When liming and revegetation contribute to the mobilisation of metals: Learning lessons for the phytomanagement of metal-polluted wetlands

The aim of this study was to identify the effectiveness of liming in combination with vegetation for the recovery of slightly acidic, saline soils of eutrophic wetlands affected by mine wastes, under fluctuating flooding conditions. Simulated soil profiles were constructed and four treatments were assayed under greenhouse conditions: control, only plant, only liming, and liming and plant. The plant species was the halophyte Sarcocornia fruticosa. Three horizons were differentiated: A (never under water), C1 (alternating flooding-drying conditions), and C2 (always under water). The pH, Eh, salinity, and the concentrations of dissolved organic carbon and soluble metals were measured regularly for 18 weeks. Liming favoured the growth of S. fruticosa, an increase in pH and a fall in Eh. The amendment was effective for reducing Mn, Zn, and Cd in pore water of bare soils, but not Cu and Pb. In the treatment with liming and plant, the growth of S. fruticosa counteracted the effect of the amendment, strongly increasing the concentrations of metals in pore water and distributing them along the soil profile. Hence, the combined use of liming and plants may increase the risk of metals mobilisation.

► Liming favoured an increase in pH and a fall in Eh in metal-polluted wetland soils.
► Liming decreased Mn, Zn, and Cd in pore water of bare soils, but not Cu and Pb.
► Sarcocornia fruticosa by itself had little effect on metal contents in pore water.
► Liming combined with S. fruticosa increased metal contents in pore water.