A new method to measure effective soil solution concentration predicts copper availability to plants

Risk assessments of metal contaminated soils need to address metal bioavail ability. To predict the bioavailability of metals to plants, it is necessar y to understand both solution and solid phase supply processes in soils. In striving to find surrogate chemical measurements, scientists have focused either on soil solution chemistry, including free ion activities, or operat ionally defined fractions of metals. Here we introduce the new concept of e ffective concentration, C-E, which includes both the soil solution concentr ation and an additional term, expressed as a concentration, that represents metal supplied from the solid phase. C-E was measured using the technique of diffusive gradients in thin films (DGT) which, like a plant, locally low ers soil solution concentrations, inducing metal supply from the solid phas e, as shown by a dynamic model of the DGT-soil system. Measurements of Cu a s C-E, soil solution concentration, by EDTA extraction and as free Cu2+ act ivity in soil solution were made on 29 different soils covering a large ran ge of copper concentrations. They were compared to Cu concentrations in the plant material of Lepidium heterophyllum grown on the same soils. Plant co ncentrations were linearly related and highly correlated with C-E but were more scattered and nonlinear with respect to free Cu2+ activity, EDTA extra ction, or soil solution concentrations. These results demonstrate that the dominant supply processes in these soils are diffusion and labile metal rel ease, which the DGT-soil system mimics. The quantify CE is shown to have pr omise as a quantitative measure of the bioavailable metal in soils.