Environmental Engineering Reference
In-Depth Information
Spartina
, as well as other plants whose roots are present in
anoxic environments, such as freshwater marshes and
swamps. As would be expected in such reducing sedimen-
tary environments, the transport of excess oxygen away from
the roots would oxidize any reduced species such as manga-
nese and iron. The measurement of a plant's ability to
transport oxygen into reduced sediments away from its
roots has been called radial oxygen loss (ROL) (Michaud
and Richardson 1989). Plants that have this trait to transport
oxygen include cattail (
Typha latifolia
) and rush (
Juncus
effusus
).
Wie
generated in reduced wetland sediments exits the subsurface
through plants through the aerenchymal tissues. Moreover,
the release of methane is not constant throughout the day but
has been reported to be higher in the early daylight hours
than at other times for
Typha domingensis
and
Typha
latifolia
in the Florida Everglades (Chanton et al. 1993).
If the oxygen enters through stomata or lenticels, how
does the CH
4
exit? In general, CH
4
emission does not appear
to be related to stomatal opening or closure (Whiting and
Chanton 1996). Conventional thinking about methane move-
ment and release to the atmosphere through aquatic plants is
that it is driven by changes in soil and air temperatures. Peak
emissions coincide with peak daily temperatures. These data
suggest that diffusion is the driving force in gaseous trans-
port from the anoxic sediment to the atmosphere through
plants. Gaseous movement processes include diffusion
(driven by concentration gradient) and convection (driving
force of high gas pressure to low gas pressure). Peak gas
transport emission did not correlate with peak stomatal
conductance.
Two types of gas-flow exchange occur in plants that grow
in anoxic sediments. The pressurized system relies on ther-
mal transpiration, or the pressure difference between the
higher oxygen pressures in the younger leaves in the atmo-
sphere to the lower pressures in the roots underground and
back to the atmosphere through older or dead leaves. Passive
diffusion relies on molecular diffusion of gas molecules
(a partial-pressure gradient).
Although CO
2
is produced by the roots during respiration,
CO
2
also is produced by the other living cells in the plant,
such as the trunk, stems, and cambium. At any given time
throughout the year, both oxygen and CO
2
can be present in
these tissues. During the winter for most trees, including
conifers, these tissues contain lower concentrations of CO
2
but higher concentrations of O
2
, as the lower temperatures
and light levels decrease photosynthesis. Cores collected
from decaying trees can emit gases that can be ignited
when exposed to a flame. This phenomenon can occur in
trees afflicted by the condition known as wetwood and is
often found in cottonwood trees. It is an infection of the
heartwood, usually by anaerobic soil bacteria that have
entered the roots. These anoxic bacteria essentially ferment
the wood and release fatty acids, which putrefy and are
expressed to the bark under localized pressures (Hiratsuka
1987). These observations support the notion that other than
the openings of lenticels, the bark of most trees is relatively
impermeable to gaseous transport.
If roots reach groundwater rendered anoxic by the pres-
ence of reduced organic contaminants, CO
2
also will be
present due to microbial oxidation of the contaminants. In
either case, this CO
2
can be taken up by roots in the
dissolved phase. This CO
2
can then be translocated in the
xylem to other parts of the plant, and possibly be used as a
ner et al. (2002) investigated the amount of oxygen
released into solution in contact with the roots of several
aquatic plants. All were found to release oxygen, at rates
between 0.01 and 1.41 mg O
2
/h, until fully oxidized
ex-situ
conditions were reached. Consumption of oxygen outside
the roots is the main factor that determines the distance that
oxygen will travel from the roots, as well as the concentra-
tion at a particular location. For microorganisms, the con-
centration of dissolved oxygen in pore water is the upper
limit to define aerobic versus anaerobic pathways. The more
reducing the solution the more oxygen is released. This
process of oxygen release will affect the rhizosphere and,
potentially,
b
the redox status of
shallow contaminated
groundwater.
One of the factors that affects the presence of oxygen in
the root zone is the level of the water table. This also can
occur when surface-water levels are high, as during a flood,
when the normally dry (and aerated) flood-plain vegetation
becomes inundated. Kozlowski (1997) provides a review
of the many processes that are affected when a plant
experiences flooded conditions. These include a decrease
in photosynthesis through stomatal closure, lack of
mycorrhizae (which are aerobic, for the most part), and an
increase in root decay. In flood-tolerant plants, however,
photosynthesis and growth can remain unaffected, because
such plants have adapted by the development of lenticels,
thick aerenchymal tissues that allow atmospheric interaction
between the roots and the air, and adventitious roots. The
effects of flooding are more severe during the growing
season relative to the dormant season.
12.3.1.2 Carbon Dioxide and Methane Diffusion
from the Root Zone
Plants cannot only transport atmospheric oxygen to roots in
anoxic environments but can transport reduced gases from
the subsurface to the atmosphere. This has been previously
demonstrated for aquatic macrophytes, such as rice, that
have roots in anoxic mud. The methane produced by micro-
bial methanogenesis, often seen as bubbles that rise when
such sediments are disturbed, can also exit to the atmosphere
through these plants. As such, these plants can act as gas
conduits or shunts. In general, 80-90% of the methane
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