Speciation of Cd and Zn in contaminated soils assessed by DGT-DIFS, and WHAM/Model VI in relation to uptake by spinach and ryegrass

A pot experiment was carried out to investigate the impact of Cd and Zn extractability in soil and speciation in pore water of industrial contaminated soils, on metal concentration in a metal sensitive species like spinach (Spinacia oleracea) and a more metal tolerant species like Italian ryegrass (Lolium multiflorum). For chemical speciation of Cd and Zn in pore water, WHAM/Model VI version 6.0 was used. The DGT technique was used to determine the effective concentration, C(E), of Cd and Zn in soils. The free ion activity in pore water correlated well with the contents in plants, and there was a linear relationship between the C(E) values and the concentration of Cd and Zn in both spinach and ryegrass in the non-toxic range. However, the C(E) values usually overestimated the plant contents when plants, particularly the spinach plants, were subjected to toxic concentration in the pore water. Metal uptake decreased in plants affected by toxicity, whereas metal binding to the Chelex resin did not. Thus, we found no linear relationship between the C(E) and metal contents in spinach, whereas a linear relationship was found between C(E)-Zn and the Zn concentration in ryegrass (r2=0.96, p<0.001). For Cd in ryegrass this relationship was weak (r2=0.53, p=0.18). This study indicates that the transport of metals from labile metal pools to the DGT-resin is linearly related to plant uptake only when plants are growing well, and that the applicability of DGT as an indicator for plant uptake seems species dependent.