WODSS SCIENCE
SNC 1DI
Name: ________________________
Date: ___________________
THE NITROGEN CYCLE
Like carbon, nitrogen is a very important element for all living things.
 All living things use nitrogen to make DNA (genetic material) and proteins (proteins are found
in HAIR, NAILS and many other body tissues).
 Plants also use nitrogen to make chlorophyll (which is used in PHOTOSYNTHESIS)
Nitrogen gas (N2) composes nearly 78% of the earth’s atmosphere, but living organisms can’t use
nitrogen in this form. In order for nitrogen to be useful to living organisms, nitrogen gas must be
converted into nitrate ions (NO3-).
NITROGEN FIXATION
Nitrogen fixation is the process which converts atmospheric nitrogen into nitrates. This can happen in
two ways:
1) Lightning
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Lightning accounts for only about 3% of the world’s nitrogen fixation
Energy from lightning causes OXYGEN and NITROGEN in the atmosphere to react,
producing nitrates.
2) Bacteria
 Bacteria convert nitrogen gas into a variety of nitrogen-containing compounds, including
nitrates, nitrites, and ammonia.
 These bacteria are found in the soil, as well as in nodules of the roots of legumes ( PEAS,
ALFALFA and SOY BEAN). These bacteria supply usable nitrogen directly to the
plant.
NITROGEN IN THE FOOD WEB
 Plants get the nitrogen they need from nitrates in the SOIL.
 Animals get the nitrogen they need by CONSUMING PLANTS.
 When organisms die, decomposers break the nitrogen-containing chemicals into
NITROGEN. Animals also excrete extra nitrogen they consume as ammonia.
DENITRIFICATION
Denitrification is the process which converts nitrates back to atmospheric nitrogen gas. This process
is done by BACTERIA.
***See Nitrogen Cycle Diagram on Back***
Fertilizers and Eutrophication
 Sometimes fertilizers are applied to fields and gardens to add extra nitrogen, as well as other
nutrients needed by plants like phosphorus.
 When the field or garden is watered, the water dissolves the fertilizer, and this provides
nutrients to the plants.
 Water from the garden runs into local waters and streams and takes the nutrients with it.
 If the stream runs into a pond or lake, the high levels of nutrients cause algae to grow rapidly.
This process is called eutrophication.
 Eutrophication is the addition of nutrients to an aquatic ecosystem causing increased growth
of plants such as algae.
The Nitrogen Cycle
Problems with Eutrophication:
Large amounts of algae growing near the surface
allow less light to penetrate so that plants below
the surface die
Large amounts of algae eventually die.
Decomposing these dead plants requires a lot of
oxygen.
The resulting low amount of oxygen in the water
may cause fish and other animals to die.
Questions:
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Describe how nitrogen in the atmosphere makes its way into plants.
How is the nitrogen in dead organisms released back into the soil?
Identify the role that bacteria play in the nitrogen cycle.
Farmers will often rotate crops by planting crops like alfalfa and soybeans one year, and then crops like corn, that
require a lot of nitrogen, the next year. Why is this a good idea?
Nitrogen-fixing bacteria survive best in oxygen-rich environments, and denitrifying bacteria survive best in oxygenpoor environments. Given this information, explain why people aerate their lawns.
Describe an advantage of using fertilizers.
Define eutrophication.
Explain why the presence of nitrogen and phosphorus in water can threaten ecosystems.