Evaluation of an electrostatic toxicity model for predicting Ni2+ toxicity to barley root elongation in hydroponic cultures and in soils

• Assessing environmental risks of metal contamination in soils is a complex task because the biologically effective concentrations of metals in soils vary widely with soil properties. • The factors influencing the toxic effect of nickel (Ni) on root growth of barley (Hordeum vulgare) were re-evaluated using published data from both soil and hydroponic cultures. The electrical potential (ψ0o) and ion activities ({Iz}0o) at the outer surfaces of root-cell plasma membranes (PMs) were computed as the basis of the re-evaluation. • The reanalyses demonstrated that root growth was related to: the Ni2+ activity at the PM surface, ({Ni2+}0o); calcium (Ca) deficiency (related to {Ca2+}0o); osmotic effects; and modification of intrinsic Ni2+ toxicity by magnesium (Mg2+; this appeared to exert an intrinsic (specific) ameliorating effect on intrinsic Ni2+ toxicity). Electrostatic toxicity models (ETM) were developed to relate root growth to these factors (R2 > 0.751). • Based on the ETM developed in soil culture and a Ni2+ solid–solution partitioning model, critical metal concentrations in soils linked to a biological effect were well predicted for 16 European soils with a wide range of properties, indicating the potential utility of ETM in risk assessment of metals in terrestrial ecosystems.