Evaluating a ‘Free Ion Activity Model’ applied to metal uptake by Lolium perenne L. grown in contaminated soils.

Abstract

We investigated several formulations of the 'free ion activity model' (FIAM) as a means of describing plant uptake of soil Cd and Zn from contaminated soils. Lolium perenne was grown on a range of urban and metal-spiked agricultural soils selected to provide a wide range of Cd and Zn concentrations, pH values and other physicochemical properties. Plants were grown under controlled conditions and above-ground biomass was harvested at regular intervals. Concentrations of Cd and Zn in the grass were compared with estimates of metal 'capacity' (total or radio-labile metal content in the soil) and 'intensity' (metal concentration in the soil solution or free divalent ion activity). The results suggested that 'capacity' terms alone were poor predictors of plant metal uptake (r 2 values between 0.001 and 0.43), while metal ion 'intensity' provided quite reasonable predictions of the variation observed for several harvests of the grass (r 2 = 0.60-0.87). Soil solution-to-plant transfer factors were highly pHdependent which may suggest significant competition between trace metals and protons for sorption sites on roots. However, resolution of this question was confounded because of the strong co-variance between pH and p(M 2+ ) in the soil pore water. Thus the influence of pH could not be separated from the effect of changing metal ion activity on uptake rate. Other possible effects on metal uptake such as dilution from increased biomass during growth and competition for uptake between different metal ions (Zn vs. Cd), or with Ca 2+ , appeared to play very minor roles in determining bioavailability. Several formulations of the FIAM failed to provide a consistently superior prediction of metal uptake when compared to purely empirical regression with pH and p(M 2+ ) within the range of the data used to parameterise the models.

Predicting availability of mineral elements to plants with the DGT technique: a review of experimental data and interpretation by modelling

“…The distribution of a metal between solid and liquid phases is then modelled with an equilibrium partitioning model, and the plant response (uptake or toxicity) is modelled from the solution composition using a FIAM or BLM approach. [42][43][44][45][46] These models give reasonable predictions of the uptake, but this does not prove that the 'equilibrium' assumptions behind the model are valid. For example, Hough et al [46] showed that the prediction using a FIAM approach was not superior to that of a simple empirical regression model.As pointed out by the authors, the derived 'reaction constants' could truly represent an equilibrium condition, in which case they describe the affinity of metal and proton ions for the root sites, but they could also be fitting parameters in a transport-limited uptake process.…”

Lability, solubility and speciation of Cd, Pb and Zn in alluvial soils of the River Trent catchment UK

“…These regression models have helped develop understanding of the robustness of the relationships between solution metal and soil characteristics 12 . More recently, the free metal ion concentration or activity has been identified as the solution species that provides robust 20 predictions of plant and microbial uptake of metals based on the Free Ion Activity Model (FIAM) or Biotic Ligand models 16,17 . Soil Critical Limits for Zn, Cd and Pb have been developed based on free ion concentrations in solution 18,19 .…”

Characterization of soil heavy metal pools in paddy fields in Taiwan: chemical extraction and solid-solution partitioning

“…The total pool is usually larger than the reactive pool, and the difference between both is considered to be nonreactive or inert, at least on a time scale of years to decades. The directly available pool can be related to uptake of metals by soil organisms and plants (Saxe et al 2001;Lofts et al 2004, Hough et al 2005Peijnenburg et al 2007;Koopmans et al 2008a). Both the total dissolved concentration and the free metal ion activity in solution have been used as a measure of the directly available pool (Saxe et al 2001;Lofts et al 2004, Hough et al 2005Peijnenburg et al 2007).…”

“…The directly available pool can be related to uptake of metals by soil organisms and plants (Saxe et al 2001;Lofts et al 2004, Hough et al 2005Peijnenburg et al 2007;Koopmans et al 2008a). Both the total dissolved concentration and the free metal ion activity in solution have been used as a measure of the directly available pool (Saxe et al 2001;Lofts et al 2004, Hough et al 2005Peijnenburg et al 2007). Since it is difficult to obtain real soil solution samples, soil extracts such as a 1:10 w/v soil-solution extract using 0.01 M CaCl 2 (Houba et al 2000) are believed to represent the directly available pool in soil (Degryse et al 2003).…”