Cycling of Matter: Nitrogen Cycle


You are probably least familiar with the nitrogen cycle, but it behaves similar to the carbon and water cycles. Nitrogen gas makes up nearly 78% of the Earth's atmosphere (see pie chart below), and its often found in organic compounds, like protein, and waste matter produced by organisms. As you explore the nitrogen cycle, refer to the diagram below; remember, with a cycle, there is no start or end point.

Air_Composition_Chart.png Nitrogen_Cycle_Compounds.png

Repairman.jpg1. As I said earlier, a lot of nitrogen exists in the air as a gas. Nitrogen gas, however, is not very useful to living organisms. To use this nitrogen, organisms must first add it to other elements and form new compounds. Only then, can the nitrogen be taken out and used by the organism. Some bacteria perform this function very well, taking nitrogen gas and changing it into ammonia. This process is known as nitrogen fixation. Ammonia is a compound used by plants and other bacteria to construct larger organic compounds. Nitrogen fixation can also result from lightning, which causes nitrogen gas and oxygen gas to react and form nitrate. Simply, to "fix" nitrogen is to combine it with other elements and construct compounds that are useful to living organisms. To help remember this, imagine nitrogen gas as something broken and useless. Then, bacteria or lightening fix it into something that is useful.

Deconstruct.png2. Some bacteria perform the opposite reaction of nitrogen fixation, and thus return nitrogen back to its gaseous state. This process is known as denitrification, and like nitrogen fixation, it is performed by some types of bacteria. Consider the prefix "de-"; we've seen it before in the word "decomposer". "De-" is often used in words that mean to break down or perform the opposite action, like "deconstruct". Therefore, it makes sense that denitrification would mean to remove nitrogen from a compound, like ammonia or nitrates, and return in back to the air.

toilet_2.jpg3. Nitrogen, like any other element found in organic matter, can travel up the food chain and pass through all trophic levels. Eventually, some of it is expelled as waste or released back to the environment when the organism dies. When an organism dies, bacteria will decompose the body and release nitrogen containing compounds into the soil. There, it can be used again by plants.

fertilizer.jpg4. Because plants need nitrogen to survive, farmers often add fertilizers rich in ammonia and nitrate to their crops. However, plants fail to use all this nitrogen before some of it is washed away by rain. When this occurs, excess ammonia and nitrate can drain into a river, lake, or ocean an cause harmful changes to an ecosystem.



Activities: Read the instructions before beginning each activity.

  • Activity 1: Class Zone: Nitrogen Cycle:
    1. Watch the animatio
    2. Complete in the nitrogen cycle handout; you may need to view the animation more than once to complete the handout.
    3. Tape the handout onto page 30 of your composition book.

  • Activity 2: BrainPop: Nitrogen Cycle
    1. Login to Brain Pop; Username: txau_ohenry, Password: aisd
    2. Watch the video at least 2 times
    3. Complete the graded quiz at the end; you may retake it if you are not happy with your score.
    4. Click "Email Your Results" and email it to

  • Activity 3: Answer the following question using complete sentences on pg 31 of your composition book
    1. Compare and contrast the nitrogen cycle with the carbon cycle by explaining how the two cycles are alike and different.
    2. Describe at least 3 similarities and 2 differences.

Extra Resources:

Extra Credit:

  • Option 1: Seach the internet and find the best soil pH for nitrogen fixation. Then explain why a low pH (high acidity) is bad for the nitrogen cycle. You may write you responses on a peice of paper, or on a MS Word document and save it to the shared drive.

  • Option 2: Over spring break, go outside and sketch a picture of an outdoor environment. Then draw how nitrogen flows through that environment. You may also take a picture with a digital camera and then use MS PowerPoint to add arrows and descriptions.