Page 201 - Soil Degradation, Conservation and Remediation

190 6 Soil Pollution

zone of hybrid poplar trees relative to an unplanted reference site. Plants may also
release exudates to the soil environment that help to stimulate the degradation of
organic chemicals by inducing enzyme systems of existing bacterial populations,
stimulating growth of new species that are able to degrade the wastes, and/or
increasing soluble substrate concentrations for all microorganisms (Barkovskii
et al. 1996 ). Plants help with microbial transformations through the following:
(1) mycorrhizal fungi and bacteria associated with plant roots metabolize the
organic pollutants, (2) plant exudates stimulate bacterial transformations (enzyme
induction), (3) buildup of organic carbon increases microbial mineralization rates
(substrate enhancement), (4) plants provide habitat for increased microbial populations
and activity, and (5) oxygen is pumped to roots ensuring aerobic transformations.
Fungi, growing in symbiotic association with the plant, have unique enzymatic
pathways that help to degrade organics that could not be transformed solely by

bacteria. In addition to soluble exudates, the rapid decay of fine root biomass can
become an important addition of organic carbon to soils that serves to retard organic
chemical transport. Microbial mineralization of atrazine is directly related to the
fraction of organic carbon in the soil (McFarlane et al. 1987 ).

6.2.12 Heavy Metal Pollution of Soils

Although variously defined (on the basis of density, atomic number, and atomic
−3
weight), metals (or metalloids) that have a density greater than 5 g cm and an
atomic mass exceeding that of calcium are generally considered as heavy metals.
The most common environmentally important heavy metals are zinc (Zn), copper
(Cu), lead (Pb), cadmium (Cd), mercury (Hg), chromium (Cr), nickel (Ni), tin (Sn),
silver (Ag), and the metalloid arsenic (As). Some heavy metals play an essential
role in plant and animal physiology and are thus required in small amounts for
normal healthy growth (Zn, Cu, and Ni for plants; Zn, Cu, Se, and Cr for animals).
They are essential micronutrients. Other heavy metals are not essential and have no
nutritional value (e.g., Cd, Pb, Hg, Sn, and As). All these metals are highly toxic at
high concentrations or at concentrations higher than required. Some heavy metals
affect the central nervous system (manganese, mercury, lead, arsenic), the kidneys
or liver (mercury, lead, cadmium, copper) or skin, bones, or teeth (nickel, cadmium,
copper, chromium) (Zevenhoven and Kilpinen 2001 ).
Soils may become polluted with heavy metals and metalloids through emissions
from the rapidly expanding industrial areas, mine tailings, disposal of metal wastes,
leaded gasoline and paints, application of fertilizers, animal manures, sewage
sludge, pesticides, wastewater irrigation, coal combustion residues, spillage of
petrochemicals, and atmospheric deposition (Khan et al. 2008 ; Zhang et al. 2010 ).
Heavy metals most commonly found at contaminated sites are lead (Pb), chromium
(Cr), arsenic (As), zinc (Zn), cadmium (Cd), copper (Cu), mercury (Hg), and nickel
(Ni) (GWRTAC 1997 ). Heavy metals do not undergo microbial or chemical