Uptake of Metals from Soil into Vegetables - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 357

Table 8.11 Soil to plant transfer models for cadmium in leek on the basis of Eq. ( 8.3 )forthedata
in Fig. 8.8 . Given for each model are the coefficients of determination ( R 2 ), and standard errors
(se). SOM

= soil organic matter content (%)

R 2 Nr. Regression models se

1 Log M plant = -1 + 0.89 log M soil 0.29 0.43

2 Log M plant = 1.0 - 0.39 pH 0.30 0.42

3 Log M plant = 1.32 + 0.95 log M soil - 0.42 pH 0.63 0.31

4 Log M plant = 1.64 + 1.21 log M soil -0.37pH

-0.87 log SOM

0.67 0.30

it can be seen that the cadmium concentrations in leek in the sandy soils in the
Belgian-Dutch Kempen region depend on the cadmium concentration of the soil
and soil pH. Using Eq. 8.3 , with soil cadmium, soil pH and soil organic matter,
most of the variation can be explained. Such a model can be used, for example, to
predict the threshold soil cadmium concentrations as a function of pH above which
the food cadmium standards would be exceeded.
An advantage of Freudlich-type soil-plant transfer relations is the simplicity and
the applicability. Most equations use variables that are available from soil investi-
gations, such as total metal content, pH, organic matter and CEC. However these
soil-plant equations should not be used for soils where concentrations of metals are
outside the range from which the regressions were derived. Römkens et al. ( 2009 )
studied the quality of the soil-plant transfer equations for rice and concluded that
only models for cadmium and zinc gave good predictions. While predictions for
cobalt and nickel where not as good, and prediction for copper and lead were not
possible.

8.5.1.3 FIAM

The free ion activity model (FIAM) (Morel 1983 ) is based on the assumption that
metal uptake or toxicity is related directly to the free ion activity in the pore water.
While early reports suggested this was a promising model (Sauvé et al. 1996 ), later
studies have questioned whether free ion activities alone can improve predictions
of plant metal uptake, given effects of diffusional limitations adjacent to and near
the plant root (Degryse et al. 2006a, b ; Hudson 2005 ; McLaughlin 2001a ) and ion
competition and other factors that can markedly affect plant metal/metalloid acqui-
sition (Section 8.4.1 ). Free metal activities could not explain nickel uptake by oats
in glasshouse experiments (Weng et al. 2003 , 2004 ) nor copper uptake and toxic-
ity to barley and tomato, also in glasshouse experiments (Zhao et al. 2004 ). Nolan
et al. ( 2005 ) examined a wide range of techniques to predict cationic metal (cad-
mium, copper, lead, and zinc) uptake by wheat (again in glasshouse trials), including
free metal ion activities, and found consideration of elemental free ion activities
did not improve the predictions. McLaughlin et al. ( 1997 ) examined the relation-
ship between free Cd 2+ ion activities and concentrations of cadmium in potato