Phytotoxicity of nickel in a range of European soils: Influence of soil properties, Ni solubility and speciation

We investigated the influence of soil properties on Ni toxicity to barley root and tomato shoot growth, using 16 European soils. The effective concentration of added Ni causing 50% inhibition (EC5) ranged from 52 to 1929 mg kg-1 and from 17 to 920 mg kg-1 for the barley and tomato test, respectively, representing 37- and 54-fold variation among soils. Soil cation exchange capacity was the best single predictor for the EC5. The EC5 based on either the Ni concentration or free Ni2+ activity in soil solution varied less among soils (7-14 fold) than that based on the total added Ni, suggesting that solubility of Ni is a key factor influencing its toxicity to plants. The EC5 for free Ni2+ activity from the barley test decreased with increasing pH, indicating a protective effect of protons. The results can be used in the risk assessment of Ni in the terrestrial environment. - Soil cation exchange capacity explains variation among soils in the toxicity threshold values of nickel to plants