Environmental Engineering Reference
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(a) one soil, different metal concentrations
(b) different soils with different metal
concentrations
8
100
non-esstential metal
micronutrient
7
90
80
6
70
5
60
4
50
deficient
adequate
toxic
3
40
30
2
20
1
10
0
0
0
2
4
6
8
20
60
100
0
40
80
Soil total metal concentration (mg/kg)
Soil total metal concentration (mg/kg)
Fig. 8.2
(
a
) Generalized concentration dependent uptake for metals:homeostatic processes main-
tain internal concentrations of essential metals at adequate supply, whereas these processes break
down at phytotoxic concentrations. Uptake of non-essential metals increases proportionally to soil
concentrations up to a level where saturation occurs. (
b
) the concentration dependent uptake of
non-essential metals is masked by differences in soil metal bioavailability in soils with different
soil properties. Data in (
a
): zinc (micronutrient) uptake by corn shoot from Zn
2+
salt spiked soil
(data of Maclean
1974
) and cadmium (non-essential) uptake in soybean shoot from Cd
2+
spiked
soil (Haghiri
1973
); in (
b
): cadmium uptake in wheat seedlings (Smolders et al.
1999
)
and tissue concentrations readily increase with soil concentrations and this increase
is generally associated with the onset of
phytotoxic conditions
. The homeostatic
mechanisms generally act stronger on shoot than on root concentrations.
The soil-plant concentration relationships depicted in Fig.
8.2
are only observed
in experiments where soils are enriched with soluble forms of the metal or metalloid.
These patterns are termed the 'metal or metalloid salt linear response relationships'
(Brown et al.
1998
). In the environment, no such clear pattern is found, because
the different bioavailabilities of the metal or metalloid in soil obscure the underly-
ing patterns. Figure
8.2b
illustrates that the concentration dependency can even be
completely invisible when merging data from various soils. Variable bioavailabil-
ity is related to differences in metal or metalloid speciation, interionic effects on ion
uptake from pore water and indirect effects of soil properties on translocation within
the plant.
The general paradigm in metal and metalloid uptake in soil is that roots absorb
elements through pore water and the concentration of the dissolved elements affects
the uptake rate. Dissolved metals and metalloids can be present in different forms
('species') and it is known that root uptake rate changes with the type of species
present. Numerous experiments have shown that uptake of cationic metals from pore
water generally decreases as the metal is complexed by chelating agents. Figure
8.3a
illustrates this for the uptake of cadmium in a plant grown in solution culture (hydro-
ponics). The addition of the cadmium chelator reduces the uptake of cadmium at
constant cadmium concentrations, illustrating that free ion is the preferred species.
This observation can be explained by the fact that ions are absorbed through their
specific uptake systems while complexes are too large to pass the root cell mem-
branes. This model is the basis of the
free ion activity model
(FIAM) that relates
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