Intro: The Nitrogen Cycle
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Nitrogen: the fourth most common element in living things
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Nitrogen is essential to life
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It is a basic building block for amino acids, proteins and DNA
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Nitrogen goes through a cycle similar to energy and carbon:
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Passes along food chains
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Circulates between biotic and abiotic parts of the environment
How do we get Nitrogen?
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Plants need nitrogen (N), phosphorus (P) and potassium (K) for growth, which
they can get through fertilizers.
Without fertilizers, plants must absorb nitrogen from the environment using
their roots.
Animals can obtain nitrogen ONLY by eating plants or other animals.
Nitrogen Gas
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Earth’s atmosphere is 80% nitrogen gas (N2). The atmosphere is our biggest “nitrogen pool” (like
carbon pools)
Nitrogen gas in the atmosphere is composed of two nitrogen atoms bound to each other.
Nitrogen gas is a form that very few organisms can use
– They can’t absorb it directly, including plants
– How do they get it?
In order to be used by organisms, nitrogen gas must be “fixed”.
Nitrogen Fixation
Nitrogen Fixation: When nitrogen gas is taken from the atmosphere and bound to other elements to make
it usable by living organisms.
There are 2 types of nitrogen fixation:
• add nitrogen to oxygen to form NO3- (nitrate)
• add nitrogen to hydrogen to form NH4+ (ammonium)
Nitrogen gas can be fixed in two basic ways:
1.
Lightning provides enough energy to "burn" the nitrogen and fix
it in the form of nitrate (NO3-).
– This process is duplicated in fertilizer factories to
produce nitrogen fertilizers.
– This is a minor source of nitrogen fixation
2. Nitrogen fixing bacteria use special
enzymes to fix nitrogen (react the nitrogen
with oxygen or hydrogen).
Nitrogen Fixing Bacteria
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The job of “fixing” nitrogen is up to certain bacteria that are found in the soil and water.
– The most important of these is Rhizobium, a bacterium that lives in nodules on the roots
of plants like legumes (peas, beans, alfalfa and clover)… what a great relationship!
(SYMBIOSIS – both the bacteria and the plant benefit)
– Before artificial fertilizers were developed, farmers used to plant legumes in their fields
to help restore the fertility of their soil.
– Nitrogen fixing bacteria in the water are called cyanobacteria, which are commonly
referred to as blue-green algae.
Plants can absorb the fixed nitrogen (called nitrates) through their roots, and then animals get
nitrogen by eating plants or other animals that have eaten plants.
So, without these bacteria, other life could not survive!
And The Cycle Begins…
1. Most plants can take up nitrates and convert them to amino acids and proteins.
2. Animals acquire all of their amino acids when they eat plants (or other animals).
3. When plants or animals die (or release waste) the nitrogen is returned to the soil.
4. The nitrogen that is returned to the soil in animal wastes or in the output of the decomposers is usually
in the form of ammonia. Ammonia is rather toxic.
5. Nitrifying bacteria in soil or water convert ammonia back into nitrates, which are taken up by plants to
continue the cycle. This is called nitrification.
So, there is a sub-cycle… nitrogen in the form of ammonia does not have to go back into the air as gas.
Instead, it can be nitrified again by nitrifying bacteria in the soil.
Denitrification
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So how does nitrogen ever get back into the atmosphere?
Some bacteria, called denitrifying bacteria, convert nitrates in soil and water back into nitrogen
gas and release it back into the atmosphere.
This is called denitrification.