Environmental Engineering Reference
In-Depth Information
widespread and affordable and contributed to widespread
use of such inorganic sources of nitrogen in commercial
fertilizers rather than manure. Since then, most of the plant
proteins, which contain nitrogen, that have been ingested by
man have been derived from manmade ammonia. Plants also
require calcium (Ca
2+
), magnesium (Mg
2+
), and sulfur (as
the oxidized anion SO
4
2-
). Because each of these compounds
constitutes more than 1% of the dry organic weight of the
plant, they are called
macronutrients
.
Each of these macronutrients is used for different pro-
cesses in which the whole usually is greater than the sum of
the parts. Nitrogen is used to synthesize proteins and co-
enzymes. Phosphorus, typically taken up as a phosphate salt,
is essential in conversion of ADP to ATP. Plants absorb
phosphorus not as elemental phosphate but as inorganic
phosphorus. Calcium is used to maintain cell membranes.
Magnesium is in the heme group of chlorophyll
a
. Potassium
helps maintain the process of osmosis discussed in Chap. 3
with regard to water uptake by root cells. Potassium in root-
cell cytoplasm and vacuoles reduces the water potential, or
concentration, in the cell and, thus, sets up conditions for the
passive entry of water.
The elements present in plants at less than 1% of dry
organic weight are called
micronutrients
and include iron (as
Fe
2+
or complexes of Fe
3+
), manganese (Mn
2+
), zinc (Zn
2+
),
boron (as BO
3
3-
or B
4
O
7
2-
), copper (Cu
+
or Cu
2+
), and
molybdenum (as MoO
4
2-
). These micronutrients all are
essential to plant life; zinc helps transfer phosphorus,
boron helps during synthesis of the growth hormone
auxin, and iron and manganese act as catalysts in many
reactions, which change the rate of reactions without them-
selves being used up. Although iron is an essential micronu-
trient, too much iron can be toxic to plants, and amounts
greater than 400 mg Fe/kg dry plant weight (equivalent to
ppm) cause toxic effects. The role of some of these macro-
and micro-nutrients is discussed in Chap. 11, with
specific emphasis on their role in the phytoremediation of
contaminated groundwater.
Even elements considered unessential and toxic to plants
can be useful. For example, sodium and chloride can upset
the osmotic gradient of the plant. But sodium and chloride
are necessary for plant survival. In arid areas where recent
recharge is essentially absent and the depth to the water table
is on the order of hundreds of feet (tens of meters), ground-
water can rise toward the land surface in response to lowered
water potentials near the soil surface caused by evaporation
and transpiration (Andraski et al. 2003). Much like the salt
residue left in a pot after water has been boiled away, in arid
parts of the United States, as the water near the land surface
evaporates, salts do not evaporate and can form a crust on the
surface soils at percent concentrations. This can be toxic to
plants or to animals that eat the plants. For example, cows in
California can become poisoned if allowed to eat grasses
that contain high levels of selenium, a salt that is enriched in
surface soils as a result of evapotranspiration of shallow
groundwater. Saline soils have been operationally defined
as having more than 4 decisiemens per meter (or 4
millisiemens per centimeter). On the other hand, some
plants, such as
Spartina spp.
and tamarisk, can tolerate
higher levels of salt, because they can keep the salt content
of their cells higher than the salt content of water, even
seawater; thus, the water content in the cells is lower than
the water content in saltwater. This tolerance explains why
these plants dominate the estuaries of the east coast and
many riparian areas of the United States, respectively.
An interesting relation between root density of plants that
are intolerant to salt and salinity levels in soils and subsur-
face water suggests that, in some cases, the salinity profile
can change over time in response to root growth. At a site in
western Australia, researchers removed cores alongside
eucalyptus trees that were planted to lower a high water
table composed of saline water (Rural Industries Research
and Development Corporation 2000; Fig.
7.7
). From obser-
vation of the cores, root density was greater near the surface
(20-50 m/dm
3
), decreased to less than 5 m/dm
3
3ft(1m)
below land surface but increased between 6 and 21 ft (2 and
7 m). Chloride concentrations also were higher between 6
and 24 ft (2 and 8 m). These relations between depth, root
density, and chloride were not constant but changed through-
out the year in response to changes in soil moisture and the
water-table elevation (Fig.
7.8
).
The overall conclusion is that tree roots intersected the
water table and took up groundwater, which tended to
concentrate chloride in the area where the root density
was the greatest. This emphasizes the advantage of transpi-
ration by plants such that dilute elements are concentrated
near their roots. Hence, the highest chloride concentrations
in a vertical profile may indicate the recent (rather than
past) location of water uptake by the trees. Moreover, live
roots were observed to be growing below the water table,
which the authors correctly assumed meant the groundwa-
ter contained dissolved oxygen which they, unfortunately,
did not measure.
There are other processes that involve plants, groundwater,
and soil salinization. The removal of forests during
clearcutting for lumber or pulp production can result in an
increase in net recharge to aquifers that previously had deep
water tables as a result of removal by tree transpiration.
Clearcutting of forests results in an increase in recharge, and
the water table rises near land surface. Thus, mineralized
groundwater can be influenced further by evaporation,
which leaves behind salt in the soil profile. Subsequent irriga-
tion of the salt-enriched soils can lead to soil salinization,
especially if the source of the irrigation water is the
mineralized groundwater, or if irrigation can lead to artificial
recharge of the water table causing it to rise and be further
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