AP Environmental Science
Mr. McDowell
The Nitrogen Cycle
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Nitrogen is an important element that is found in both the organic (living things) and the inorganic (nonliving) parts
of the Earth system. Most of the nitrogen on Earth is found in the atmosphere. It comprises approximately 80% of our
atmosphere where it exists as N2 gas. Nitrogen can also be found in a variety of forms in plants, animals, soils, ocean,
and other reservoirs in the environment. All plants and animals need nitrogen to make amino acids, proteins, and DNA,
but the nitrogen in the atmosphere is not in a form that they can use. This gas must first be converted into a usable form
during a process known as nitrogen fixation. Only specialized bacteria in soil and certain types of algae in water can fix
nitrogen. Lightning strikes can also result in some nitrogen fixation.
Plants get the nitrogen that they need from the soil or water in which they live. This nitrogen is usually in the form
of inorganic nitrate (NO 3-) . Nitrate is easily dissolved in water and often leaches out of the soil. Animals get the nitrogen
that they need by consuming plants or other animals which contain nitrogen within organic molecules. When organisms
die, their bodies decompose bringing the nitrogen into soil or into the oceans. As these dead organisms decompose,
nitrogen is converted into inorganic forms such as ammonium salts (NH 4+) by a process known as mineralization.
These ammonium salts are absorbed by the clay in the soil and are chemically altered by bacteria into nitrite (NO 2-) and
then nitrate (NO 3-). The different paths in which nitrogen may follow as it cycles throughout the earth is known as the
nitrogen cycle.
Human activities have had a huge impact in global nitrogen cycles by causing changes in the amount of nitrogen
stored in reservoirs. The use of nitrogen-rich fertilizers can lead to nitrates from the fertilizers washing into waterways.
This increase in nitrate level can cause the rapid growth of aquatic plants during a process known as eutrophication.
These plants will eventually die, decompose and deplete the water of available dissolved oxygen which can have
disastrous effects on the entire food chain. Additionally, humans are altering the nitrogen cycle by burning fossil fuels and
forests, which release nitric oxide, nitrous oxides, and other by-products into the atmosphere where they combine with
water to form acid rain and enhance the greenhouse effect.
The nitrogen cycle contains several stages:
Nitrogen fixation
Atmospheric nitrogen occurs primarily in an inert form (N2) that few organisms can use; therefore it must be converted to
an organic - or fixed - form in a process called nitrogen fixation. Most atmospheric nitrogen is 'fixed' through biological
processes. First, nitrogen is deposited from the atmosphere into soils and surface waters, mainly through precipitation.
Once in the soils and surface waters, nitrogen undergoes a set of changes: its two nitrogen atoms separate and combine
with hydrogen to form ammonia (NH4+). This is done by microorganisms that fall into three broad categories: bacteria
living in symbiotic relationships with certain plants, free anaerobic bacteria, and algae. Crops, such as alfalfa and beans,
are often planted in order to remedy the nitrogen-depletion in soils, and nitrogen-fixing bacteria employ an enzyme, known
as nitrogenase, to split atmospheric nitrogen molecules into individual atoms for combination into other compounds.
A small amount of nitrogen is 'fixed' through a process of high energy fixation that occurs primarily as lighting
strikes converting atmospheric nitrogen into ammonia (NH4+) and nitrates (NO3-). Nitrogen can also be fixed through manmade processes, primarily industrial processes that create ammonia and nitrogen-rich fertilizers.
Nitrification
While ammonia can be used by some plants, most of the nitrogen taken up by plants is converted by bacteria from
ammonia - which is highly toxic to many organisms - into nitrite (NO2-), and then into nitrate (NO3-). This process is called
nitrification, and these bacteria are known as nitrifying bacteria.
Assimilation
Nitrogen compounds in various forms, such as nitrate, nitrite, ammonia, and ammonium are taken up from soils by plants
which are then used in the formation of plant and animal proteins.
Ammonification
When plants and animals die, or when animals emit wastes, the nitrogen in the organic matter reenters the soil where it is
broken down by other microorganisms, known as decomposers. This decomposition produces ammonia which is then
available for other biological processes.
Denitrification
Nitrogen makes its way back into the atmosphere through a process called denitrification, in which nitrate (NO3-) is
converted back to gaseous nitrogen (N2). Denitrification occurs primarily in wet soils where the water makes it difficult for
microorganisms to get oxygen. Under these conditions, certain organisms - known as denitrifiying bacteria - will process
nitrate to gain oxygen, leaving free nitrogen gas as a byproduct.
Test Your Understanding
1. Livestock farming creates large amounts of animal waste. Explain the effects on the nitrogen cycle?
2. What would happen if a farmer used too much fertilizer? What adverse affects can this have on the
environment? Be specific!
3. How can burning fossil fuels affect the nitrogen cycle?
4. Explain what nitrogen fixation is, and how it occurs.
5. Explain why nitrogen fixation has to occur in order to cycle nitrogen.
6. Explain what eutrophication is, and how it occurs.
7. What form on nitrogen do plants use?
8. What form of nitrogen does animal waste contain?
9. Explain what mineralization does, and the forms of Nitrogen that are affected.
10. Describe: a) the major events, b) the reactants and products, and c) any biological requirements for each of the
steps of the Nitrogen Cycle.
11. On a sheet of white paper, diagram the major events of the Nitrogen Cycle, including any reactants and products.
Include a, b, and c from Question #10 in your diagram
DO NOT simply copy the diagram from the top of this assignment!