Plant-induced changes in the bioavailability of heavy metals in soil and biosolids assessed by DGT measurements

Purpose This study investigated the effects of plants on the available pools of heavy metals and their re-supply potential in contaminated substrates in a short-term experiment using five metal-accumulating willow and poplar species/cultivars and in a longer-term experiment for Salix x reichardtii. Material and methods Five species of willow and poplar were grown in either soil or biosolids for short-term experiment (4 months). Further investigations of longer-term effects of plant on metal availability were conducted with S. x reichardtii grown in biosolids in a column (100 cm height and 37.5 cm diameter) experiment over a period of 12 months. Samples collected before and after experiments were determined for pH and bioavailability of metals using diffusive gradients in thin films (DGT). Various pools of metals in biosolids were determined by sequential extraction. Concentrations of heavy metals in plant material were determined. Results and discussion The concentration of metals determined by DGT (C (DGT)) and concentration of metals in pore water (C (SOL)) of Ni, Cu, Zn, and Cd in soil and biosolids generally decreased significantly compared to the initial measurements and were usually lower than those of the controls. However, C (DGT) and C (SOL) were higher in planted soil compared to those in the controls. There was a negative correlation between Ni, Zn, and Cd in plant shoots and C (DGT) in both soil and biosolids. The R values, the ratio of C (DGT)/C (SOL) calculated for Ni, Cd and Zn of planted substrates, were significantly higher than the corresponding R values of initial substrates. By contrast, R values for Cu showed little change. R values for Ni, Zn, and Cd were higher in planted biosolids compared to the unplanted biosolids. While S. x reichardtii leaf Cd, Ni, and Zn concentrations increased significantly over time, leaf Cu concentration declined. The patterns of plant uptake for the metals reflected the patterns observed by DGT and soil solution measurements of R. Sequential extraction of heavy metals from biosolids after 12 month's experimentation confirmed that Cu was predominantly in the organic fraction. Conclusions The short-term effects of plants on the bioavailability of metals in soils and biosolids were different. The R values of cultivated treatments varied between species but were not significantly different from the control in most of the cases. The longer-term experiment indicated that both C (DGT) and C (SOL) of Ni, Zn, and Cd decreased significantly over time in both planted and unplanted treatments. The results of this study demonstrated that R values measured by DGT may be useful in assessing the potential bioavailability of heavy metals in soil and biosolids.