THE NITROGEN CYCLE
Nitrogen is an important component of DNA and proteins, which are essential
for the life processes that take place inside cells
- Nitrogen is essential for muscle function in animals and for growth in
plants
Where nitrogen is stored
The largest store of nitrogen is the ______________________,
where it exists as nitrogen gas (N2)
- Other major stores include ____________________an
organic matter in _______________
- Living organisms, __________ and _________________
also store nitrogen (but in smaller amounts)
How nitrogen is cycled through ecosystems
78% of Earth’s atmosphere is nitrogen gas, but most organisms cannot use this form of nitrogen
- Much of the nitrogen cycle involves processes that make nitrogen available to plants and
eventually to animals
_________________________________ = the process in which nitrogen gas (N2) is converted into
compounds that contain nitrate (NO3-) or ammonium (NH4+)
Nitrogen fixation occurs in three ways:
*
*
*
Atmospheric nitrogen fixation = occurs when ____________________
_______________________________________

This nitrate and other nitrogen-containing compounds enter
terrestrial and aquatic ecosystems through the ____________
Nitrogen fixation in the soil occurs when ___________________________________ that lives in
the root nodules of legumes and other plants convert ____________________________________
-
The plants supply the bacteria with food (_______________) and the bacteria supply the
host with ________________ needed for growth
Nitrogen fixation in water bodies: Certain species of ____________________ (blue-green bacteria
that manufacture their own food during photosynthesis) fix __________ into _____________.
- Nitrogen-fixing cyanobacteria make ammonium available to plants in the surface waters of
the oceans, wetlands, and lakes
Nitrification and uptake
Nitrification = the process when
_____________ is converted into
_____________.
This process occurs in two steps:
1. Certain species of
______________________
______________________
______________________
2. Different species of
nitrifying bacteria in the
soil then convert the
_________ into ________
Once nitrates are made available they can enter __________________________ and eventually be
incorporated into plant ______________________
How nitrogen is returned to the atmosphere
_____________________________= the process of how nitrogen is returned to the atmosphere;
 In a series of chemical reactions, ______________________________ convert nitrate (NO3)
back into _______________________
 Nitrogen is also returned to the atmosphere as ammonia (NH3) in _____________________
and nitrogen oxide gases such as nitrogen oxide and nitrogen dioxide
How nitrogen is removed from
ecosystems
Excess nitrate and ammonium that
are not taken up by plants mix with
______________________ and are
washed from the soil into
_______________________________
 This unused nitrogen may
settle to ocean, lake or river bottoms
in sediments
 Eventually these sediments
will form _____________ and the
nitrogen will not be available
Human Activities and the Nitrogen Cycle
Human activities have ______________ the amount of nitrogen in the biosphere in the past 50
years
 Millions of tonnes of nitrogen are added to the atmosphere annually in the form of
nitrogen oxide (NO) and nitrogen dioxide (NO2) as a result of ________________________

________________________________________________________________ also releases
trapped nitrogen into the atmosphere
o These compounds eventually return to the terrestrial and aquatic ecosystems as
_____________________

_________________________________________ that fix N2 into nitrogen compounds that
crops can use are used in agriculture
o Excess nitrogen (ammonium and nitrate) that is not used can escape back into the
_____________________ or can be leached (leak out of) from the soil and end up in
_____________________________
o This increased amounts of dissolved nitrogen causes _________________________
which deprives other aquatic plants of sunlight and of oxygen as the algae
undergoes cellular respiration
o This deprives aquatic animals of _____________________ and can lead to the death
of all fish in a lake