It's fairly well-known that data from sequential selective extractions can be difficult to interpret, since the fractions obtained are somewhat operational and don't necessarily reflect actual chemical or mineralogical species. In addition, there are not really consistent relationships between the concentration of pollutants in a particular fraction and the bioaccessibility or bioavailability of that pollutant.

From my understanding of this field, the best relationships of chemically-extracted metals with bioavailability (metals are the pollutants I'm most familiar with) are between plant uptake and metals extracted with electrolyte solutions. Such extractants probably target ion-exchangeable and weakly adsorbed forms of metal ions in soils.

Metal ion uptake may, in some instances, be related to the diffusion-limited rate with which metal ions can be released from soil solids into the soil pore water; the evidence for this would be the ability of DGT soil analyses to predict plant uptake in some studies. In polluted soils, a biotic ligand approach (essentially an extension of the free-ion activity model) seems to work.

If metal pollutants are your interest, you might want to check this meta-analysis: Smolders, E., Oorts, K., van Sprang, P., Schoeters, I., Janssen, C.R., McGrath, S.P., McLaughlin, M.J., 2009. Toxicity of trace metals in soil as affected by soil type and aging after contamination: using calibrated bioavailability models to set ecological soil standards. Environmental Toxicology and Chemistry, 28, 1633-1642.

Hi Andrew, thanks a lot for your knowledgeable perspectives on this confusing issue. As plant uptake is not my familiar topic, these information are really inspiring. Metals are indeed my interest. What is DGT as you mentioned? Ion exchanged portion of soil pollutants are the most weakly bonded species. But the mineralized portion is supposed to be dissolved by plant-soil interactions since ligands are usually involved in such reactions. By the way, does the biotic ligand mean biologically produced small organic chemicals?

Very good points, thank you.

Hi Zhenze, DGT is diffusive gradients in thin films - see Davison, W., Zhang, H., 1994. In situ speciation measurements of trace components in natural waters using thin-film gels. Nature 367, 546-548.

The biotic ligand model assumes that toxicity is related to the amount of metal ion adsorbed to the root surface of plants or other biological surface (it was originally developed to describe toxicity of metals to fish). See:

Di Toro, D.M., Allen, H.E., Bergman, H.L., Meyer, J.S., Paquin, P.R., Santore, R.C., 2001. Biotic ligand model of the acute toxicity of metals. 1. Technical basis. Environmental Toxicology and Chemistry, 20, 2383-2396.

Thakali, S., Allen, H.E., Di Toro, D.M., Ponizovsky, A.A., Rooney, C.P., Zhao, F.J., McGrath, S.P., 2006. A Terrestrial Biotic Ligand Model. 1. Development and application to Cu and Ni toxicities to barley root elongation in soils. Environmental Science and Technology, 40, 7085-7093.

regards...

Thanks Andrew, I will have a look at these recommended papers and further the discussions.

Best