Agriculture Reference
In-Depth Information
7.3 TRACE ELEMENTS
7.3.1 GLOBAL CYCLING OF TRACE ELEMENTS
Wetlands are important in the cycling of trace metals and metalloids, firstly
because they are often in high rainfall areas and receive correspondingly high
additions by wet deposition; secondly because fluxial and phreatic wetlands
receive large volumes of surface and ground water bearing dissolved and par-
ticulate trace elements; and thirdly because of their particular biogeochemistry
which results in transformations and accumulation of trace elements.
Concentrations of trace elements in surface and ground waters are controlled
by deposition from the atmosphere and dissolution from soils and bedrock.
Concentrations in the atmosphere arise from anthropogenic sources—fossil fuel
combustion, cement production, extractive metallurgy—as well as through nat-
ural processes—windborne soil, volcanic ejecta, forest fires, biogenic processes.
Depending on the metal, transport through the atmosphere and subsequent wet
or dry deposition may exceed transport through surface and ground water. Ele-
ments for which atmospheric transport predominates are termed atmophile and
those for which transport by water predominates are termed lithophile (Stumm
and Morgan, 1996). Many atmophile metals are volatile or can become volatile
through methylation, especially the B-type metals (Section 3.1) Hg, As and Pb.
By contrast the A-type metals—Mn, Co, Cr, V and Ni—are lithophile. Hence B-
type metals tend to be enriched in the environment through diffuse atmospheric
pollution (Table 7.5). They also tend to be pernicious toxins because of their
tendency to react with soft bases, such as -SH and -NH groups in enzymes.
7.3.2 TRANSPORT OF TRACE ELEMENTS THROUGH SOIL AND INTO
PLANT ROOTS
The factors controlling the transport through soil include:
•
the concentration of the free ion in solution;
•
complexation with organic and inorganic ligands in solution;
•
redox reactions;
•
sorption on organic matter, clay minerals and oxides in the soil solid in outer-
sphere and highly insoluble inner-sphere complexes;
•
precipitation and co-precipitation in insoluble compounds, particularly hydrox-
ides, carbonates, sulfides or phosphates;
•
sorption, precipitation and co-precipitation in suspended colloids;
•
conversion to volatile forms.
Because the importance of these factors differs between the different trace
elements, predicting mobilities is complicated. The tendency to form organic
