Nitrogen Cycling in Ecosystems
Animation: http://www.teachersdomain.org/asset/lsps07_int_nitrogen/
Nitrogen is the most common element in Earth's atmosphere, yet it is also one of the
most limiting factors for growth in plants and animals. Most nitrogen exists as a gas. In this
common molecule, two nitrogen atoms are bound by a strong triple bond that makes them all
but completely unavailable to any other atom, ion, compound, or organism. Yet without
sufficient levels of available nitrogen, organisms would be unable to create their structures or to
perform vital functions. Nitrogen is a key building block in a number of important molecules,
such as nucleic acids, amino acids, and proteins. Without it, life as we know it would be
impossible.
Two natural forces are responsible for most of the gaseous nitrogen that is "fixed," or made
available to, plants and animals. Electricity in the form of lightning is one such force. When a
bolt of lightning travels through the atmosphere it breaks the triple bond holding the nitrogen
gas molecule together, enabling free nitrogen atoms to bond with oxygen in the air to form
nitrogen oxides. These compounds dissolve in atmospheric moisture to form nitrates that then
fall as rain.
The other force, although less spectacular than lightning, is no less energetic. Countless
bacteria, including those living freely in the soil and those found on the roots of some types of
plants, fix more than ten times the nitrogen released by lightning strikes. These single-celled
organisms break the bonds in nitrogen gas molecules and combine free nitrogen atoms with
hydrogen to form ammonium, an ion that is readily absorbed by plants. Some types of
nitrogen-fixing bacteria have formed symbiotic relationships with certain types of plants.
Legumes, such as peas and beans, support colonies of bacteria called rhizobium, in special
structures called nodules, which appear directly on their roots. While the bacteria provide the
plants with nitrogen, the plants provide the bacteria with energy-rich carbohydrates and a moist
environment in which they can thrive.
Human activities have severely altered the nitrogen levels in some ecosystems. Although
nitrogen is typically a limited resource in many environments, it has been made available in
massive quantities as a result of agricultural and industrial practices. Sometimes described as
"nutrient pollution," the result has often been catastrophic. For instance, when waste such as
nitrogen-rich sewage and fertilizers pours into ponds, lakes, and streams, the result is an
overabundance of algae (also know as an algal bloom). The eventual death of these
microscopic plants leads to their decomposition by bacteria—a process that uses vast quantities
of oxygen. Following an algal bloom, decomposing bacteria in lakes and ponds become so
abundant and oxygen depletion so complete that fish and other aquatic life may die. In the
years since scientists discovered this connection, cities and farmers have taken measures to
control the flow of nitrogen into ecosystems, so that this powerful element is available at the
right quantity where it is needed and less pervasive where it is not.
Discussion Questions
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The atmosphere is 80% nitrogen: why do you think plants and animals can't use nitrogen as it is found in
the atmosphere?
Explain what is meant by nitrogen fixation.
What is the role of bacteria in the nitrogen cycle?
Why don't legumes need nitrogen-containing fertilizers?
Why is nitrogen so important for living things?
Nitrogen Fixation:
Nitrogen Fixation is the conversion of atmospheric nitrogen (N2) into reactive compounds such as
ammonia (NH3) and nitrate (NO3-). The breaking of the bonds between the nitrogen atoms requires a
great deal of energy and occurs naturally in two primary ways:
1. Abiotic Fixation: Nitrate is the result of high energy fixation in the atmosphere from
lightning and cosmic radiation. In this process, N2 is combined with oxygen to form nitrogen
oxides such as NO and NO2, which are carried to the earth’s surface in rainfall as nitric acid
(HNO3). This high energy fixation accounts for approximately 10% of the nitrate entering
the nitrogen cycle.
2. Biological fixation: Biological fixation is accomplished by a series of soil microorganisms such as aerobic and anaerobic bacteria. Often, symbiotic bacteria such as
Rhizobium are found in the roots of legumes and provide a direct source of ammonia to the
plants. In root nodules of these legumes, the bacteria split molecular nitrogen into two free
nitrogen atoms, which combine with hydrogen to form ammonia (NH3). The following
plants are common examples of legumes: clover, alfalfa, soy beans, and chick peas. The
breakdown of these legumes by bacteria during ammonification actually returns excess
nitrogen not utilized by the plant to the surrounding soil. Therefore, to promote sustainable
soil fertility, it is beneficial to use these agricultural crops in rotation with other plants, such
as corn, that are more profitable but deplete the available nitrogen in the soil-Some freeliving aerobic bacteria, such as Azotobacter, and anaerobic bacteria, like Clostridium, freely
fix nitrogen in the soil and in aquatic environments. Some members of the photosynthetic
Cyanobacteria phylum fix nitrogen in aquatic environments as well.
Nitrification:
Nitrification is the process by which ammonia is oxidized to nitrite ions (NO2-) and then to nitrate
ions (NO3-), which is the form most usable by plants. The two groups of micro-organisms involved
in the process are Nitrosomas and Nitrobacter. Nitrosomas oxidize ammonia to nitrite and
Nitrobacter oxidize nitrite to nitrate.
Assimilation:
Nitrates are the form of nitrogen most commonly assimilated by plants through root hairs. Since
heterotrophic organisms cannot readily absorb nitrogen as plants do, they rely on acquiring
nitrogen-based compounds through the food they eat. Since plants are the base of the food chain, the
nitrogen-based compounds they have assimilated into their tissue will continue to pass from one
organism to another (through consumption) as matter and energy transfers through the ecosystem’s
food web.
Ammonification:
In ammonification, a host of decomposing microorganisms, such as bacteria and fungi, break down
nitrogenous wastes and organic matter found in animal waste and dead plants and animals and
convert it to inorganic ammonia (NH3) for absorption by plants as ammonium ions. Therefore,
decomposition rates affect the level of nutrients available to primary producers.
Denitrification:
Denitrification is the process by which nitrates are reduced to gaseous nitrogen (N2) and lost
to the atmosphere. This process occurs by facultative anaerobes in anaerobic environments.
Farmers with waterlogged fields and soils that have high clay content are especially
vulnerable to nitrogen losses due to denitrification.