Nitrogen Cycle Lab
Introduction:
Nitrogen is one of the most important elements that are found in nature. It is a key part of our DNA and
our proteins, so without it, we would not be here. It is a good thing, then, that 78% of our atmosphere
is made up of nitrogen gas. There is just one problem: this gas is not in a form that plants or animals can
use. Other organisms, mainly bacteria, have to convert (change) this nitrogen gas so that all of the other
living things on the planet can use it. After these plants and animals have used the nitrogen, then the
usable nitrogen gets returned to different bacteria in the soil that turn it back into nitrogen gas. And,
the process starts all over again. Because nitrogen keeps taking this grand circle tour, it is called the
Nitrogen Cycle.
Nitrogen in
Atmosphere (N2)
Denitrification
Nitrogen used and
returned by plants
and animals
Nitrogen Fixation
& Nitrification
Usable Nitrogen
(NH4 & NO3)
So why can’t we use nitrogen gas? And what are the steps in the Nitrogen Cycle? That is what we are
going to explore today.
Lab Procedure
Supplies:
1. 2 Nitrogen molecules (large marshmallows)
2. 8 Hydrogen molecules (mini-marshmallows)
3. 6 Oxygen molecules (gumdrops)
4. 10 Chemical Bonds (toothpicks)
5. 2 Graham crackers
**DO NOT EAT OR BREAK ANY OF THE MATERIALS!! IF YOU DO, THE EXPERIMENT WILL BE OVER FOR
EVERYONE.**
Procedure
Step #1: Build your nitrogen gas (N2) molecule
Use your 2 large marshmallows and three of your toothpicks to build something that looks like this:
Take a look at your nitrogen gas molecule and answer the questions below using complete sentences.
1. How stable is your molecule? Can it stand up on its own? Will it fall apart if you didn’t hold it?
2. How easy (or hard) do you think it would be to break all three bonds? (Do not break your
toothpicks; you will cause the experiment to stop for everyone)
3. Design an experiment that will show how hard or easy it is to break all three toothpicks.
Step #2: Nitrogen Fixation: Making Nitrogen gas (N2) into Ammonium (NH4)
Carefully take your N2 molecule and pull it apart. Use 8 toothpicks and 8 of your mini-marshmallows
(Hydrogen atoms) and build 2 molecules of NH4. Your molecules should look something like this:
This is what a nitrogen-fixing bacterium does for the nitrogen cycle.
1. Do you think that the bonds in this NH4 molecule will break more easily than in your N2
molecule? Why or why not?
2. If you think that the bonds will break more easily, explain how having nitrogen in this form
might be more useful to living things. If you thought the bonds would be harder to break, then
explain how this wouldn’t be useful to living things.
Step #3: Nitrification: Making NH4 into NO3
Carefully take one of your NH4 molecules and pull it apart. Use 3 toothpicks and 3 of your gumdrops
(Oxygen atoms) and build 1 molecule of NO3. Your molecule should look something like this:
This is what a nitrifying bacterium does for the nitrogen cycle.
1. How have you modified (changed) your ammonium (NH4) molecule? How might this benefit a
plant or animal?
2. Why do you think some organisms need NH4 and some need NO3 (Make an educated guess
here?)
Step #4: Uptake of Nitrogen (both NH4 and NO3) by plants and animals
Once nitrogen gas (N2) has been turned into ammonia (NH4) or (NO3), it can be taken up by plants
through their roots. When these plants get eaten, they pass that nitrogen on to whatever eats them,
and this nitrogen is passed up the food chain.
Carefully take your ammonium molecule (NH4) and your Nitrate molecule (NO3) and attach each to a
graham cracker with a toothpick. You should end up with two graham crackers that look like this:
1. Why do plants and animals need nitrogen? What do they make with the nitrogen once they
take it in?
2. Based on what plants and animals do with nitrogen, how easy is it for plants and animals to
return it to the environment?
Step #5: Returning the Nitrogen to the Environment
Plants and animals can return nitrogen to the environment two ways: they can eliminate wastes (like
urine) or they can die and have their bodies decomposed so that all of their molecules of their body,
including the nitrogen-containing DNA and protein, return to the soil.
Carefully detach your ammonium (NH4) and nitrate (NO3) molecules from the graham crackers. You
should end up with 2 bare graham crackers and 2 molecules.
This is what decomposers do for the Nitrogen Cycle
1. Why do you have to have decomposers in the Nitrogen cycle? What would happen to the
nitrogen if you didn’t?
2. Why do you think that cow poop is a good fertilizer? (Think about what wastes contain, and
how that might benefit plants)
Step #6: Denitrification: Turning both ammonium (NH4) and (NO3) back into nitrogen gas (N2)
Carefully pull your ammonium and nitrate molecules apart and reform your nitrogen gas. You should
end up with something that looks like this:
This is what a denitrifying bacterium does for the nitrogen cycle.
1. Why is this step necessary for the Nitrogen cycle? Why do you think we need to replace the N2
gas?
2. What would happen to the Nitrogen Cycle if this step didn’t happen?