Plant Induced Changes to Rhizosphere Characteristics Affecting Supply of Cd to Noccaea caerulescens and Ni to Thlaspi goesingense

Changes in soil rhizosphere properties after growing the Cd hyperaccumulator Noccaea caerulescens and the Ni hyperaccumulator Thlaspi goesingense were investigated. Dissolved organic carbon content increased in the rhizosphere, but there were no significant changes in the solution concentrations of Cd and Ni. Concentrations of these metals extracted by NH4Cl and EDTA decreased in the rhizosphere, as did DGT-measured concentrations, indicating a depletion of labile metal in the solid phase. The results could be explained by the increased DOC in the rhizosphere maintaining a higher proportion of the labile metal in solution through complexation, with the overall depletion of metals only manifest in the solid phase. The DGT induced fluxes in soils and sediments (DIFS) model was used to provide key kinetic information on soil processes and labile pool data more limited metal supply in the rhizosphere after the growth of hyperaccumulators was due to both depletion of the solid phase pool and a lower rate constant of supply from solid phase to solution. The effect on the rate constant, which could be rationalized by the plant sequentially accessing and consuming the more labile pools of metal, was most marked for Cd, which had the highest accumulation factors.