Page 215 - Soil Degradation, Conservation and Remediation

204 6 Soil Pollution

Mercury Uptake

The availability of mercury in soil to plants is low. When absorbed, mercury tends
to accumulate in roots, which indicate that the roots serve as a barrier to mercury
translocation. Mercury concentration in aboveground parts of plants depends largely
0
on foliar uptake of Hg volatilized from the soil. Uptake of mercury has been found

to be plant specific in bryophytes, lichens, wetland plants, woody plants, and crop
plants. Factors that affect uptake of mercury by plants include organic matter
content, cation exchange capacity, oxide and carbonate content, redox potential, and
total metal content of soil. With lower levels of mercury pollution, the amounts in
crops are below the permissible levels. Mercury concentrations in the plants (stems
and leaves) are always greater when the metal is introduced in organic form.
Mercury-vapor uptake by leaves of the C3 species oats, barley, and wheat is fi ve
times greater than that by leaves of the C4 species corn, sorghum, and crabgrass.
Such differential uptake by C3 and C4 species is largely attributable to internal
resistance to mercury-vapor binding. Airborne mercury thus seems to contribute
significantly to the mercury content of crops and thereby to its intake by humans as

food (Patra and Sharma 2000 ).

Nickel Uptake

Plants absorb Ni through the roots by passive diffusion and active transport (Seregin
and Kozhevnikova 2006 ). The ratio of uptake between active and passive transport
varies with the species, form of Ni, and concentration in the soil (Vogel-Mikus et al.
2+
2005 ). The overall uptake of Ni by plants depends on the concentration of Ni ,
plant metabolism, the acidity of soil or solution, the presence of other metals, and
organic matter composition (Chen et al. 2009 ). However, uptake of Ni usually
declines at higher pH values of the soil solution due to the formation of less soluble
2+
complexes (Temp 1991 ). For example, the uptake of Ni by Lathyrus sativus report-
edly increased with increasing pH up to 5.0 and decreased as the pH is increased
2+
further up to 8.0 (Pandaa et al. 2007 ). Moreover, Ni ion may also compete with
other essential metal ions when it is absorbed by roots. The uptake of heavy metals
2+
from the soil solution is strongly affected by calcium ion. Ca lowered the absorption
2+
of Ni in Arabidopsis bertolonii , an endemic plant of serpentine soils, but promoted
2+
Ni absorption in Berkheya coddii (Boyd and Martens 1998 ). The inhibitory effect
2+
of various metal ions on absorption and translocation of Ni from roots to shoots
2+
2+
+
+
+
3+
2+
varied as Fe > Co > Ca > Mg > NH 4 > K > Na (Temp 1991 ). Besides being
absorbed by roots, Ni can also enter into the plants via leaves. When a radioisotope
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of Ni was applied on the leaves of Helianthus annuus , 37 % of the total amount
was translocated to other plant organs (Sajwan et al. 1996 ). Similar trend was also
observed when oat, soybean, tomato, and eggplant leaves were sprayed with Ni
salt solution (Hirai et al. 1993 ). The path of Ni transport in plants is from root to
shoot and makes an exit through transpiration stream (Neumann and Chamel 1986 )
via the xylem.