A terrestrial biotic ligand model. 1. Development and application to Cu and Ni toxicities to barley root elongation in soils.

A Terrestrial Biotic Ligand Model (TBLM) was developed using noncalcareous soils from Europe based on Cu and Ni speciation and barley (Hordeum vulgare cv. Regina) root elongation bioassays. Free metal ion (M2+) activity was computed by the WHAM VI model using inputs of soil metal, soil organic matter, and alkali and alkaline earth metals concentrations, and pH in soil solution. The TBLM assumes that metal in soil and in the solution are in equilibrium. Metal ions react with the biotic ligand, the receptor site, and inhibit root elongation. Other ions, principally H+, Ca2+ and Mg2+, compete with M2+ and, therefore, affect its toxicity. Toxicity is correlated only to the fraction of the total biotic ligand sites occupied by M2+. Compared to other models using either the soil metal concentration or M2+ activity as the toxic dose, the TBLM provides a more consistent method to normalize and compare Cu and Ni toxicities to root elongation among different soils. The TBLM was able to predictthe EC50 soil Cu and Ni concentrations generally within a factor of 2 of the observed values, a level of precision similar to that for the aquatic Biotic Ligand Model, indicating its potential utility in metals risk assessment in soils.

Terrestrial biotic ligand model. 2. Application to Ni and Cu toxicities to plants, invertebrates, and microbes in soil.

The Terrestrial Biotic Ligand Model (TBLM) is applied to a number of noncalcareous soils of the European Union for Cu and Ni toxicities using organisms and endpoints representing three levels of terrestrial organisms: higher plants, invertebrates, and microbes. A comparison of the TBLM predictions to soil metal concentration or free metal ion activity in the soil solution shows that the TBLM is able to achieve a better normalization of the wide variation in toxicological endpoints among soils of disparate properties considered in this study. The TBLM predictions of the EC50s were generally within a factor of 2 of the observed values. To our knowledge, this is the first study that incorporates Cu and Ni toxicities to multiple endpoints associated with higher plants, invertebrates, and microbes for up to eleven noncalcareous soils of disparate properties, into a single theoretical framework. The results of this study clearly demonstrate that the TBLM can provide a general framework for modeling metals ecotoxicity in soils.