Assessment of Zn mobilization in the rhizosphere of Thlaspi caerulescens by bioassay with non-accumulator plants and soil extraction

This study used co-cultivated plants as a bioassay to test the hypothesis that the roots of the zinc-hyperaccumulating plant Thlaspi caerulescens mobilize Zn from less-available pools in the soil. Thlaspi caerulescens was grown in uncompartmentalised pots, or pots that were divided by solid or mesh barriers to limit the extent of root intermingling (rhizosphere interaction) with co-cultivated Thlaspi arvense or Festuca rubra. Thlaspi caerulescens did not increase the concentration of Zn in either indicator species, suggesting that T. caerulescens does not strongly mobilize Zn in its rhizosphere. The increase in the shoot mass of T. arvense when its roots were permitted to intermingle with those of T. caerulescens was explained by greater intensity of competition of T. arvense compared to T. caerulescens. There was no effect of co-cultivation with T. caerulescens on the shoot biomass of F. rubra. Despite the absence of increased Zn-availability to the co-cultivated species, the mass of Zn accumulated by T. caerulescens was 3-times greater than the mass of Zn depleted from the pool of extractable-Zn in the soil, measured by extraction with 1 M ammonium nitrate. The results are consistent with the hypothesis that the rapid Zn-uptake systems in the roots of T. caerulescens deplete the soluble-Zn at a rate equal to, or faster than that at which Zn is replenished to the soil solution via plant/microbially mediated mobilization or the Zn-buffering capacity of the soil.