Phytoremediation of cadmium-contaminated soil through multipurpose tree species

Phytoremediation of heavy metal-contaminated sites using trees may be preferred over crops due to capacity of trees to accumulate heavy metals in the wood which is biologically non-hazardous. An investigation was undertaken to assess the cadmium (Cd) phytoextraction potential of four multipurpose tree species viz. eucalyptus (Eucalyptus tereticornis Sm.), subabul [Leucaena leucocephala (Lam.) de Wit], dhrek (Melia azedarach L.) and shisham (Dalbergia sissoo Roxb.) at six rates of Cd applied in solution form through CdCl2 center dot H2O (0, 10, 20, 40, 80 and 120 mg Cd kg(-1) soil) and harvested after 18 months of growth in an alkaline loamy sand soil in earthen pots. Dry matter (DM) of leaves, stems and roots, Cd content of these constituents and that of soil before planting and after harvesting the tree species were determined. Subabul had the highest and eucalyptus the lowest total DM production at all levels of Cd. Total DM did not decrease significantly up to application of 40 mg Cd kg(-1) soil (P > 0.05). The highest Cd level resulted in 49, 41, 26 and 43% reduction in total DM of eucalyptus, subabul, dhrek and shisham, respectively over control. Cadmium concentration was higher in leaves whereas uptake was higher in stems than the other components. Subabul had significantly highest uptake (0.05-71.4 mg pot(-1)) of Cd at various levels due its higher biomass whereas shisham had the lowest (0.05-28.2 mg pot(-1)) (P 0.05). Available Cd in the soil after harvesting of plants was the lowest in subabul and the highest in shisham pots. The upper critical level of available Cd in the soil at 20% reduction in DM was the highest for dhrek (85 mg kg(-1)) and the lowest for eucalyptus (46 mg kg(-1)). Therefore, dhrek was more tolerant to Cd but subabul had the greatest potential for phytoextraction of Cd from the soil.