Environmental Engineering Reference
In-Depth Information
or
dissimilatory
denitrification by nitrate reduction as an
alternative electron acceptor during the oxidation of organic
matter in the absence of oxygen, such as
bilayers. Phosphates also are used by the plant to buffer
against rapid changes in pH. But the most important need
for phosphorus is for the synthesis of ATP; without phos-
phorus, plants cannot synthesize ATP. Unlike carbon and
nitrogen, which have the atmosphere as their source, phos-
phorus does not have a gaseous phase at environmentally
relevant temperatures and pressures and is primarily found
in soils derived from marine sedimentary rocks that contain
phosphorus-rich organic matter, rocks, or minerals. Thus,
the flow of phosphorus through an ecosystem starts with
the weathering of rocks and the entrance of phosphorus
into the hydrologic cycle. Phosphorus also can enter the
hydrologic cycle by runoff.
Much as plants require inorganic nitrogen, so plants
require inorganic phosphorus, or oxidized phosphorus as
the orthophosphate ion. Phosphorus needs of plants are
about one-tenth that of N
2
. Too little phosphorus results in
stunted plant growth, but too much causes excessive growth,
especially in aquatic plants that grow in surface waters that
receive phosphorus-rich wastes—this scenario is one of the
challenges facing managers of the Florida Everglades.
Unlike the flows of carbon or nitrogen, the flow of phos-
phorus is limited because much of phosphorus is locked up
in the anoxic layers of buried geologic material (Fig.
11.6
).
Plants take up the phosphate ion (PO
4
3
). Immobile phos-
phate tends to result from binding with sodium, calcium,
CH
2n
O
þ
NO
3
!
CO
2
þ
H
2
O
þ
N
2
:
(11.7)
The presence of denitrification in anaerobic soils that are
characterized by high organic content and wet conditions,
such as wetlands or swamps, reduces the fertility of the area
for plant growth, as it produces nitrogen in a form that plants
cannot directly use, and the nitrogen returns to the atmo-
sphere. This is why carnivorous plants, such as pitcher plants
or the Venus fly traps found in these environments use
animals and insects as a nitrogen source.
The effect of endocrine-disrupting (ED) compounds on
nitrogen fixation was investigated by Fox et al. (2001). They
found that nitrogen fixation by alfalfa (
Medicago sativa
) was
altered by the presence of EDs such as the pesticide DDT
and the herbicide 2,4-D. Apparently, the presence of these
chemicals altered the signal between the plant and the
rhizospheric bacteria.
One of the reasons behind the habit of many gardeners and
farmers to plow plant materials back into the soil is to stimu-
late the nitrogen cycle for a localized area. This process puts
plant material into the ground for microbes to break down into
ammonia and increases oxygen diffusion in soil to be used to
oxidize ammonium to nitrate. In fact, this technique of
plowing to accelerate the nitrogen cycle was used by the
French forces during the Napoleonic wars to overcome
shortages of nitrate used to manufacture gunpowder—soil
andmanureweremixedandfrequentlyturnedtoaccelerate
the conversion of ammonia to nitrate, called 'nitrate gardens.'
The interaction between plants and the flow of nitrogen is
perhaps best seen in what happens to local nitrogen dynam-
ics when trees are removed from an area. This is best done
with analysis of the differences in nitrogen-stable isotopes in
the annual growth rings of trees. The lighter
14
N atoms react
faster than the heavier
15
N
isotopic signature, of stable nitrogen isotopes can be used,
therefore, to delineate the predominant source of nitrogen in
plants or to represent changes in nitrogen sources over time.
This approach was used by Bukata and Kyser (2005) to show
that the
15
N atoms. The difference, or
d
15
N in the annual rings of trees measured gets
heavier by 1.5-2.5 per mil with tree-ring age. The authors
attributed this shift in heavier
d
15
N isotope in tree rings to
increased bacterial nitrification of NH
4
+
d
to NO
3
and
subsequent leaching from the soil.
11.1.5 The Flow of Phosphorus
Similar to the plant need for nitrogen is the plant need for
phosphorus. Plant cell membranes contain phospholipid
Fig. 11.6
A representation of the flow of phosphorus. Shown are
oxygen (O
2
); phosphate (PO
4
); ferrous phosphate (Fe(II)PO
4
).
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