Nitrogen Cycle
One important nutrient in soil that can be depleted is nitrogen. Nitrogen is a necessary building block in
the matter that makes up living things. Since the air around you is about 78 percent nitrogen gas, you
might think that it would be easy for living things to obtain nitrogen. However, most organisms cannot
use the nitrogen gas in the air. Nitrogen gas is called “free” nitrogen, meaning it is not combined with
other kinds of atoms. Most organisms can use nitrogen only once it has been “fixed,” or combined with
other elements to form nitrogen-containing compounds. You can follow this process, called the nitrogen
cycle, in the diagram below. The nitrogen cycle is the process by which nitrogen is removed from
the atmosphere, fixed in the soil by bacteria, incorporated in other living things, and then
released back into the atmosphere.
The Nitrogen Cycle
In the nitrogen cycle,
nitrogen moves from
the air to the soil, into
living things, and back
into the air.
Nitrogen Fixation
The process of
changing free
nitrogen gas into a
usable form of
nitrogen is called
nitrogen fixation.
Most nitrogen
fixation is performed
by certain kinds of
bacteria. Some of
these bacteria live in
bumps called nodules (NAHJ oolz) on the roots of certain plants. These plants, known as legumes, include
clover, beans, peas, alfalfa, and peanuts. Another way nitrogen gets fixed to soil is through lightening.
The relationship between the bacteria and the legumes is an example of mutualism. A relationship in
which both species benefit is called mutualism. Both the bacteria and the plant benefit from this
relationship: The bacteria feed on the plant’s sugars, and the plant is supplied with nitrogen in a usable
form.
To enrich their fields, many farmers make use of the nitrogen-fixing bacteria in legumes. Every few
years, a farmer may plant a legume such as alfalfa in a field. The bacteria in the alfalfa roots build up a
new supply of nitrogen compounds in the soil. The following year, the new crops planted in the field
benefit from the improved soil.
Return of Nitrogen to the Environment Once the nitrogen has been fixed into chemical compounds,
it can be used by organisms to build proteins and other complex substances. Decomposers break down
these complex compounds in animal wastes and in the bodies of dead organisms. This returns simple
nitrogen compounds to the soil. Nitrogen can cycle from the soil to producers and consumers many
times. At some point, however, bacteria break down the nitrogen compounds completely in a process
called dentrification. These bacteria release free nitrogen back into the air. Then the nitrogen cycle starts
again.
Nitrogen Cycle
Nitrogen is also key in the existence of ecosystems and food chains. Nitrogen forms about 78% of the air on
earth. But plants do not use nitrogen directly from the air. This is because nitrogen itself cannot be used by
green plants to make protein. Nitrogen gas therefore needs to be converted into nitrate compound in the soil by
nitrogen-fixing bacteria in soil, root nodules or lightning. When nitrogen in the air is converted to ammonia, it
is called nitrogen fixation.
Go to this website:
http://www.classzone.com/books/ml_science_share/vis_sim/em05_pg20_nitrogen/em05_pg20_nitrogen.html
Work your way through the diagram and complete the following notes:
Plants and animals _________________ use pure _______________ gas, N2 . It must first be
_____________ into a usable form. ______________ fix N2 and form NO3 (when combined with oxygen.)
NO3 enters the _____________ and is absorbed into ________________ through the roots. Bacteria in the
roots of plants also fix the nitrogen so plants can use it. ______________, like the vole, ___________ the
plant to get ____________________ they can use. As organic matter _____________________, the nitrogen
returns to the soil. Bacteria in the soil break it down again forming nitrogen _________________ and
____________________ to the _______________.
1. Can plants and animals use pure nitrogen gas, N2 as it is found in the atmosphere? ______________
2. What must happen to the nitrogen before they can use it? ____________________
3. What happens to N2 when lightning “fixes” it? ____________________________________________
4. What happens to the NO3? ____________________________________________________________
5. How do animals get nitrogen they need? _________________________________________________
6. How does nitrogen get back into the soil? ________________________________________________
7. How does the nitrogen turn back into N2? ________________________________________________
8. What happens to the nitrogen once it turns back into N2? ____________________________________
Sources for review:
http://eschooltoday.com/ecosystems/thenitrogen-cycle.html
http://studyjams.scholastic.com/studyja
ms/jams/science/ecosystems/nitrogencycle.htm
Write a summary about the nitrogen cycle include:
What is the nitrogen cycle? Why is it important? From your picture, how does the animal get its
nitrogen? What is one way that the animal can give nitrogen back to the soil? Is it good or bad
for plants to have nitrogen fixing bacteria on their roots? Include evidence. Explain why it is a
good idea for a farmer to plant legumes every few growing seasons? Why would it be bad for
them never to plant them? Explain the relationship between the bacteria and the legumes?
Explain how nitrogen returns to the atmosphere.
Define nitrogen fixation.