Bioavailability of Metals in the Soil Plant Environment
and its Potential Role in Risk Assessment
R. Naidu, S. Rogers, V.V.S.R. Gupta, R.S. Kookana, N.S. Bolan and D.C.
Adriano
INTRODUCTION
Metal contamination of soil environments and the assessment of its potential
risk to terrestrial and aquatic biota and public health is one of the
most challenging tasks confronting scientists today. This is partly due
to the plethora of soil biochemical and physical processes controlling
the fate of metals and partly because of the difficulty associated with
the assessment of their availability. A credible method for the assessment
of "bioavailability" of metals and other contaminants is lacking.
The definition of "bioavailability" and the concept on which
it is based are unclear, the methods adopted vary throughout the world,
and there- fore there is no single standard technique for the assessment
of either plant availability of metals or their ecotoxicological impacts
on soil biota. 'Some of the difficulties probably arise because of confusion
with the understanding of the bioavailability concept (see Chapter 1)
and relating this to the measurement of metal availabilities. For example,
a number of investigators relate bioavailability to that fraction of extractable
metals that correlates with the total metal uptake by plants. Although
this has been quite successful with certain plan and soil types, its applicability
has often been found to be fraught with limitations. Firstly, as bioavailability
is often assessed by chemical extractions, it is likely that extract ants
may release those pools of metals which the plant roots and microorganisms
do not exploit m the soil solution. This is supported by the definition
proposed in Chapter 1 which suggests that the concept of bioavailability
is fundamental to toxicology and it is not the same as bioavailable fraction,
which is largely a measure of solubility. Secondly and more importantly,
chemical extraction methods do not consider physiological and biochemical
factors associated with plants and microbes, e.g. transport of metals
across the cell membranes. The parameters that control the membrane transport
of metals are species-specific and possibly dictate the rate of uptake
by different plant and microbes from the same bioavailable pool of metals
in soils.
SREL Reprint #2760
Naidu, R., S. Rogers, V. V. S. R. Gupta, R. S. Kookana, N. S. Bolan and D. C. Adriano. 2003. Bioavailability of metals in the soil plant environment and its potential role in risk assessment. p. 21-57. In Bioavailability, Toxicity and Risk Relationships in Ecosystems, edited by R. Naidu, V. Gupta, S. Rogers, R. Kookana, N. Bolan and D. Adriano. Science Publishers, Inc.
