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The Nitrogen Cycle
Nitrogen is essential for many processes; it is crucial for all life on Earth. It is in all amino acids,
is incorporated into proteins, and is present in the bases that make up nucleic acids (DNA and
RNA).
In plants, much of the nitrogen is used in chlorophyll molecules, which are essential for
photosynthesis and further growth. Although Earth’s atmosphere consists largely of nitrogen
(78%), most is relatively unusable by plants. Chemical or natural processes are necessary to
convert gaseous nitrogen into forms usable by living organisms. This makes nitrogen a crucial
part of food production. The abundance or scarcity of the usable forms of nitrogen dictates how
much food can be grown on a piece of land.
The nitrogen cycle is the process by which nitrogen is converted between its various chemical
forms. This transformation can be carried out via both biological and non-biological processes.
Important processes in the nitrogen cycle include fixation, assimilation, ammonification,
nitrification, and denitrification.
Nitrogen Fixation
Nitrogen fixation is the natural process by which nitrogen ( N 2 ) in the atmosphere is converted
into ammonium ions ( NH 4+ ).
This process is essential for life because fixed nitrogen is required to create the basic building
blocks of life, e.g., nucleotides for DNA and RNA and amino acids for proteins. Some fixation
occurs in lightning strikes, but most is done by free-living or symbiotic bacteria.
The conversion of nitrogen ( N 2 ) from the atmosphere into a form that is readily usable by plants
can occur in four ways:
1. Biological Fixation
• bacteria in soil and plants are able to fix nitrogen
2. Industrial Fixation
• in the production of fertilizer, nitrogen gas ( N 2 ) and hydrogen gas ( H 2 ) are reacted
to form ammonia ( NH 3 )
3. Combustion of Fossil Fuels
• car engines and coal power plants release various nitrogen oxides ( NOx ) as waste
products
4. Other Processes
• nitrogen gas ( N 2 ) and oxygen gas ( O2 ) will react to form nitrogen monoxide ( NO )
in the presence of lightning
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Assimilation
Plants take nitrogen from the soil, by absorption through their roots in the form of either nitrate
ions or ammonium ions. Animals get their nitrogen by eating plants (or other animals that have
eaten plants).
Ammonification
When a plant or animal dies, or an animal expels waste, decomposers convert the organic
nitrogen within the remains into ammonium ( NH 4+ ). This process is called ammonification.
Nitrification
Nitrification is the conversion of ammonium into nitrates ( NO3− ). This two-step process is
carried out primarily by soil-dwelling bacteria.
In the first step, nitrifying bacteria combine ammonium with oxygen gas ( O2 ) to form nitrite
( NO2− ). In the second step, other bacteria combine nitrite with oxygen gas to form nitrate ( NO3− ).
Denitrification
Denitrification is the conversion of nitrates back into nitrogen gas, completing the nitrogen
cycle. This process is performed by certain bacteria in anaerobic conditions (no oxygen
available).
The nitrogen cycle is of particular interest to ecologists because nitrogen availability can affect
the rate of key ecosystem processes, including primary production (plant growth) and
decomposition.
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The diagram below shows the nitrogen cycle.
Under normal conditions, there is a balance to the exchanges of nitrogen between the various
reservoirs. This balance means that the amount of nitrogen in each reservoir remains
approximately the same all the time, since the amount being removed and the amount being
returned are equivalent.
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Disturbing the Nitrogen Cycle
Humans add nitrogen to the environment in two ways: agricultural processes and industrial
processes.
Agricultural Processes
Plants require nitrogen in order to grow. The number of plants that can grow in a field is
determined by the supply of nitrogen in that field. Up until the early 1900s, that supply was
determined by natural processes alone.
The invention of fertilizer provided farmers with an artificial way to increase the amount of
nitrogen in soil. This, in turn, increased the number of plants that could be grown in a field.
Industrial Processes
When fossil fuels are burned, the nitrogen they contain is released into the air as nitrogen
compounds. Eventually these compounds dissolve in moisture in the air and fall back to Earth.
Modern farming, the burning of fossil fuels, and other human activities have more than doubled
the amount of nitrogen being moved through the environment every year. In fact, we are adding
nearly 140 million more tons of nitrogen than ecosystems can absorb and use. The result is
nitrogen overload.
The next few sections describe some of the effects of nitrogen overload on the world.
Effects on Soil
Excessive amounts of nitrogen in soil can result in an increase in the acidity of the soil. This
increased acidity damages tree roots, stunts tree growth, and causes needles on spruce trees and
other conifers to turn yellow and fall.
In addition, acidic soil will dissolve toxic metals in the soil. These toxic metals cause further
damage to plant roots and organisms in the soil that absorb the groundwater.
Excess nitrogen in soil can also wash from the soil into streams, affecting aquatic ecosystems.
Effects on the Atmosphere
Car exhaust and the smoke from factories both contain nitrogen. When these gases dissolve in
moisture in the air, they form acid rain.
When acid rain falls to Earth, it can harm a variety of organisms:
•
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acid rain raises the acidity of lakes, which can kill fish, birds, amphibians, and other
organisms
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•
acid rain damages the waxy covering on some plant leaves, exposing the leaves to
disease
•
acid rain leaches nutrients from soil, hindering plant growth
Effects on Fresh-Water Ecosystems
Extra nitrogen enters lakes in a number of ways:
•
fertilizer runoff from farms
•
acid rain
•
sewage systems (solid waste contains nitrogen)
The buildup of nitrogen in an aquatic ecosystem is called eutrophication, and goes something
like this:
•
Nitrogen enters the lake as runoff from farms and sewer systems.
•
Increased growth of plants on the surface of the water blocks sunlight from penetrating
deeper into the water.
•
Plants below the surface are unable to carry out photosynthesis. They begin to die out and
stop producing oxygen.
•
As the plants die, the population of decomposers explodes. The decomposers consume
oxygen in the water (cellular respiration).
•
The reduced oxygen levels in the water kill fish and other animals that require high
oxygen levels.
Increased nitrogen in fresh-water can also have a direct impact on humans. Nitrogen in the form
of nitrates reduce the ability of blood to carry oxygen (this is known as anemia). Thus, if your
drinking water contains excessive amounts of nitrates, it may increase your risk of anemia. This
is more of a problem in rural communities where water is not treated before being used.
Effects on Marine Ecosystems
Excess nitrogen in seawater initially results in a population explosion of algae in the warm
surface waters. This is called an algal bloom. As the algae die, they sink down to cooler waters.
Decomposers break down the algae, consuming oxygen in the process.
Because there is little mixing between warm and cold layers in seawater, the cold layer
eventually may contain very little oxygen. This results in the death of many oxygen-requiring
organisms in this area.
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Worksheet
1. What are nitrogen fixing bacteria?
2. What organisms are involved in the movement of nitrogen between the air and soil, and back
to the air?
3. How do animals obtain usable nitrogen?
4. What is denitrification?
5. Livestock farming creates large amounts of animal waste. How would this affect the nitrogen
cycle?
6. What would happen if a framer used too much fertilizer? What adverse affects can this have
on the environment? Be specific!
7. How can burning fossil fuels affect the nitrogen cycle?
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