Phytoremediation: Metal decontamination of soils after the sequential forestation of former opencast coal land

Abstract

Phytoremediation through forestry may be an effective means for reducing the metal loading in lands reclaimed after surface-coal-mining in the UK. Planted with mixed woodland, the soil loading of 5 key metals (Zn, Cd, Mn, Pb and Cu) decreased, significantly and progressively, compared to soils left as grassland through a 14 year forestation chronosequence on land reclaimed from the former Varteg opencast coalmine, South Wales. Fourteen years after initial tree planting, soil metal loadings decreased by 52% for Cd (4.3 mg∙kg−1 per year), 48% for Cu (2.1 mg∙kg−1 per year), 47% for Zn (7.3 mg∙kg−1 per year), 44% for Pb. (7.1 mg∙kg−1 per year) and 35% for Mn (45 mg.kg-1 per year). Analysis of metal loadings in the leaves of Alnus glutinosa (L. Gaertn) (Common Alder) and Betula pendula (Roth) (Silver Birch) found both to be involved in metal uptake with birch taking up more Cd, Cu, Zn and Mn and Alder more Pb. Concentrations of Zn, Mn and Cd (Birch only) increased significantly in leaves from, but not in soils, under older plantings. Since different tree species take up metals at different rates, mixed plantings may be more effective in forest phytoremediation.