Efficiency of a phytoimmobilisation strategy for heavy metal contaminated soils using white lupin

White lupin (Lupinus albus L.) has been shown to be a useful plant species for phytoimmobilisation of heavy metal contaminated soils. However, since it is an annual species, the heavy metals taken up by the plant and stored in roots can be incorporated back into the soil during root degradation, after plants have been harvested. In this work the efficiency of metal immobilisation by roots of white lupin has been studied in three metal polluted soils (calcareous, neutral and acid) after collection of the aerial part of the plants through an incubation experiment using the intact roots colonised soil. The pattern of C mineralisation in the soil allowed the estimation of the soil microbial activity and the degradation of the root tissues that remained in the soil. The proportion of root tissue degraded in soil was from 47% to 61%, the highest value being found in the calcareous soil. Heavy metal amounts released into the soil after root degradation were very low in comparison with the total soil metal concentration. Soluble metal concentrations in the calcareous and neutral soils were not affected by root degradation, and in the acid soil, soil conditions had a greater effect on heavy metals fractionation than root degradation. These results confirm the feasibility of the use of L. albus for metal phytoimmobilisation techniques.

► The phytostabilisation efficiency of white lupin was evaluated after harvesting.
► Roots degradation and metal release were studied in an incubation experiment.
► Degradation of lupin root tissues was not related to soil metal concentrations.
► Soil soluble metal concentrations were not affected by root degradation.
► Growing conditions alter soil metal solubility in a greater extent than plant roots.