investigated. Thlaspi caerulescens from Prayon, Belgium, and Clough Wood, UK, were grown in pots containing unenriched soil (35 µg Zn g⁻¹), or five treatments enriched with Zn compounds of different solubility (ZnS, Zn3(PO₄)₂, ZnO, ZnCO₃, and ZnSO₇·7H₂O). The Zn-enriched treatments had similar total Zn contents (1000 µg Zn g⁻¹), but differed greatly in their concentrations of extractable-Zn. In the treatments with little extractable-Zn (unenriched and ZnS-enriched) T. caerulescens accessed Zn fractions that were not initially soluble; the mass of Zn accumulated in the shoots on Day 90 was greater than the mass of ammonium nitrate extractable-Zn in the soil on Day 0. Moreover, the decrease in ammonium nitrate extractable-Zn in the unenriched treatment after growth accounted for only 50 and 24% of the Zn accumulated by plants of the Clough Wood and Prayon populations, respectively. Despite accumulation of Zn from the previously non-labile fraction in soil, Zn hyperaccumulation from the unenriched and ZnS-enriched treatments was less than from the four treatments with highly extractable-Zn. The mechanisms involved in the solubilization of Zn were therefore not strong. The dissolution of Zn in the soil might have resulted from the very high root density in the pots either enhancing weak mobilization mechanisms, and/or highly efficient uptake in to the roots coupled with replenishment of the Zn taken up through the soil buffering capacity.