Download Explanation of Nitrogen Cycle

THE NITROGEN CYCLE Nitrogen (N) is an
element like carbon. All creatures need
nitrogen to survive. There are huge amounts
of nitrogen gas in the atmosphere, but most
animals and plants have no way of using it.
It needs to be fixed (put into a biologically
useful compound). After it is fixed, it can
then start to move through the cycles and
organisms in an ecosystem.
WHERE CAN YOU FIND IT? Let's start with
the main sources of nitrogen. Nitrogen gas is
the most abundant element in our
atmosphere. The other main source of
nitrogen is in the nitrates of soil. The
nitrogen in the atmosphere cannot be used
while the nitrates in the soil can be used by
plants. Nitrogen can be converted into useful
nitrate compounds by bacteria, algae, and
even lightning. Once in the soil, the nitrogen
becomes biologically accessible.
BORROWING NITROGEN Plants are the main users of
nitrogen in the soil. They are able to take in the nitrates
through their root system. Once inside the plant, the nitrates
are used in organic compound that let the plant survive.
Organic compounds have carbon atoms. Those compounds
might be proteins, enzymes, or nucleic acids. Once the
plants have converted the nitrogen, the element can be
returned to the soil or taken up by animals. Herbivores eat
plants and convert many of the amino acids into new
proteins. Omnivores that eat both plants and animals are
able to take in the nitrogen rich compounds as well. The
nitrogen compounds are only borrowed. Nitrogen atoms are
returned to the soil in poop and dead organisms. Once in
the soil, the whole process can start again.
Nitrogen is essential to all living systems, which makes the nitrogen cycle one of
Earth's most important nutrient cycles.
Eighty percent of Earth's atmosphere is made up of nitrogen in its gas phase.
Atmospheric nitrogen becomes part of living organisms in two ways. The first is
through bacteria in the soil that form nitrates out of nitrogen in the air. The second is
through lightning. During electrical storms, large amounts of nitrogen are oxidized
and united with water to produce an acid that falls to Earth in rainfall and deposits
nitrates in the soil.
Plants take up the nitrates and convert them to proteins that then travel up the food
chain through herbivores and carnivores. When organisms excrete waste, the
nitrogen is released back into the environment. When they die and decompose, the
nitrogen is broken down and converted to ammonia. Plants absorb some of this
ammonia; the remainder stays in the soil, where bacteria convert it back to nitrates.
The nitrates may be stored in humus or leached from the soil and carried into lakes
and streams. Nitrates may also be converted to gaseous nitrogen through a process
called denitrification and returned to the atmosphere, continuing the cycle.
Human activities and the
nitrogen cycle
Human activities cause increased
nitrogen deposition in a variety of
ways, including
* burning of both fossil fuels and
forests, which releases nitrogen
into the atmosphere
* fertilizing crops with nitrogenbased fertilizers, which then enter
the soil and water
* ranching, during which livestock
waste releases ammonia into the
soil and water
* allowing sewage and septic
tanks to leach into streams, rivers,
and groundwater
Harmful effects of nitrogen deposition
The consequences of human-caused nitrogen deposition are profound and influence many aspects of the
Earth system, including
*ecosystems: Nitrogen additions to the soil can lead to changes that favor weeds over native plants,
which in turn reduces species diversity and changes ecosystems. Research shows that nitrogen levels are
linked with changes in grassland species, from mosses and lichens to grasses and flowers.
*precipitation: Nitrogen oxides react with water to form nitric acid, which along with sulfur dioxide is a
major component of acid rain. Acid rain can damage and kill aquatic life and vegetation, as well as corrode
buildings, bridges, and other structures.
*air quality: High concentrations of nitrogen oxides in the lower atmosphere are a precursor to
tropospheric ozone which is known to damage living tissues, including human lungs, and decrease plant
production.
*water quality: Adding large amounts of nitrogen to rivers, lakes, and coastal systems results in
eutrophication, a condition that occurs in aquatic ecosystems when excessive nutrient concentrations
stimulate blooms of algae that deplete oxygen, killing fish and other organisms and ruining water quality.
Parts of the Gulf of Mexico, for example, are so inundated with excess fertilizer that the water is clogged
with algae, suffocating fish and other marine life.
*carbon cycle: The impacts of nitrogen deposition on the global carbon cycle are uncertain, but it is likely
that some ecosystems have been fertilized by additional nitrogen, which may boost their capture and
storage of carbon. Sustained carbon sinks are unlikely, however, because soil acidification, ozone
pollution, and other negative effects eventually compromise nitrogen-enhanced carbon uptake.