The nitrogen cycle
The nitrogen cycle is one of the most difficult of the cycles to learn,
simply because there are so many important forms of nitrogen, and because
organisms are responsible for each of the interconversions. Remember that
nitrogen is critically important in forming the amino portions of the amino
acids which in turn form the proteins of your body. Proteins make up skin
and muscle, among other important structural portions of your body, and all
enzymes are proteins. Since enzymes carry out almost all of the chemical
reactions in your body, it's easy to see how important nitrogen is.
The chief reservoir of nitrogen is the atmosphere, which is about 78%
nitrogen. Nitrogen gas in the atmosphere is composed of two nitrogen
atoms bound to each other. It is a pretty non-reactive gas; it takes a lot of
energy to get nitrogen gas to break up and combine with other things, such
as carbon or oxygen.
Nitrogen gas can be taken from the atmosphere (fixed) in two basic
ways. First, lightning provides enough energy to "burn" the nitrogen and fix
it in the form of nitrate, which is a nitrogen with three oxygens attached.
This process is duplicated in fertilizer factories to produce nitrogen
fertilizers. The second process is biological nitrogen fixation, the process
wherein N2 is converted to ammonium, essential because it is the only way
that organisms can attain nitrogen directly from the atmosphere. Certain
bacteria, for example those among the genus Rhizobium, are the only
organisms that fix nitrogen through metabolic processes. Nitrogen fixing
bacteria often form symbiotic relationships with host plants. This symbiosis
is well-known to occur in the legume family of plants (e.g. beans, peas, and
clover). In this relationship, nitrogen fixing bacteria inhabit legume root
nodules and receive carbohydrates and a favorable environment from their
host plant in exchange for some of the nitrogen they fix. There are also
nitrogen fixing bacteria that exist without plant hosts, known as free-living
nitrogen fixers. In aquatic environments, blue-green algae (really a bacteria
called cyanobacteria) is an important free-living nitrogen fixer. All of these
fix nitrogen, either in the form of nitrate or in the form of ammonia
(nitrogen with 3 hydrogens attached). Most plants can take up nitrate and
convert it to amino acids. Animals acquire all of their amino acids when
they eat plants (or other animals). When plants or animals die (or release
waste) the nitrogen is returned to the soil.
The usual form of nitrogen returned to the soil in animal wastes or in the
output of the decomposers, is ammonia. Ammonia is rather toxic, but,
fortunately there are nitrite bacteria in the soil and in the water which take
up ammonia and convert it to nitrite, which is nitrogen with two oxygens
through a process called nitrification. Nitrite is also somewhat toxic, but
another type of bacteria, nitrate bacteria, take nitrite and convert it to nitrate,
which can be taken up by plants to continue the cycle.
We now have a cycle set up in the soil (or water), but what returns
nitrogen to the air? It turns out that there are denitrifying bacteria which
take the nitrate and combine the nitrogen back into nitrogen gas. Through
denitrification, oxidized forms of nitrogen such as nitrate and nitrite (NO2-)
are converted to dinitrogen (N2) and, to a lesser extent, nitrous oxide gas.
Denitrification is an anaerobic process that is carried out by denitrifying
bacteria.
Early in the 20th century, a German scientist named Fritz Haber figured
out how to short circuit the nitrogen cycle by fixing nitrogen chemically at
high temperatures and pressures, creating fertilizers that could be added
directly to soil. This technology has spread rapidly over the past century,
and, along with the advent of new crop varieties, the use of synthetic
nitrogen fertilizers has led to an enormous boom in agricultural
productivity. This agricultural productivity has helped us to feed a rapidly
growing world population, but the increase in nitrogen fixation has had
some negative consequences as well.
Not all of the nitrogen fertilizer applied to agricultural fields stays to
nourish crops. Some is washed off of agricultural fields by rain or
irrigation water, where it leaches into surface or ground water and can
accumulate. In groundwater that is used as a drinking water source, excess
nitrogen can lead to cancer in humans and respiratory distress in infants.
The U.S. Environmental Protection Agency has established a standard for
nitrogen in drinking water of 10 mg per liter nitrate-N. Unfortunately,
many systems (particularly in agricultural areas) already exceed this level.
By comparison, nitrate levels in waters that have not been altered by
human activity are rarely greater than 1 mg/L. In surface waters, added
nitrogen can lead to nutrient over-enrichment, particularly in coastal
waters receiving the inflow from polluted rivers.
Nitrogen cycle questions:
1) Why is nitrogen so important to us?
2) The most important reservoir for nitrogen in the nitrogen cycle is
_________________________
3) Two ways in which atmospheric nitrogen is fixed into a form that can be
used by living things are:
1._______________________________________________________
2._______________________________________________________
4) Nitrogen can be fixed by three different groups of microbes. List there
three groups and briefly describe them.
a) ______________________________________________________
b) ______________________________________________________
c) ______________________________________________________
5) What happens during the process of nitrification?
6) What happens during the process of denitrification?
7) How have humans affected the nitrogen cycle?