The transfer of heavy metals to barley plants from soils amended with sewage sludge with different heavy metal burdens

PURPOSE: Our main aim objective was to evaluate the transfer of Cd, Cr, Cu, Ni, Pb and Zn to barley (Hordeum vulgare) grown in various soils previously amended with two sewage sludges containing different concentrations of heavy metals. This allowed us to examine the transfer of heavv metals to barley roots and shoots and the occurrence of restriction mechanisms as function of soil type and for different heavy metal concentration scenarios. MATERIAL AND METHODS: A greenhouse experiment was performed to evaluate the transfer of heavy metals to barley grown in 36 agricultural soils from different parts of Spain previously amended with a single dose (equivalent to 50 t dry weight ha⁻¹) of two sewage sludges with contrasting levels of heavy metals (common and spiked sludge: CS and SS). RESULTS AND DISCUSSION: In soils amended with CS, heavy metals were transferred to roots in the order (mean values of the bio-concentration ratio in roots, BCFRₒₒₜₛ, in brackets): Cu (2.4) ~ Ni (2.3) > Cd (2.1) > Zn (1.8) > Cr (0.7) ~ Pb (0.6); similar values were found for the soils amended with SS. The mean values of the soil-to-shoot ratio were: Cd (0.44) ~ Zn (0.39) ~ Cu (0.39) > Cr (0.20) > Ni (0.09) > Pb (0.01) for CS-amended soils; Zn (0.24) > Cu (0.15) ~ Cd (0.14) > Ni (0.05) ~ Cr (0.03) > Pb (0.006) for SS-amended soils. Heavy metals were transferred from roots to shoots in the following order (mean values of the ratio concentration of heavy metals in shoots to roots in brackets): Cr (0.33) > Zn (0.24) ~ Cd (0.22) > Cu (0.19) > Ni (0.04) > Pb (0.02) for CS-amended soils; Zn (0.14) > Cd (0.09) ~ Cu (0.08) > Cr (0.05) > Ni (0.02) ~ Pb (0.010) for SS-amended soils. CONCLUSIONS: Soils weakly restricted the mobility of heavy metals to roots, plant physiology restricted the transfer of heavy metals from roots to shoots, observing further restriction at high heavy metal loadings, and the transfer of Cd, Cu and Zn from soils to shoots was greater than for Cr, Ni and Pb. Stepwise multiple linear regressions revealed that soils with high sand content allowed greater soil-plant transfer of Cr, Cu, Pb and Zn. For Cd and Ni, soils with low pH and soil organic C, respectively, posed the highest risk.