An Introduction to the Nitrogen Cycle

Background for Teachers

The nitrogen cycle is a complex biogeochemical cycle. There are many forms of nitrogen and chemical notation of each can be a challenge. There are numerous nitrogen cycle diagrams in textbooks and on the Web that give the basics, for example:

Figure 1:

But, none deal specifically with our landscape and all focus on the entire nitrogen cycle.  It is important that students put the nitrogen cycle into the context of the local landscape, that their focus be directed to the most important species of nitrogen (NO3- and NH4+) and that they understand the nitrogen cycle in their own words. For this activity students will watch Dr. Ivan Fernandez video on the nitrogen cycle as it pertains to the Northeastern United States forested landscape and then create a location-specific reference diagram of the nitrogen cycle to use for the rest of the project. The diagram should be location specific (in other words, if there is not a solitary cow outside your classroom then there should not be one in the diagram).

There are two key paradoxes in the nitrogen cycle:

Paradox 1: There is a lot of nitrogen in the atmosphere, but almost all of it is in a form that plants cannot use. In fact, relatively speaking, in the nitrogen cycle there is very little of the nitrogen that plants can use. The scarcity of useable nitrogen limits plant growth.

However, plants have adapted to this scare amount of nitrogen and there are upper limits to how much nitrogen plants can take up. Above that point plants either cannot take up additional nitrogen or the effects of taking up the nitrogen are deleterious (as you will see in Activity 3).

Paradox 2: If nitrogen were always a limiting resource then the upper limit of plant uptake would not matter. However, because of the burning of fossil fuel and the industrial manufacture and non-specific application of nitrogen-rich fertilizer there is now a great deal more bioavailable nitrogen in the environment. All of this excess nitrogen does not benefit the nitrogen-limited plants.

It may be easy to use a water analogy:

If a tree has adapted to using a certain amount of water then altering its environment so that it has an excess of water will not help it thrive. (Here’s an example: if I underwater my basil plants they die, if I overwater them they die… they are adapted to a certain range of water requirements and overwatering is just as deadly as underwatering. In the same way over-nitrogening is no better than under-nitrogening.)

Paradox #1 led, in part, to Paradox #2: When we discovered that nitrogen was a limiting nutrient, but that we could industrially nitrify the abundant N2 in the atmosphere, we were able to increase crop production and feed more people. However, the industrially-nitrified fertilizer did not stay within the areas where it was placed and that has led to Paradox #2.


Students understand:

  • The basic nitrogen cycle
  • That there are different forms of nitrogen
  • The role microorganisms play in changing the forms of nitrogen
  • The importance of nitrogen to living things
  • That plants and animals can use only certain forms of nitrogen
  • The natural and human-made inputs to the nitrogen cycle


    Students will produce a nitrogen cycling diagram that is specific to their location (or to the field site, if one has already been picked)

Where does this lesson happen in the Project?

This is the first lesson in the project.

Getting Ready

Ask your students to tell you anything they can about nitrogen- how much of the atmosphere is made of nitrogen molecules, what is ammonia, why do we need nitrogen. List all student information on a board.

Explain that, for this project, the students will need to understand the nitrogen cycle and that they will be diagramming the nitrogen cycle for themselves and then creating a reference diagram for the class to use while they are working on this project.


If the students are creating a classroom nitrogen cycle diagram then you will need a large sheet of paper and markers (or a whiteboard or smartboard).

Student Prerequisites

  • Students should be able to explain a natural cycle
  • Students should understand that chemical elements, like nitrogen, exist in a closed system. This is the basis for the key concept of conservation of mass. Nitrogen can neither be created nor destroyed, but it is cycled

Time Needed

Two class periods (one for viewing and creating individual nitrogen cycles, one for creating the class nitrogen cycle)

Doing the Activity

Watch the video.

After watching the video students should create their own diagram of the nitrogen cycle and answer questions about the video.

Students should share diagrams with each other and then make a classroom nitrogen cycle diagram- to be used as a reference as this project moves forward.



Have your students explain their nitrogen diagrams to each other, have students write down the questions that they have about the nitrogen cycle and their diagrams.

Having listened to Dr. Ivan Fernandez’ video it will be possible for students to think about the seasonality of nitrate flushing into a watershed stream. Have students draw a year-long time series graph of nitrogen, as nitrate, flushing out of a watershed. This should be a ‘back of the envelope’ graph—a sketch with no real numbers or data, but just showing the pattern they would expect.


Have students explain the nitrogen cycle to their parents, siblings or another class.

Lesson Extensions and Supplements

Students read: Hubbard Brook Research Foundation’s Nitrogen Pollution: From the Sources to the Sea (2-page fact sheet, there is also a 29-page document available)

Students read: Ecological Society of America’s “Issues in Ecology: Human Alteration of the Global Nitrogen Cycle: Causes and Consequences”