Does specific parameterization of WHAM improve the prediction of copper competitive binding and toxicity on plant roots?

We aimed at assessing whether the binding and rhizotoxicity of metal cations such as copper that exhibit high affinity for plant roots could be adequately predicted using the Windermere Humic Aqueous Model (WHAM) default parameterization. Accordingly, we first compared the ability of the default parameterization of WHAM and a specific parameterization for terrestrial higher plants (WHAM-THP) to model the competitive binding of copper on wheat (Triticum aestivum L.) and tomato (Solanum lycopersicum L.) roots. Secondly, in an external dataset, we evaluated the ability of WHAM-THP to predict the copper concentration and toxicity to pea (Pisum sativum L.) roots relative to WHAM. WHAM-THP estimates generated a slightly better fit for the competitive binding of copper on wheat and tomato roots (log of the root-mean-square error, RMSE = 0.15) than WHAM estimates (RMSE = 0.24). WHAM-THP estimates slightly better fitted the copper concentration in pea roots (RMSE ≤ 0.49) than WHAM estimates (RMSE ≤ 0.67) at low copper exposure and pH ≤ 5. However, WHAM-THP did not at all improve the prediction of copper toxicity to pea roots (RMSE = 13% as also for WHAM). We thus conclude that, although the default parameterization of WHAM does not neatly predict the binding of metal cations on roots, it could however be used with confidence in predictive ecotoxicology for terrestrial higher plants without any specific parameterization.