Effects of zinc chemistry on phytoextraction in Thlaspi caerulescens-vermiculite system

Zinc phytoextraction capability of the hyperaccumulator is usually limited by zinc content in the soil. To investigate the effects of different zinc fractions (water-soluble, exchangeable, and non-exchangeable) on phytoextraction of Thlaspi caerulescens from medium of vermiculite, pot experiments were performed for various zinc application treatments (0-2000 mg kg^-1). Relationships between zinc quantity accumulated in the plants and zinc content in the medium were investigated. Zinc content accumulated in the shoot of T. caerulescens increased and approached a constant value with zinc content in vermiculite. The analysis of zinc balance in vermiculite before and after the growth of T. caerulescens revealed that more than 40% of the zinc content accumulated by T. caerulescens is contributed by exchangeable zinc and that a high linear correlation exists between the zinc content in shoot of T. caerulescens and the initial exchangeable zinc in vermiculite. Moreover, 17.73-32.94% of the total zinc uptake by T. careulescens was from non-exchangeable fraction. Thus, it was suggested that exchangeable zinc predominantly influenced the zinc uptake by plants and that T. caerulescens could contribute to the mobilization of non-labile zinc fraction in vermiculite.