Validation of Cu toxicity to barley root elongation in soil with a Terrestrial Biotic Ligand Model developed from sand culture

Constants for a Terrestrial Biotic Ligand Model (TBLM) to predict the Cu toxicity to barley root elongation (RE) were developed from controlled sand culture experiments. These constants were used to predict RE in soil culture. The competition of H+, Ca2+, and Mg2+ to Cu2+ toxicity were studied individually and independently, and linear relationships between EC50 free Cu2+ and H+, Ca2+, and Mg2+ activities were found, meaning that the cations H+, Ca2+, and Mg2+ will alleviate the toxicity of Cu2+ in solutions. Toxicity accompanying increasing concentration of solution ions other than Cu2+ was observed and modeled as an osmotic effect which improved soil culture toxicity prediction. The Root Mean Square Error (RMSE) of %RE and EC50 (50% effective concentration) for soil toxicity prediction using TBLM parameters developed from sand culture are 13.0 and 0.23 respectively, which are as good as that of 14.0 and 0.24 using parameters that developed from soil culture itself. A model including the activity at the root plasma membrane surface was tested and found not to provide improvement over the use of bulk solution activity to predict metal toxicity. TBLM parameters obtained from water solution culture were unable to accurately predict the EC50s in soils whereas the parameters obtained from sand culture were able to predict the toxicity in soils. Including the toxicity of CuOH+ was found to improve the toxicity prediction slightly.