The Nitrogen Cycle
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 know 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.
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 man-made 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.