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What Is the Nitrogen Cycle and Why Is It
Key to Life?
ABSTRACT
Nitrogen, the most abundant element in our atmosphere, is crucial to life
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It is also essential to life: a key building block of DNA, which determines
our genetics, is essential to plant growth, and therefore necessary for the food we
grow
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Plants that do not have enough nitrogen
become yellowish and do not grow well and can have smaller flowers and fruits
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Understanding the Nitrogen
Cycle—how nitrogen moves from the atmosphere to earth, through soils, and
back to the atmosphere in an endless Cycle—can help us grow healthy crops
and protect our environment
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Nitrogen is in the soil under our feet, in the water we drink, and in
the air, we breathe
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It plays a key role in plant growth:
too little nitrogen and plants cannot thrive, leading to low crop yields; but
too much nitrogen can be toxic to plants
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WHY IS NITROGEN IMPORTANT?
The delicate balance of substances that is important for maintaining life is
an important area of research, and the balance of nitrogen in the
environment is no exception
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Farmers may add fertilizers containing nitrogen to their crops, to increase
crop growth
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But we need to know how much nitrogen is necessary for plant
growth, because too much can pollute waterways, hurting aquatic life
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and RNA
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Eutrophication happens when too much nitrogen enriches the water,
causing excessive growth of plants and algae
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(see Figure 1)! When the phytoplankton dies, microbes in the
water decompose them
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Organisms in the dead zone die from lack of oxygen
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​ Figure 1 - Eutrophication at a waste water outlet in the
Potomac River, Washington, D
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​ The water in this river, is bright green because it has
undergone eutrophication, due to excess nitrogen and
other nutrients polluting the water, which has led to

increased phytoplankton and algal blooms, so the water
has become cloudy and can turn different colors, such as
green, yellow, red, or brown, depending on the algal
blooms
Figure 2 shows the stages of Eutrophication

​
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Figure 2 - Stages of eutrophication
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(2) Some
nutrients become dissolved in water and leach or leak into deeper
soil layers
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(3) Some nutrients run off from over the soils and
ground directly into the water
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(5) Sunlight becomes blocked by the algae
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(7) Next, the algae bloom dies and
falls to the bottom of the water body
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(8) The decomposition process causes the water to have
reduced oxygen, leading to “dead zones
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The water body has now undergone
eutrophication
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They can re-reroute
excess nutrients away from lakes and vulnerable costal zones, use
herbicides (chemicals used to kill unwanted plant growth) or algaecides
(chemicals used to kill algae) to stop the algal blooms, and reduce the
quantities or combinations of nutrients used in agricultural fertilizers, among
other techniques [5]
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Once a lake has undergone eutrophication, it is even harder to do damage
control
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For example, water managers can introduce organisms that eat
phytoplankton, and these organisms can help reduce the amounts of
phytoplankton, by eating them!

WHAT EXACTLY IS THE NITROGEN CYCLE?
The nitrogen cycle is a repeating cycle of processes during which nitrogen
moves through both living and non-living things: the atmosphere, soil,
water, plants, animals, and bacteria
Microscopic living organisms that usually contain only one cell and are
found everywhere
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...
In the atmosphere, nitrogen exists as a gas (N2), but in the
soils, it exists as nitrogen oxide, NO, and nitrogen dioxide, NO2, and when
used as a fertilizer, can be found in other forms, such as ammonia, NH3,
which can be processed even further into a different fertilizer, ammonium
nitrate, or NH4NO3
...
In this image, microbes in the soil turn
nitrogen gas (N2) into what is called volatile ammonia (NH3), so the fixation
process is called volatilization
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is where certain forms of nitrogen (such as nitrate, or NO3) becomes
dissolved in water and leaks out of the soil, potentially polluting waterways
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Earth’s
atmosphere contains a huge pool of nitrogen gas (N2)
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To be used by plants, the N2
must be transformed through a process called nitrogen fixation
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A small amount of nitrogen can be fixed when lightning provides the energy
needed for N2 to react with oxygen, producing nitrogen oxide, NO, and
nitrogen dioxide, NO2
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Nitrogen can also be fixed through the industrial process that
creates fertilizer
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Most nitrogen fixation occurs naturally, in the soil, by bacteria
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Some bacteria attach to plant roots and have a symbiotic
(beneficial for both the plant and the bacteria) relationship with the plant
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The fixed nitrogen is then carried to
other parts of the plant and is used to form plant tissues, so the plant can
grow
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These bacteria can also create forms of nitrogen
that can be used by organisms
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The Nitrogen Cycle: Nitrogen cycling through the various forms in soil
determines the amount of nitrogen available for plants to uptake
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Nitrogen moves from organic materials,
such as manure or plant materials to an inorganic form of nitrogen that
plants can use
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This becomes important in the second stage of the
nitrogen cycle
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All
plants under cultivation, except legumes
A member of the pea family: beans, lentils, soybeans, peanuts, and peas,
are plants with seed pods that split in half
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Legumes get
nitrogen through fixation that occurs in their root nodules, as described
above
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The NH3 in the soil then reacts with water to form
ammonium, NH4
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STAGE 3: NITRIFICATION
The third stage, nitrification, also occurs in soils
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Nitrates can be used
by plants and animals that consume the plants
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Although nitrite is not usable by plants and
animals directly, other bacteria can change nitrites into nitrates—a form that
is usable by plants and animals
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The bacteria that we are talking about are
called nitrosomonas and nitrobacter
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Both kinds of bacteria can act
only in the presence of oxygen, O2 []
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STAGE 4: IMMOBILIZATION
The fourth stage of the nitrogen cycle is immobilization, sometimes
described as the reverse of mineralization
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Just like plants, microorganisms
An organism, or living thing, that is too tiny to be seen without a
microscope, such as a bacterium
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These soil
microorganisms pull nitrogen from the soil when the residues of
decomposing plants do not contain enough nitrogen
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Immobilization, therefore, ties up nitrogen
in microorganisms
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STAGE 5: DENITRIFICATION
In the fifth stage of the nitrogen cycle, nitrogen returns to the air as nitrates
are converted to atmospheric nitrogen (N2) by bacteria through the process
we call denitrification
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NITROGEN IS CRUCIAL FOR LIFE
The cycling of nitrogen through the ecosystem is crucial for maintaining
productive and healthy ecosystems with neither too much nor too little
nitrogen
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Understanding how the plant-soil nitrogen cycle
works can help us make better decisions about what crops to grow and
where to grow them, so we have an adequate supply of food
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Certain plants can uptake more nitrogen or other
nutrients, such as phosphorous, another fertilizer, and can even be used as
a “buffer,” or filter, to prevent excessive fertilizer from entering waterways
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) held up to 84% of the nitrate,
preventing it from entering the river
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Farmers and
communities need to work to improve the uptake of added nutrients by
crops and treat animal manure waste properly
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But, our current patterns of clearing trees to build roads and
other construction worsen this problem, because there are fewer plants left
to uptake excess nutrients
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We also need to find other ways to fix or avoid the problem of
excess nitrogen spilling over into aquatic ecosystems
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