Recycling in the Biosphere
How does matter move through the biosphere?
Matter moves through the biosphere differently than the
way in which energy moves. Solar and chemical energy are captured
by primary producers and then pass in a one-way fashion from one trophic
level to the next—dissipating in the environment as heat along the way.
But while energy in the form of sunlight is constantly entering the biosphere,
Earth doesn’t receive a significant, steady supply of new matter from space.
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Unlike the one-way flow of energy, matter is recycled
within and between ecosystems.
Elements pass from one organism to another and among parts
of the biosphere through closed loops called biogeochemical cycles,
which are powered by the flow of energy as shown in Figure 3–13.
Biogeochemical Cycles
Processes in which elements, chemical compounds, and
other forms of matter are passed from one organism to another and one part
of the biosphere to the other.
As that word suggests, cycles of matter involve biological
processes, geological processes, and chemical processes. Human activity
can also play an important role. As matter moves through these cycles,
it is transformed. It is never created or destroyed—just changed.
There are many ways in which the processes involved in
biogeochemical cycles can be classified. Here, we will use the following
guidelines:
Biological Processes
Biological processes consist of any and all activities
performed by living organisms. These processes include eating, breathing,
“burning” food, and eliminating waste products.
Geological Processes
Geological processes include volcanic eruptions, the
formation and breakdown of rock, and major movements of matter within and
below the surface of the earth.
Chemical and Physical Processes
Chemical and physical processes include the formation
of clouds and precipitation, the flow of running water, and the action
of lightning.
Human Activity
Human activities that affect cycles of matter on a global
scale include the mining and burning of fossil fuels, the clearing of land
for building and farming, the burning of forests, and the manufacture and
use of fertilizers.
These processes, shown in Figure 3–14, pass the same atoms
and molecules around again and again. Imagine, for a moment, that
you are a carbon atom in a molecule of carbon dioxide that has just been
shot out of a volcano. The leaf of a blueberry bush in a nearby mountain
range absorbs you during photosynthesis. You become part of a carbohydrate
molecule in a blueberry. A caribou eats the fruit, and within a few
hours, you pass out of the animal’s body. You are soon swallowed
by a dung beetle, which gets eaten by a hungry shrew. You are combined
into the body tissues of the shrew, which is then eaten by an owl.
You are released back into the atmosphere when the owl exhales carbon dioxide,
dissolve in a drop of rainwater, and flow through a river into the ocean.
This could just be part of the never-ending cycle of a
carbon atom through the biosphere. Carbon atoms in your body may
once have been part of a rock on the ocean floor, the tail of a dinosaur,
or even part of a historical figure such as Julius Caesar!
How does matter move through
the biosphere?
REVIEW & DO
NOW
Answer the following questions: |
How does matter move through the biosphere differently
than energy?
What are biogeochemical cycles? |
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What are four ways that biogeochemical cycles can be
classified?
How are they alike and how are they different? |
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The Water Cycle
How does water cycle through the biosphere?
Every time you see rain or snow, or watch a river flow,
you are witnessing part of the water cycle.
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Water continuously moves between the oceans, the atmosphere,
and land—sometimes outside living organisms and sometimes inside them.
As Figure 3–15 shows, water molecules typically enter the
atmosphere as water vapor, a gas, when they evaporate from the ocean or
other bodies of water. Water can also enter the atmosphere by evaporating
from the leaves of plants in the process of transpiration.
Water vapor may be transported by winds over great distances.
If the air carrying it cools, water vapor condenses into tiny droplets
that form clouds. When the droplets become large enough, they fall
to Earth’s surface as precipitation in the form of rain, snow, sleet, or
hail. On land, some precipitation flows along the surface in what
scientists call runoff, until it enters a river or stream that carries
it to an ocean or lake. Precipitation can also be absorbed into the
soil and is then called groundwater. Groundwater can enter plants
through their roots, or flow into rivers, streams, lakes, or oceans.
Some groundwater penetrates deeply enough into the ground to become part
of underground reservoirs. Water that re-enters the atmosphere through
transpiration or evaporation begins the cycle anew.
How does water cycle through
the biosphere?
REVIEW & DO
NOW
Answer the following questions: |
What are two ways that water vapor enters the atmosphere? |
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What are three types of precipitation? |
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Nutrient Cycles
What is the importance of the main nutrient cycles?
The chemical substances that an organism needs to sustain
life are called nutrients.
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Every organism needs nutrients to build tissues and carry
out life functions. Like water, nutrients pass through organisms
and the environment through biogeochemical cycles. The three pathways,
or cycles that move carbon, nitrogen, and phosphorus through the biosphere
are especially critical for life.
Another element, oxygen, participates in parts of the carbon,
nitrogen, and phosphorus cycles by combining with these elements and cycling
with them through parts of their journeys. Oxygen gas in the atmosphere
is released by one of the most important of all biological activities:
photosynthesis. Oxygen is used in respiration by all multicellular
forms of life, and many single-celled organisms as well.
The Carbon Cycle
Carbon is a major component of all organic compounds,
including carbohydrates, lipids, proteins, and nucleic acids. In
fact, carbon is such a key ingredient of living tissue and ecosystems that
life on Earth is often described as “carbon-based life.” Carbon in the
form of calcium carbonate (CaCO3) is an important component of many different
kinds of animal skeletons and is also found in several kinds of rocks.
Carbon and oxygen form carbon dioxide gas (CO2), which is an important
component of the atmosphere and is dissolved in oceans.
Some carbon-containing compounds that were once part of
ancient forests have been buried and transformed by geological processes
into coal. The bodies of marine organisms containing carbon have
been transformed into oil or natural gas. Coal, oil, and natural
gas are often referred to as fossil fuels because they are essentially
“fossilized” carbon. Major reservoirs of carbon in the biosphere
include the atmosphere, oceans, rocks, fossil fuels, and forests.
Figure 3–17 shows how carbon moves through the biosphere.
Carbon dioxide is continuously exchanged between the atmosphere and oceans
through chemical and physical processes. Plants take in carbon dioxide
during photosynthesis and use the carbon to build carbohydrates.
Carbohydrates then pass through food webs to consumers. Many animals—both
on land and in the sea—combine carbon with calcium and oxygen as the animals
build skeletons of calcium carbonate. Organisms release carbon in
the form of carbon dioxide gas by respiration. Also, when organisms
die, decomposers break down the bodies, releasing carbon to the environment.
Geologic forces can turn accumulated carbon into carbon-containing rocks
or fossil fuels. Carbon dioxide is released into the atmosphere by
volcanic activity or by human activities, such as the burning of fossil
fuels and the clearing and burning of forests.
Scientists know a great deal about the biological, geological,
chemical, and human processes that are involved in the carbon cycle, but
important questions remain. How much carbon moves through each pathway?
How do ecosystems respond to changes in atmospheric carbon dioxide concentration?
How much carbon dioxide can the ocean absorb? Later in this unit, you will
learn why answers to these questions are so important.
The Nitrogen Cycle
All organisms require nitrogen to make amino acids, which
are used to build nucleic acids, which combine to form DNA, RNA, and proteins.
Many different forms of nitrogen occur naturally in the biosphere.
Nitrogen gas (N2) makes up 78 percent of Earth’s atmosphere. Nitrogen-containing
substances such as ammonia (NH3), nitrate ions (NO3?), and nitrite ions
(NO2?) are found in soil, in the wastes produced by many organisms, and
in dead and decaying organic matter. Dissolved nitrogen also exists
in several forms in the ocean and other large water bodies. Figure
3–18 shows how different forms of nitrogen cycle through the biosphere.
Although nitrogen gas is the most abundant form of nitrogen
on Earth, only certain types of bacteria can use this form directly.
These bacteria live in the soil and on the roots of certain plants, such
as peanuts and peas, called legumes. The bacteria convert nitrogen
gas into ammonia, in a process known as nitrogen fixation.
Other soil bacteria convert that fixed nitrogen into nitrates and nitrites.
Once these forms of nitrogen are available, primary producers can use them
to make proteins and nucleic acids. Consumers eat the producers and
reuse nitrogen to make their own nitrogen-containing compounds. Decomposers
release nitrogen from waste and dead organisms as ammonia, nitrates, and
nitrites that producers may take up again. Other soil bacteria obtain
energy by converting nitrates into nitrogen gas, which is released into
the atmosphere in a process called denitrification. A relatively
small amount of nitrogen gas is converted to usable forms by lightning
in a process called atmospheric nitrogen fixation. Humans add nitrogen
to the biosphere through the manufacture and use of fertilizers.
Excess fertilizer is often carried into surface water or groundwater by
precipitation.
The Phosphorus Cycle
Phosphorus is essential to living organisms because it
forms a part of vital molecules such as DNA and RNA. Although phosphorus
is of great biological importance, it is not abundant in the bio sphere.
Unlike carbon, oxygen, and nitrogen, phosphorus does not enter the atmosphere
in significant amounts. Instead, phosphorus in the form of inorganic
phosphate remains mostly on land, in the form of phosphate rock and soil
minerals, and in the ocean, as dissolved phosphate and phosphate sediments,
as seen in Figure 3–19.
As rocks and sediments gradually wear down, phosphate
is released. Some phosphate stays on land and cycles between organisms
and soil. Plants bind phosphate into organic compounds when they absorb
it from soil or water. Organic phosphate moves through the food web,
from producers to consumers, and to the rest of the ecosystem. Other
phosphate washes into rivers and streams, where it dissolves. This
phosphate may eventually makes its way to the ocean, where marine organisms
process and incorporate it into biological compounds.
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REVIEW & DO
NOW
Answer the following questions: |
What are nutrients?
What are the three pathways that move nutrients through
the biosphere?
What role does oxygen play in the nutrient cycles?
Why is carbon important to life?
What are two important carbon compounds, and why are they
important?
Why is nitrogen important to life? |
|
How much of Earth's atmosphere is composed of nitrogen
gas?
What are three major nitrogen compounds, and where are
they found?
What is nitrogen fixation and why is it important?
What is denitrification?
Why is phosphorus important to living things?
Does phosphorus enter the atmosphere? |
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Nutrient Limitation
How does nutrient availability relate to the primary
productivity of an ecosystem?
Ecologists are often interested in an ecosystem’s primary
productivity—the rate at which primary producers create organic material.
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If ample sunlight and water are available, the primary
productivity of an ecosystem may be limited by the availability of nutrients.
If even a single essential nutrient is in short supply, primary
productivity will be limited. The nutrient whose supply limits productivity
is called the limiting nutrient.
Nutrient Limitation in Soil
In all but the richest soil, the growth of crop plants
is typically limited by one or more nutrients that must be taken up by
plants through their roots. That’s why farmers use fertilizers! Most
fertilizers contain large amounts of nitrogen, phosphorus, and potassium,
which help plants grow better in poor soil. Micronutrients such as
calcium, magnesium, sulfur, iron, and manganese are necessary in relatively
small amounts, and these elements are sometimes included in specialty fertilizers.
(Carbon is not included in chemical fertilizers because plants acquire
carbon dioxide from the atmosphere during photosynthesis.) All nutrient
cycles work together like the gears in Figure 3–20. If any nutrient
is in short supply—if any wheel “sticks”—the whole system slows down or
stops altogether.
Nutrient Limitation in Aquatic
Ecosystems
The open oceans of the world are nutrient-poor compared
to many land areas. Seawater typically contains only 0.00005 percent
nitrogen, or 1/10,000 of the amount often found in soil. In the ocean
and other saltwater environments, nitrogen is often the limiting nutrient.
In streams, lakes, and freshwater environments, phosphorus is typically
the limiting nutrient.
Sometimes, such as after heavy rains, an aquatic ecosystem
receives a large input of a limiting nutrient—for example, runoff from
heavily fertilized fields. When this happens, the result can be an
algal bloom.
Algal Bloom
A dramatic increase in the amount of algae and other
primary producers that results from a large input of limiting nutrients,
discoloring the water, producing foul odors and bad taste, or sometimes
producing toxins or using up the free oxygen in the water, leading to fish
die-off.
Why can runoff from fertilized fields produce algal blooms?
More nutrients are available, so producers can grow and reproduce more
quickly. If there are not enough consumers to eat the algae, an algal
bloom can occur, in which case algae can cover the water’s surface and
disrupt the functioning of an ecosystem.
How does nutrient availability
relate to the primary productivity of an ecosystem?
REVIEW & DO
NOW
Answer the following questions: |
What is a limiting nutrient?
What are five important micronutrients?
What is the major limiting nutrient in seawater? |
|
What is the major limiting nutrient in freshwater?
What is an algal bloom?
In what ways can an algal bloom be harmful? |
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