Environmental Engineering Reference
In-Depth Information
to water. This overload of plant nutrients can change or
impair natural ecological processes that purify water.
earth's average temperature by adding excess carbon
dioxide to the atmosphere.
Since 1800, and especially since 1950, we have been in-
tervening in the earth's carbon cycle in two ways that
add carbon dioxide to the atmosphere.
First,
we clear
trees and other plants that absorb CO
2
through photo-
synthesis faster than they can grow back.
Second,
we
add large amounts of CO
2
by burning fossil fuels (Fig-
ure 3-26, p. 56) and wood.
Computer models of the earth's climate systems
suggest that increased concentrations of atmospheric
CO
2
and other gases could enhance the planet's
natural
greenhouse effect,
which helps warm the lower atmos-
phere (troposphere) and the earth's surface (Figure 3-7).
The resulting
global warming
could disrupt global food
production and wildlife habitats, alter temperature and
precipitation patterns, and raise the average sea level in
various parts of the world.
The Carbon Cycle
Carbon recycles through the earth's air, water, soil,
and living organisms.
Carbon, the basic building block of the carbohydrates,
fats, proteins, DNA, and other organic compounds
necessary for life, circulates through the biosphere in
the carbon cycle shown in Figure 3-25 (p. 56).
The
carbon cycle
is based on carbon dioxide
(CO
2
) gas, which makes up 0.038% of the volume of
the troposphere and is also dissolved in water. Car-
bon dioxide is a key component of nature's thermo-
stat. If the carbon cycle removes too much CO
2
from
the atmosphere, the atmosphere will cool; if it gener-
ates too much CO
2
, the atmosphere will get warmer.
Thus, even slight changes in this cycle can affect
climate and ultimately the types of life that can exist
on earth.
Terrestrial producers remove CO
2
from the atmos-
phere, and aquatic producers remove it from the wa-
ter. They then use photosynthesis to convert CO
2
into
complex carbohydrates such as glucose (C
6
H
12
O
6
).
The cells in oxygen-consuming producers, con-
sumers, and decomposers then carry out aerobic res-
piration. This process breaks down glucose and other
complex organic compounds and converts the carbon
back to CO
2
in the atmosphere or water for reuse by
producers. This linkage between
photosynthesis
in pro-
ducers and
aerobic respiration
in producers, con-
sumers, and decomposers circulates carbon in the
biosphere. Oxygen and hydrogen—the other ele-
ments in carbohydrates—cycle almost in step with
carbon.
Some carbon atoms take a long time to recycle.
Over millions of years, buried deposits of dead plant
matter and bacteria are compressed between layers of
sediment, where they form carbon-containing
fossil
fuels
such as coal and oil (Figure 3-25). This carbon is
not released to the atmosphere as CO
2
for recycling
until these fuels are extracted and burned, or until
long-term geological processes expose these deposits
to air. In only a few hundred years, we have extracted
and burned fossil fuels that took millions of years to
form. This is why fossil fuels are nonrenewable re-
sources on a human time scale.
The Nitrogen Cycle: Bacteria in Action
Different types of bacteria help recycle nitrogen
through the earth's air, water, soil, and living
organisms.
Nitrogen is the atmosphere's most abundant element,
with nitrogen gas (N
2
) making up 78% of the volume
of the troposphere. The N
2
in the atmosphere is a
stable molecule that does not readily react with other
elements, so it cannot be absorbed and used directly as
a nutrient by multicellular plants or animals.
Fortunately, atmospheric electrical discharges in
the form of lightning and certain types of bacteria in
aquatic systems, in the soil, and in the roots of some
plants can convert N
2
into compounds useful as nutri-
ents for plants and animals as part of the
nitrogen cy-
cle,
depicted in Figure 3-27 (p. 58).
The nitrogen cycle consists of several major steps.
In
nitrogen fixation,
specialized bacteria in soil and
aquatic environments convert (or fix) gaseous nitrogen
(N
2
) to ammonia (NH
3
) that can be used by plants.
Ammonia not taken up by plants may undergo
ni-
trification.
In this two-step process, specialized soil bac-
teria convert most of the ammonia in soil first to
nitrite
ions
(NO
2
), which are toxic to plants, and then to
ni-
trate ions
(NO
3
), which are easily taken up by the
roots of plants. Animals, in turn, get their nitrogen by
eating plants or plant-eating animals.
Plants and animals return nitrogen-rich organic
compounds to the environment as wastes, cast-off par-
ticles, and dead bodies. In
ammonification,
vast armies
of specialized decomposer bacteria convert this detri-
tus into simpler nitrogen-containing inorganic com-
pounds such as ammonia (NH
3
) and water-soluble
salts containing ammonium ions (NH
4
).
Effects of Human Activities
on the Carbon Cycle
Burning fossil fuels and clearing photosynthesizing
vegetation faster than it is replaced can increase the
