Metal Availability and Chemical Properties in the Rhizosphere of Lupinus albus L. Growing in a High-Metal Calcareous Soil

Abstract

Chemical processes in the rhizosphere play a major role in the availability of metals to plants. The objective of this study was to assess the potential of white lupin (Lupinus albus L.) for the phytoimmobilisation of heavy metals in a calcareous soil with high levels of Zn and Pb (2,058 and 2,947 μg g−1 , respectively) by evaluating the chemical changes in the rhizosphere, relative to bulk soil, which modify the solubility of heavy metals. Plants were cultivated for 74 days in specially designed pots (rhizopots) in which rhizosphere was sampled easily under controlled conditions. White lupin accumulated high concentrations of Mn in the shoots (average of 4,960 μg g−1 ), well above the normal concentration in plants (300 μg g−1 ). But the metal concentrations found in shoots were not at toxic levels. Rhizosphere soil showed a significantly greater redox potential (245 mV) and water-soluble organic carbon content (34.6 μgCg−1 ) than bulk soil (227 mV; 27.6 μg C g−1 ). Root activity decreased EDTA-extractable Pb, Zn and Fe and promoted their precipitation as insoluble compounds in the residual fraction (acid digestion), hardly available to plants. These results indicate the suitability of this annual N2-fixing species for the initial phytoimmobilisation of heavy metals in contaminated soils.