Environmental Engineering Reference
In-Depth Information
an important factor in achieving remediation. Root density is
different for different types of plants, and this knowledge
can be used to add the correct plant to remediate different
distributions of contaminants. For example, shallow
contaminated sediment could best be remediated by grasses,
which tend to have fibrous roots, where the roots are
distributed in the upper parts of the soil column such as the
O-horizon. Conversely, deeper sediment contamination or
contamination of the water table or capillary fringe needs to
come into contact with plant roots that are distributed more
deeply, such as poplar trees and willows, or perhaps a deeply
rooted prairie grass.
that a nutrient culture solution that contained anions such as
nitrate became more basic as the nitrate was depleted. This
would occur as the plant roots release bicarbonate ions
(HCO
3
) to maintain electrical neutrality at the root surface.
In roots, this occurs according to the reaction
NO
3
þ
OH
(12.54)
R
OH
þ
8H
!
R
NH
2
þ
3H
2
O
þ
As nitrate is reduced in the plant, hydroxide ion is
released and must be excreted by the plant to maintain
internal pH consistency. In shoots, it is possible for it to be
excreted as an organic anion that may or may not be stored
within the plant itself. The converse also can hold true for
some plants that sequester more cations relative to anions,
and these roots release hydrogen ions (H
+
) to maintain
electrical neutrality. This happens if nitrogen is made avail-
able as ammonium, such that as this cation is taken up, H
+
ion is released (Miller et al. 1970).
Plant root respiration releases CO
2
into the rhizosphere.
This CO
2
can also lead to changes in soil pH, primarily when
soils become flooded, because the removal of CO
2
would be
limited to the solubility of CO
2
in water. In the unsaturated
zone, CO
2
can migrate readily away from the root source by
diffusion.
12.2.2 Rhizotron Methodology
Roots are inherently difficult to study directly because they
are located underground. In many cases, to observe roots
requires excavation or removal of dirt from around the roots
in situ
, both of which are invasive techniques. Noninvasive
techniques, however, have been developed to enable root-
growth patterns to be studied directly. One such method
involves the observation of roots through an enclosure, or
rhizotron, that consists of at least one clear panel. These
macrocosms can be multiple feet in dimension, are neces-
sarily expensive, and be used to monitor root growth for one
or many plants.
Root-growth enclosures on a smaller scale more applica-
ble to phytoremediation projects use smaller devices called
minirhizotrons. These consist of tubes of clear material such
as plastic, glass, acrylic, and butyrate, a few inches in diam-
eter, that can be inserted into the root zone. A camera can be
lowered down the tube to view the soil and roots in contact
with the tube walls (Taylor et al. 1990).
The advantage of both methods is that the roots are not
destroyed during observation and measurements at the same
location over time can be made. One disadvantage of the
minirhizotron method, that roots tend to grow in the space
between the tube and soil, can be overcome using an inflat-
able minirhizotron as described by Gijsman et al. (1991).
12.2.4 Release of Root Exudates and Increased
Bioavailability
The release of organic substance by plant roots in the rhizo-
sphere can be described as a passive or active process,
depending upon when the organics are released. For exam-
ple, the release of organic matter that follows plant death
is a passive release. The active release of carbohydrates,
proteins, sugars, tannins, mucigel, and ethylene occurs
when bacteria exude the genes to promote the plant to
release such organic material, such as during an infestation
or allelopathic encounter. In this instance, the release of
certain exudates may be a result of the plant needing the
rhizosphere bacteria as protection from naturally occurring
threats to the plant. The protection may be derived from the
physical barrier the growth permits, or by actual secretions
by the fungi, such as organic acids or chelators. Under
natural conditions where plants are exposed to terpenes and
alkaloid compounds as the result of allelopathic competition
for resources, plant roots secrete pectins or lignitic
compounds that act to sorb the toxicant prior to plant entry.
The amount of organic matter that is actively released by
living roots is a subject of great controversy. On one hand, it
is believed that plants actively release tens of percent of their
GPP into the subsurface. However, the photosynthate used
to create these exuded compounds is no longer available to
12.2.3 Root-Zone Changes in Subsurface
Sediment Chemistry
Roots can not only influence contaminants dissolved in
groundwater but can also influence soil chemistry and, there-
fore, influence the fate and behavior of redox-sensitive
solutes. For example, root respiration releases CO
2
as some
percentage of gross primary production (GPP), the uptake of
ions by roots alters the ion concentration remaining in solu-
tion, O
2
can enter and leave the subsurface, and roots can
release organic compounds. A report by Sachs (1875) stated
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