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Advanced Biology: Bahe & Deken
Biogeochemical
Cycles
Chapter 5
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Advanced Biology: Bahe & Deken
5.1 Nutrients in an Ecosystem
vapor in the atmosphere, 84%, comes from
the oceans.
An ecosystem needs more than energy to
function. It also needs matter that is used by
organisms in the ecosystems for life
processes such as growth. Most ecosystems
need greater than 20 elements for life
processes. They are so important to living
things that they are called nutrients.
We already know that energy is lost along a
food chain. Little or no energy is left at the
end of the food chain to be recycled back to
producers. However, this is not the case for
nutrients. Nutrients flowing through the
food chain are returned to the producers.
Producers get their nutrients from the soil,
water and air. Herbivores get most of their
nutrients when they eat producers.
Carnivores also get the same nutrients when
they eat the herbivores. Then decomposers
break down animal wastes and dead
organisms. This releases the nutrients back
into the soil, water and air so producers can
use them again. In this way, nutrients are
recycled through an ecosystem. The path
each nutrient follows is called a nutrient
cycle, or biogeochemical cycle, and each
cycle is subject to disruption by human
activity. The important cycles, all driven
directly or indirectly by incoming solar
energy and gravity, include water, carbon,
nitrogen, and phosphorus cycles.
The amount of water vapor air can hold
depends on its temperature, with warm air
holding more water vapor than cold.
Relative humidity is the amount of water
vapor in a certain mass of air, expressed as a
percentage of the maximum amount it could
hold at that temperature. For example, a
relative humidity of 60% at 27°C (80°F)
means that each kilogram of air contains
60% of the maximum amount of water
vapor it could hold at that temperature.
In the cool upper atmosphere this vapor
condenses, forming clouds or fog. In time,
enough water collects in the clouds to cause
precipitation. In order for precipitation to
occur, the air must contain condensation
nuclei, tiny particles on which droplets of
water vapor can collect such as volcanic ash,
soil dust, smoke, sea salts and particulate
matter from factories, coal-burning power
plants and automobiles. When rain, sleet,
and snow occur, some of the water falling
on the ground runs along the surface of the
ground to a stream, pond or ocean where it
can resume the cycle. This water is called
surface runoff. Some of the water also
soaks into the ground by percolation
5.2 The Water Cycle
The global water cycle is driven by heat
energy from the sun. Water is moved
continuously between land, oceans and the
atmosphere by several processes. Water
enters the atmosphere through transpiration
from vegetation. Transpiration is the loss of
water through pores in the leaves of plants.
Water also enters the atmosphere by
evaporation from oceans, streams, and lakes
as well as from soil. The majority of water
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Advanced Biology: Bahe & Deken
through the soil and permeable rock
formations to groundwater storage areas
called aquifers. Groundwater can also run
along bedrock back to lakes and rivers that
eventually carry it back to the oceans to be
evaporated and cycle again.
Some of the water in the soil moves up to
the roots of plants by capillarity. The roots
absorb the water and distribute the water to
the rest of the plant. This is how most plants
get the water they need. Animals obtain
water by eating plants or animals and by
drinking it directly from a body of water.
When plants and animals die, they
decompose and the water in their tissues is
released into the environment.
Besides replenishing streams and lakes,
surface runoff can also cause soil erosion.
Because the water dissolves many nutrient
compounds, it is a major medium for
transporting nutrients within and between
ecosystems.
During the water cycle, many natural
processes act to purify water. When water
evaporates it leaves behind impurities.
Water flowing above ground through
streams and lakes and below ground in
aquifers is naturally filtered and purified by
chemical and biological processes. The
water cycle is a cycle of natural renewal of
water quality.
Humans affect the water cycle in a number
of ways. One of the main sources of
atmospheric water is transpiration from the
dense vegetation making up tropical rain
forests. The destruction of these forests for
agriculture, timber and mining, which is
occurring rapidly today, will change the
amount of water vapor in the air. This in
turn will likely alter local, and possibly,
global weather patterns. In addition such
deforestation increases runoff, reduces
infiltration that recharges groundwater
supplies, increases flooding, and accelerates
soil erosion.
Another change in the water cycle results
from pumping large amounts of
groundwater to the surface to use for
irrigation. This practice can increase the
rate of evaporation over land, and unless this
loss is balanced by increased rainfall over
land, groundwater supplies can be depleted.
The Midwestern United States, the
Southwestern American desert, parts of
California and areas bordering the Gulf of
Mexico currently face this problem.
Withdrawing large quantities of fresh water
from underground sources can also lead to
intrusion of ocean salt water into the
underground water supplies.
Humans are also modifying water quality
through the addition of nutrients, especially
phosphates and nitrates from fertilizers, and
other pollutants.
5.3 The Carbon Cycle
Carbon is the basic building block of all
living things. Carbohydrates, fats, proteins
and nucleic acids all require carbon.
Carbon dioxide (CO2) in the atmosphere
(only 0.03% carbon dioxide by volume)
forms the basis of the carbon cycle. Carbon
dioxide is taken in by terrestrial and aquatic
plants, algae, and cyanobacteria to make
food through photosynthesis. These
producers are the source of all carbon for all
biotic components of the ecosystem.
Herbivores eat some of the plants and
carnivores eat some of the herbivores. Now
the carbon is in animals. Both plants and
animals respire. Their respiration returns
carbon dioxide to the atmosphere and water.
Detritus feeders and decomposers
breakdown dead plants, leaf litter, dead
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Advanced Biology: Bahe & Deken
Name:
animals, and animal wastes. This also
returns carbon dioxide to the atmosphere
where it can be taken in again by producers
for photosynthesis.
The resulting global warming could disrupt
global food production and wildlife habitats
and raise sea levels in various parts of the
world.
Some organic matter does not decompose
readily. Instead, it builds up in the earth’s
crust. Over millions of years, these buried
deposits of dead plants and bacteria are
compressed between layers of sediment,
where they form carbon-containing fossil
fuels such as coal and oil. This stored
carbon is not released back into the carbon
cycle until the fossil fuels are burned. Over
the past few hundred years, people have
extracted and burned fossil fuels that took
millions of years to form.
Carbon dioxide is a key component of
nature’s thermostat. If the carbon cycle
removes too much CO2 from the
atmosphere, the atmosphere will cool. If it
generates too much, the atmosphere will get
warmer. Slight changes in the carbon cycle
can affect climate and ultimately the types
of life that can exist on various parts of the
planet.
Questions (answer on a separate sheet of
paper)
Since 1800 and especially since 1950
humans have been intervening in the earth’s
carbon cycle in two ways that add carbon
dioxide to the atmosphere:
Clearing trees and other plants that
absorb CO2 through photosynthesis
Adding large amounts of CO2 by
burning fossil fuels and wood
Computer models of the earth’s climate
systems suggest that increased
concentrations of atmospheric CO2 and
other gases we are adding to the atmosphere
could enhance the planet’s natural
greenhouse effect that helps warm the
lower atmosphere and the earth’s surface.
Trace a carbon atom through the carbon
cycle.
1. In what chemical form is carbon in the
air?
2. How does a carbon atom enter the food
chain?
3. In what chemical form might the
carbon atom be obtained by a consumer?
4. What chemical process puts carbon
atoms back into the atmosphere?
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Advanced Biology: Bahe & Deken
5.4 The Nitrogen Cycle
Plants and animals need nitrogen to make
amino acids, proteins and DNA. The
atmosphere contains a huge reservoir of
nitrogen. Almost 80% of the atmosphere is
N2. However, because of the strong triple
bonds holding N2 together neither plants nor
animals can use atmospheric nitrogen
directly. Plants must absorb nitrogen in the
form of nitrates (NO3-) or ammonium ions
(NH4+) with their roots. Lightning forms
some nitrate ions by causing oxygen and
nitrogen gas in the atmosphere to join.
These nitrate ions reach the soil in
precipitation and dust. Rhizobium bacteria
in the soil can also convert molecular
nitrogen into nitrates. These bacteria live on
the roots of legumes, such as soybeans,
alfalfa, peas, clover, and beans. In addition,
many free-living soil bacteria and aquatic
cyanobacteria can form nitrates. The
changing of molecular nitrogen (N2) into
nitrates (NO3-) is called nitrogen fixation.
Plants use the nitrates that they absorb to
make plant DNA and proteins. In turn,
animals get the nitrogen they need to make
proteins by eating plants or other animals.
When plants and animals die and
decompose, bacteria change their nitrogen
containing organic molecules into ammonia
(NH3), which quickly becomes ammonium
ions (NH4+). The nitrogen in urine and fecal
matter of animals is also changed to
ammonia by bacteria, keeping the nitrogen
available for use by plants. The pungent
odor of outhouses, chicken pens, hog yards,
cat litter boxes and wet baby diapers is
ample evidence of this fact. Ammonia, in
turn, is converted to nitrites and then to
nitrates by bacteria. This completes the
main part of the cycle. Denitrifying bacteria
(mostly anaerobic bacteria in waterlogged
soil or in the bottom sediments of lakes,
oceans, swamps, and bogs) convert some
nitrites and nitrates to atmospheric nitrogen
(N2) to complete the total cycle. The
nitrogen cycle need not and often does not
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involve this last denitrification step.
Humans, however, have altered the nitrogen
cycle balance in many ecosystems. Sewagetreatment facilities usually empty large
amounts of dissolved inorganic nitrogen
compounds into rivers and streams. Farmers
routinely apply large amounts of inorganic
nitrogen fertilizers, mainly ammonium
compounds and nitrates to their fields.
Lawns and golf courses receive sizable
doses of fertilizers, and denitrifying bacteria
convert some into atmospheric nitrogen.
But chemical fertilizers usually exceed the
soil’s natural recycling capacity. The excess
nitrogen compounds often enter streams,
lakes, soil, and groundwater.
In lakes and streams, these nitrogen
compounds continue to fertilize, causing
heavy growth of algae (nitrogen is normally
a limiting nutrient). The subsequent
breakdown of dead algae and other aquatic
plants can deplete the water of dissolved
oxygen and disrupt aquatic ecosystems by
killing some types of fish and other oxygenusing organisms. Ground water pollution by
nitrogen fertilizers is a serious problem in
agricultural areas. Nitrates in drinking water
are converted to nitrites, which can be toxic,
in the human digestive tract.
Humans also add large amounts of nitric
oxide (NO) into the atmosphere when we
burn fuels (N2 + O2 --> 2NO). In the
atmosphere, this nitric oxide combines with
oxygen to form nitrogen dioxide gas (NO2),
which can react with water vapor to form
nitric acid (HNO3). Droplets of HNO3,
dissolved in rain or snow are components of
acid deposition, commonly called acid
rain. Nitric acid, along with other air
pollution, can damage and weaken trees,
corrode metals and damage marble, stone
and other building materials.
Some bacteria in the soil will also convert
fertilizer and livestock waste to nitrous
oxide (N2O) that enters the atmosphere. In
the atmosphere, N2O reaches the
stratosphere where it can help warm the
atmosphere by enhancing the natural
greenhouse effect and contributing to the
depletion of the earth’s ozone shield, which
filters out harmful ultraviolet radiation from
the sun.
5.5 The Phosphorus Cycle
Plants and animals need phosphorus for the
production of nucleic acids (DNA, RNA,
and ATP) and phospholipids. Many animals
also need phosphorus for teeth, bones,
and/or shells. The atmosphere does not
contain phosphorus and thus all the
phosphorus available for plants and animals
comes from rocks and soil. Because
phosphorus does not enter the atmosphere it
is called a sedimentary cycle.
Rocks and sediment at the ocean floor
contain the largest reservoir of phosphorus.
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Terrestrial rocks upon geological uplift and
weathering exposes phosphate ions (PO43and HPO42-) that are absorbed into the soil.
Plants take up phosphorus from the soil and
then consumers incorporate phosphorus by
eating plants and other animals. As the
plants and animals die and decompose
phosphorus is again put back into the soil.
The weathering of rock and soil runoff also
puts phosphorus into aquatic ecosystems.
As the aquatic biota die and decompose
phosphorus gets trapped in ocean floor
sediments to return phosphorus back to rock.
Like nitrogen humans have altered the
phosphorus cycle. Phosphate mining for
fertilizers and detergent production put
excess phosphorus into the system.
Phosphorus, like nitrogen, is a limiting
nutrient for plant and algae growth.
Increased phosphorus and nitrogen into
aquatic ecosystems can results in
eutrophication (over-enrichment of water).
Eutrophication can lead to algal blooms,
apparent by a green scum on top of the
water. When the algae die off, decomposers
use up all the available oxygen during
cellular respiration. Because there is no
oxygen available fish and other aquatic life
begin to all die off. An abundance of
phosphate in the water can also be linked to
red tide that can produce deadly toxins.
5.6 Disruption of Biogeochemical Cycles
Obtaining an accurate picture of chemical
cycling requires long-term studies. One
experiment that continues today was begun
in 1963 by F. H. Borman from Yale and
Gene Likens from Cornell at the HubbardBrook Experimental Forest in the White
Mountains of New Hampshire.
The site has a number of watersheds; small
valleys that are each drained by a stream that
is a tributary of Hubbard Brook. Thick rock
is close to the soil surface and water does
not seep into the rock from the soil. As a
result, water drains out of each watershed
only via its own stream.
The research team began by carrying out a
controlled experiment to compare the loss of
water and nutrients from an uncut forest
ecosystem (the control system) with one
that was stripped of its trees (the
experimental system).
To do this, V-shaped concrete catchment
dams were built across the creeks at the
bottom of six valleys. The dams were
anchored on impenetrable bedrock so all
surface water that leaves each watershed had
to flow across the dams, where scientists
could measure its volume and dissolved
nutrient content.
When monitoring began, in the control
system about 60% of the water that fell as
rain and snow exited through the streams,
and the remaining 40% was lost by
transpiration from plants and evaporation
from the soil. Preliminary data also
indicated that the flow of nutrients into and
out of the watershed was nearly balanced,
and was relatively small compared with the
quantity of nutrients being recycled within
the forest itself. Thus, an undisturbed
mature forest ecosystem is very efficient at
retaining chemical nutrients.
In the next experiment the scientists
disturbed the system and observed any
changes that occurred. One winter,
investigators cut down all trees and shrubs
in one valley, left them where they fell and
sprayed them with herbicide for the next
three years to prevent regrowth. They then
compared the inflow and outflow of water
and nutrients in this modified experimental
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valley with those in the control valley for
three years.
With no plants to absorb and transpire water
from the soil, water runoff in the deforested
valley increased 30-40%. As this excess
water ran rapidly over the surface of the
ground, it eroded soil and carried nutrients
out of the ecosystem. Overall, the loss of
minerals from the cut forest was six to eight
times that in a nearby-undisturbed forest.
Chemical analysis of the water flowing
through the dams in the experimental valley
showed a 60-fold rise in the concentration of
nitrate (NO3-) ions. So much nitrogen was
lost in the experimental valley that the water
flowing out of the valley was unsafe to drink
and the over-fertilized stream below this
valley became covered with populations of
cyanobacteria and algae. After a few years,
however, vegetation slowly grew back and
nitrate levels began to return to normal.
Addition of large amounts of chemical
nutrients to an aquatic ecosystem can lead to
pollution where too much of a good thing is
bad. One can trace the chain of events that
occur when a freshwater lake, for instance,
receives an overload of nutrients from the
surrounding terrestrial ecosystems. A
natural lake typically has a moderate growth
of algae and plants and often a rich diversity
of fish and invertebrates. When a lake
receives an excess of mineral nutrients, its
entire trophic structure can change very
quickly. A lake today may receive runoff of
inorganic matter from fertilizers from
agriculture, from sewage, from factory and
animal wastes, and from pastures and
stockyards. The water becomes polluted
with these materials - actually, overfertilized - and the lake’s photosynthetic
organisms, especially algae, multiply
rapidly. The lake will undergo accelerated
eutrophication. More nutrients stimulate
the growth of algae, cyanobacteria, water
hyacinths and duckweed. Heavy plant
growth increases oxygen production during
the day but greatly reduce oxygen levels at
night when they respire. As the plants and
animals die and accumulate at the bottom of
the lake, the bacteria decomposing them can
use up much of the oxygen dissolved in deep
waters and near the edges. This oxygen
depletion can kill fish and other aerobic
aquatic animals. If excess nutrients continue
to flow into a lake, anaerobic bacteria take
over and produce gaseous decomposition
products like smelly, highly toxic hydrogen
sulfide and flammable methane. When this
happens, the lake may lose most of its
species diversity, with only the organisms
tolerant of low oxygen conditions surviving.
Human-induced eutrophication, for
example, wiped out commercially important
fish in Lake Erie in the 1950s and 1960s.
Since then, tighter regulations on the
dumping of wastes into the lake have
enabled some fish populations to rebound,
but many of the native species of fish and
invertebrates have not recovered. Today,
accelerated eutrophication is the most
common problem affecting lakes throughout
the world.
Ways to prevent or reduce this culturally
induced eutrophication include using
advanced waste treatment to remove nitrates
and phosphates, banning and limiting
phosphates in household detergents and
other cleaning agents and using soil
conservation land-use control to reduce
nutrient runoff.
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Advanced Biology: Bahe & Deken
Name:
Questions
Each of the statements below describes a situation in a healthy ecosystem. Next to each sentence
briefly describe the corresponding situation in an unhealthy system affected by human
disturbance.
1. The amount of CO2 released into the
atmosphere by respiration equals the CO2
used in photosynthesis.
2. Over time, nutrients from the
surrounding land gradually accumulate in a
lake, and the lake becomes more productive,
a process called eutrophication.
3. Much of the water that falls on tropical
forests is returned to the atmosphere by
transpiration.
4. Organic fertilizers release nitrogen and
phosphorus gradually, so they are absorbed
by crop plants and do not run off to pollute
rivers and lakes.
Multiple Choice
5. Most plants get nitrogen from
a. nitrates in the soil.
b. N2 gas in the air.
c. proteins.
d. ammonium in the soil.
e. rainfall.
6. Bacteria are especially important in
a. the water cycle.
b. the nitrogen cycle.
c. ecological succession.
d. the phosphorus cycle.
e. the calcium cycle.
7. The biggest difference between the flow
of energy and the flow of chemical nutrients
in an ecosystem is that
a. the amount of energy is much greater
than the amount of nutrients.
b. energy is recycled, but nutrients are not.
c. organisms always need nutrients, but
they don’t always need energy.
d. nutrients are recycled, but energy is not.
e. organisms always need energy, but they
don’t always need nutrients.
8. An ecosystem is unlikely to be limited by
the supply of ______ because it is obtained
from the air.
a. water
b. carbon
c. phosphorus
d. nitrogen
e. calcium
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