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Carbon and Nitrogen Cycles
Interdependence within Environmental Systems
Carbon the Element
The element carbon is one of the most essential elements on our planet. All living organisms
contain carbon, making it a critical component of all life on planet earth. In fact, the element carbon
makes up close to half of the “dry weight” of all living things!
Carbon is also recycled through the
decomposition of organic materials in soil.
Carbon sources may also be stored under
ground as coal or petroleum, two forms of fossil
fuels. When carbon sources are released to the
atmosphere, this is known as carbon
emissions. The growing amount of the number
of carbon atoms emitted as a result of chemical
combustion has had an effect on global climate
change, which is the belief that earth's climate
is changing at a rapid pace as a result of an
imbalance in human contributions to the
atmosphere.
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Carbon and Nitrogen Cycles
Interdependence within Environmental Systems
Carbon Cycle
The carbon cycle is one critical cycle that links the atmosphere, the hydrosphere, and the
lithosphere. The element carbon is a form of matter that is recycled through these “spheres”
fairly evenly, and it is essential that this element remain balanced in the biosphere.
Plants have a high demand for the carbon dioxide availability in the atmosphere for the process
of photosynthesis. The rate of removal of carbon dioxide from the atmosphere is roughly
balanced by the cellular respiration that occurs in both plant and animal cells, which releases
carbon dioxide back into the atmosphere.
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Carbon and Nitrogen Cycles
Interdependence within Environmental Systems
Carbon Cycle
The element carbon cycles through systems on planet earth in these processes:
•  From the atmosphere to plants (photosynthesis)
•  From plants to animals (food web)
•  From plants and animals to the ground (decomposition and waste)
•  From living things to the atmosphere (respiration and metabolism)
•  From fossil fuels to the atmosphere when fuels are burned (combustion)
•  From the atmosphere to the oceans, and vice versa (diffusion)
•  From volcanic eruptions and other geothermal processes that release gases into the
atmosphere
Carbon may be found in these non-living forms:
•  In the atmosphere as carbon dioxide (CO2)
•  Dissolved in water as hydrogen carbonate ions (HCO3−)
•  In limestone, or as calcium carbonate in rocks (CaCO3)
•  In sedimentary deposits (coal, oil, etc.) made from once-living organisms
•  As dead organic matter, such as found in peat moss in the soil
Carbon moves from the atmosphere to the
terrestrial biosphere (hydrosphere and lithosphere)
by these means:
•  Plants use the carbon found in carbon dioxide (CO2)
and convert it to sugars (CH2O)n through the process
of photosynthesis.
•  Animals eat the plants, and the carbon moves
through the food chain.
•  When living organisms die, their bodies return to the
soil through the process of decay, releasing carbon
(C) and other carbon byproducts directly into the
ground.
Carbon returns to the atmosphere by these means:
•  As carbon dioxide (CO2) through respiration or
burning (of organic matter or fossil fuels).
•  From the process of decay as carbon dioxide (CO2)
or methane (CH4).
•  By volcanic eruptions or other emissions from the
interior of the planet.
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Carbon and Nitrogen Cycles
Interdependence within Environmental Systems
Carbon and the Atmosphere
Carbon makes up a very small portion of earth's
atmosphere, yet this element can have dramatic
effects on Earth's climate. Carbon dioxide (CO2) is
considered a greenhouse gas as it has the ability to
absorb infrared radiation. At pre-industrial levels
(around 280 ppm), this compound's ability to hold
heat was critical to supporting life on earth, as this
effect helped trap the sun's energy in the form of heat.
However, at elevated levels (around 400 ppm as of
2012), carbon dioxide's ability to absorb heat could
contribute to elevated temperatures on earth's
surface, which may potentially cause global climate
change.
Sources of CO2 emissions may be natural,
such as volcanoes, forest fires, the decay of
organic matter, or animal respiration. At normal CO2
atmospheric concentrations, the oceans are able
to absorb and remove carbon from the atmosphere.
Sources that remove carbon are known as carbon
sinks. Plants are another carbon sink. Since the
industrial revolution, the burning of fossil fuels has
added CO2 to the atmosphere at a rate faster than the sinks can keep up with. Deforestation,
caused by industry, forest fires or pine beetle kill* has also removed a large part of the Earth's
carbon sink system, making the problem of above normal CO2 levels in the atmosphere
worse.
*The mountain pine beetle carries a fungus on its body. When it bores into trees, the fungus
will interfere with the water transport system of the trees, thereby killing the trees. This has
become an increasing problem in the northern Rocky Mountain region over the past decade.
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Carbon and Nitrogen Cycles
Interdependence within Environmental Systems
Carbon and the Lithosphere
Carbon dioxide (CO2) taken from the atmosphere is used
for photosynthesis in the leaves during the day, creating
sugars (CH2O)n. At night, the leaves release carbon
dioxide (CO2) back into the atmosphere as a product of
cellular respiration. When leaves or fruit fall from a plant, or
when the plant dies, carbon (C) is released into the ground
through decomposition.
Carbon is ingested by animals when they eat plants. The
carbon could be in the form of sugars and carbohydrates
((CH2O)n), nucleotides (the building blocks of DNA and
RNA), amino acids (the building blocks of proteins), or fatty
acids. As animals eat sugars and other carbon sources,
they produce carbon dioxide (CO2) as a waste product of
cellular respiration and release this back into the
atmosphere.
Animals also produce (feces) which releases carbon (C) back into the ground. When animals
breathe, they release carbon dioxide (CO2) back into the atmosphere. Livestock (and other
animals) release methane gas (CH4) to the atmosphere. When animals die, solid carbon (C) is
released into the ground through decomposition, and methane gas (CH4) is released to the
atmosphere. Microorganisms and fungi also release carbon dioxide (CO2) as they decompose
organic matter.
Fossil fuels are often locked underground for millions of years. Fossil fuels are found in
sedimentary rocks. These types of rocks may be found between the surface of earth and the upper
mantle, both on land and at the bottom of the ocean as marine sediments. The types of fossil fuels
found in sedimentary rocks include oil, coal (C), natural gas or methane (CH4), and peat. Burning
fossil fuels (coal, oil, natural gas) releases carbon dioxide (CO2) that has been stored for millions of
years back into the atmosphere .
Volcanoes and other geothermal systems release carbon stored in earth's crust and mantle to the
atmosphere in the form of carbon dioxide (CO2 ).
Forests can store up to 86% of the total carbon found on the surface of earth, and they store up to
73% of the carbon found in the soil of earth. When forests burn, whether caused by man or nature,
massive amounts of carbon dioxide (CO2) are released to the atmosphere.
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Carbon and Nitrogen Cycles
Interdependence within Environmental Systems
Carbon and the Hydrosphere
The upper portion of the ocean is known as the “mixed layer” of the oceans. It ranges from the
surface to 75 m water depth (below the surface). The mixed layer is where most life exists in the
oceans and includes marine plants that use carbon dioxide (CO2) for photosynthesis. The mixed
layer of the ocean “soaks up” a lot of the carbon dioxide (CO2) from the atmosphere through the
process of diffusion. Phytoplankton also live in this layer (microscopic plants and algae) and
absorb most of this CO2. Marine organisms eat the marine plants, keeping the food chain going.
Sediments from erosion and weathering flow from the land (lithosphere) to the rivers to the oceans.
Organic materials (C) from dead or decaying matter are carried to the oceans via rivers. Many
minerals and salts make their way to the oceans in this manner, but calcium carbonate (CaCO3) is
one of the more important compounds to reach the ocean. When these sediments reach the
oceans, they dissolve into carbonate ions (CO32-). Many sea creatures use the carbonate ions as
the main building blocks for shell and exoskeleton production. In fact, many marine organisms'
shells are made from calcium carbonate (CaCO3).
When carbon dioxide (CO2) diffuses into seawater, it creates a type of acid known as carbonic acid
(H2CO3). This acid releases hydrogen ions into the water, which increases the acidity of the water.
The free hydrogen ions will combine with the carbonate ions (CO32-), creating bicarbonate (HCO3-).
This reaction makes the required carbonate ions less available to marine organisms and greatly
affects their ability to produce protective shells. As CO2 levels increase in the atmosphere, the
carbonic acid (H2CO3) concentration in the oceans also increases.
The bottom portion of the ocean is the
“deep ocean,” and it ranges from 75 m
to 3700 m. The deep ocean is where
much of the decaying matter falls. As
marine organisms (such as an Orca
whale) die, they float to the bottom of
the ocean creating sediments that
eventually turn into limestone (CaCO3).
The majority of this carbon comes from
the bones and the shells of the marine
organisms.
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Carbon and Nitrogen Cycles
Interdependence within Environmental Systems
Carbon Cycle Disruptions
A disruption is an act that delays or interrupts the continuity of a cycle. A disruption in a cycle of
matter leads to an imbalance of elements within an ecosystem, or in one of the “spheres” which
could lead to an imbalance in the biosphere.
The carbon cycle is disrupted when the element is no longer in balance between the lithosphere,
the hydrosphere, and the atmosphere. The amount of carbon containing compounds in the
atmosphere has been steadily increasing in recent decades. The consequences of the increase in
carbon emissions are not yet fully known, but there have been noticeable changes in weather
patterns, species distribution, and the warming of the poles. All of these events together may be
contributing factors to global climate change. This change is reflected as a change in the balance
of the biosphere.
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Carbon and Nitrogen Cycles
Interdependence within Environmental Systems
Nitrogen the Element
We all need nitrogen. It is found in our atmosphere as N2. In fact, most of earth's atmosphere, over
78% of it, is composed of nitrogen. As another essential element, it is no surprise that nitrogen
cycles within the biosphere as well. It is required by all organisms for the basic processes of life to
make proteins, to grow, and to reproduce. It is also a major part of chlorophyll and the green color
of plants. Furthermore, the nitrogen in the soil is the most important element for plant
development, as it is required in large amounts by plants.
Even though our atmosphere consists
mainly of nitrogen, it is in a form that
biological organisms cannot use. In other
words, only certain forms of nitrogen are
available to biological organisms. The
nitrogen from the atmosphere must first be
“fixed” into a useable form. "Fixation" is
the term used to describe the conversion
of atmospheric nitrogen to nitrogen-rich
compounds that plants can use. Once the
nitrogen-rich compounds are taken up by
the plants and incorporated into the plant
tissues, animals eat the plants and can
access the nitrogen to produce proteins.
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Carbon and Nitrogen Cycles
Interdependence within Environmental Systems
Nitrogen Cycle
As nitrogen is moved through the nitrogen cycle, it is combined with hydrogen or oxygen to form a
variety of compounds needed by plants and animals. Most of these chemical reactions are
performed by bacteria in the soil, in plants cells, and in the digestive systems of animals. Besides
atmospheric nitrogen, the most common nitrogen substances involved in the nitrogen cycle include
the molecule ammonia, as well as nitrate ions, nitrite ions, and ammonium ions.
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Carbon and Nitrogen Cycles
Interdependence within Environmental Systems
Nitrogen Cycle
The element nitrogen cycles through systems on planet earth in these ways:
•  From the atmosphere to plants (nitrogen fixing legumes)
•  From the atmosphere to the ground (lightening strikes)
•  Within the soil (microorganisms cycle the nitrogen in soil)
•  From microorganisms to the atmosphere (denitrifying bacteria)
Nitrogen may be found in these non-living forms:
•  In the atmosphere as “free” nitrogen, a form of nitrogen that is not bound to another
atom (other than another nitrogen atom).
•  Biological organisms have no way to directly access this form of nitrogen, even though it makes
up 78% of the atmosphere. However, certain microorganisms can access atmospheric nitrogen
at, through chemical processes, make it accessible to other biological organisms.
Nitrogen moves from the atmosphere to the terrestrial biosphere:
•  Free nitrogen in the atmosphere can combine with oxygen under extreme heat, such as a
lightening strike, to form nitrogen monoxide.
•  Legumes, such as beans or lupines, have specialized nodules on their roots that convert free
atmospheric nitrogen to nitrates, a form of nitrogen that biological organisms can use.
Nitrogen moves within the terrestrial:
•  Certain bacteria found in soil are able to convert free
nitrogen to ammonium in a process called
ammonification, then convert ammonium to nitrites,
and finally, in a process called nitrification, to a form
that biological organisms can use called nitrates.
•  Nitrates can be used to create amino acids. Nitrates
are also produced from urea by certain bacteria found
in soil. Both amino acids and urea contain what is
called organic nitrogen.
•  When living organisms die, their bodies release
ammonium into the ground.
Legumes
Microorganisms
Nitrogen then returns to the atmosphere:
•  Denitrifying bacterial break nitrogen containing
organic compounds back down to free nitrogen.
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Carbon and Nitrogen Cycles
Interdependence within Environmental Systems
Nitrogen and the Atmosphere
Nitrogen makes up the largest component of earth's atmosphere, yet this form of nitrogen can not
be directly used by biological organisms. This form of nitrogen consists of two nitrogen atoms
bonded together. The bond between them is called a triple bond, and is energetically very difficult
to break. This bond can be broken in the atmosphere by the combination of oxygen and the high
heat caused by lightening. This atmospheric fixation is one way that nitrogen is “fixed,” or
transformed into a form that biological organisms can use. This process converts N2 into a
molecule known as nitrogen monoxide (NO). This molecule is carried by rain into the soil, where it
is broken down into nitrates (NO3-), a form of nitrogen ions that biological organisms can use. This
atmospheric nitrogen fixation accounts for roughly 5-10% of the fixed nitrogen available to
organisms.
Nitrogen is returned to the atmosphere through the process of denitrification. This process occurs
from microorganisms that live in the soil. These microorganisms do not use oxygen to respire.
Instead, they use nitrogen in their metabolic processes, converting the available nitrates (NO3-) in
the soil back to nitrogen gas (N2), which is released back into the atmosphere.
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Carbon and Nitrogen Cycles
Interdependence within Environmental Systems
Nitrogen and the Lithosphere
There are three ways that nitrogen can be directly fixed from the atmosphere to the lithosphere.
Lightening can convert “free” atmospheric nitrogen (N2) into ammonia (NH3) through the process
of atmospheric fixation. Nitrogen can also be fixed by industrial means, known as industrial fixation.
In this process, atmospheric nitrogen (N2) and hydrogen gas react at high temperatures to produce
ammonia (NH3).
There are several ways that nitrogen can be fixed through biological processes. Bacterial action in
the roots of legumes (beans, peas, lupines, soybeans, and alfalfa) can convert free nitrogen (N2)
directly to the ammonium ion (NH4+), making it available for plants to use in the process of
biological fixation. Certain trees, such as alders, can also biologically fix nitrogen directly from the
atmosphere. There are a few nitrogen-fixing bacteria that live freely in the soil. Finally,
cyanobacteria, an aquatic microorganism, can directly fix nitrogen from the atmosphere.
During the process of nitrification, two different types of nitrifying bacteria in the soil oxidize the
molecule ammonia (NH3) to nitrate ions (NO3-). One group of bacteria, known as nitrosomonas, will
oxidize the ammonia (NH3) to nitrites (NO2-). Then another group of bacteria, known as
nitrobacter, will oxidize the nitrites to nitrates (NO3-). The nitrate ions can be absorbed by plants
and used to make amino acids and proteins. The nitrogen found in proteins (amino acids) and urea
is known as organic nitrogen. In a process known as assimilation, the nitrates (NO3-) are taken in
by plants for biological function and passed through the food chain.
In the process of ammonification, decomposition of
organic nitrogen back to both the ammonium ion (NH4+)
and the molecule ammonia (NH3) by decomposers
cycles a large nitrogen source within the soil. Urea, a
form of animal waste (a component of urine) and an
organic form of nitrogen, cycles ammonia (NH3) back
to the soil.
The process of denitrification removes nitrogen from
the soil, and thereby the lithosphere. As bacteria obtain
the oxygen they need for metabolism from nitrate ions
(NO3-), they convert the nitrate ions back to nitrogen
gas (N2).
The overuse of fertilizers can increase the nitrate (NO3-) levels in soil, as well as decrease the
amounts of other types of fixed nitrogen that many soil microorganisms require.
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Carbon and Nitrogen Cycles
Interdependence within Environmental Systems
Nitrogen and the Hydrosphere
The hydrosphere will cycle nitrogen in many similar
ways as the lithosphere. Nitrogen gas (N2) in the
atmosphere can be directly converted to nitrogen
monoxide (NO) by lightning fixation (atmospheric
fixation) and can enter the hydrosphere (lakes, streams,
and oceans) through precipitation. The nitrogen
monoxide (NO) is transformed to nitrogen dioxide (NO2)
in the atmosphere. When the nitrogen dioxide reaches
the water, it reacts with the hydrogen in the water and is
transformed into nitric acid (HNO3). The nitric acid will
dissociate into ions in the water, providing a main source
of nitrate ions (NO3-), but will also release hydrogen
ions. If too much nitric acid is created, this can lower the
pH of the water, which raises the acidity of the water.
Nitrogen that has been biologically fixed in the lithosphere can be transported to the hydrosphere
through the processes of erosion and weathering. The nitrates (NO3-) in the soil can be transported
by these processes to lakes, streams, and eventually the oceans. Also, the waste products of living
organisms on land, namely ammonia (NH3), will travel into nearby waterways. The nitrogen
released from decaying land organisms, including plants and animals, will enter the hydrosphere
as well.
Within the hydrosphere, there are many organisms that also contribute to the nitrogen cycle.
Organisms such as fish, aquatic insects, and marine organisms deposit waste in the form of
ammonia (NH3) directly into the water. The ammonia (NH3) is broken down by bacteria in the water
into nitrates (NO3-) that can be used by these organisms in much the same way as land organisms.
Aquatic plants depend on this nitrogen source for growth and development.
Specific microorganisms called crenarchaeota can convert ammonia (NH3) directly to nitrates
(NO3-). They are found in both the lithosphere and the hydrosphere, but their presence in the
hydrosphere is especially important. There are more of these types of microorganisms than the
nitrifying bacterial found on land, and these microorganisms play a critical role in the balance of the
nitrogen cycle. The process of evaporation will eventually return nitrogen gas (N2) back to the
atmosphere.
If too many nitrates find their way into the hydrosphere through runoff and erosion, often due to
over-fertilization, waterways can become choked with algal blooms and increased plant growth.
This can fill in water ways in a process called eutrification.
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Carbon and Nitrogen Cycles
Interdependence within Environmental Systems
Nitrogen Cycle and Disruptions
A disruption is an act that delays or interrupts the continuity of a cycle. A disruption in a cycle of
matter leads to an imbalance of elements within an ecosystem, or in one of the “spheres”, which
could lead to an imbalance in the biosphere. Nitrogen is generally disrupted in two ways
Fertilizers are high in nitrates, which causes a boost in plant growth and biomass, leading to
waterways that are choked of oxygen (due to plant matter) and large algal blooms. Water runoff,
erosion, and irrigation and can negatively affect aquatic areas that are infiltrated with nitrates.
In large agricultural areas, new fertilizers may be used to add nitrogen to soils that have been
sterilized, or stripped, of all microorganisms through chemical means. This results in a decrease of
fixed nitrogen in the soil for plants to use. This also influences the rainfall and soil acidity through a
process called atmospheric deposition.
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