Nitrogen: too much of a good thing

Background for Teachers

This is an adaptation of Dr. Serita Frey’s “Forest Ecosystem Responses to Chronic Nitrogen Additions” from the Ecological Society of America’s Teaching Issues and Experiments in Ecology ( As the students’ research projects progress the students will need to support their own claims with data, they will also need practice in reading graphs. This activity focuses on reading graphs that were used to support research into how additional nitrogen affects forests. Talk through the questions in the hypothetical first (if N additions do affect X, what would the resultant graph look like?) You may want to have your students draw a picture of the experimental set-up. This will help them understand what is going on in the experimental forest, and may help them think through their own experimental design for their own research (in Unit 4).


Students should:

  • Understand that plants are adapted to certain nitrogen conditions
  • Understand that additional nitrogen may not improve overall forest conditions
  • Understand that we cannot always measure nitrogen levels directly, but that we can measure effects
  • Read information from graphs
  • Make statements based on interpretations of graphs


  • Students will develop answers to the questions that follow the research graphs and become more fluent in reading graphs

Where does this lesson happen in the Project?

This is an optional activity that follows Activity 1 and 2 in Unit 1: Background Understanding.

Getting Ready

Discuss If/Then statements:

  • If N limits tree growth and N increases because of human additions, then what would you expect to see happen to trees?
  • If N is used by soil microbes and N increases because of human additions, then what would you expect to see happen to soil microbes?

Explain how tree growth and microbe biomass is measured (see teacher resource sheet).

  • Saturation
  • Net primary productivity- the rate of plant growth.
  • Tree photosynthetic capacity- the rate of carbon dioxide consumption by the leaves or needles of a tree.
  • Soil respiration- how much CO2 comes out of the soil.
  • Soil microbial biomass- the mass of all the microbes living in the forest soil.
  • Biota
  • Microbial


  • Student Nitrogen Additions packet
  • Graphs
  • Student Worksheet

Student Prerequisites

Students need to:

  • Understand that living things need nitrogen to survive
  • Understand that microbes make nitrogen available to other living things
  • Have general knowledge of a forested watershed/forest ecosystem

Time Needed

Two class periods (one for reading material and graphs and one for group interpretation)

Doing the Activity

Students will read: Selections from Frey’s “Forest Ecosystem Responses to Chronic Nitrogen Additions” and use the graphs provided to answer the open-ended questions.



Asking questions:

  • Students apply what they have learned from interpreting this research to conditions at their study site. For example, what kinds of trees dominate their site?


  • What is the microbial community like?
  • How has their site been affected by human-made increases in nitrogen?
  • What organisms thrive with increased N?
  • How does that alter the ecosystem?

Research other controls (temperature, wetness) on microbial communities.


Have students create a very brief (5-slide?) presentation on their interpretation of these research results to present to the school community (another class, post on Vimeo?)

 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 the ESA issues paper.
  • Students research nitrogen-fixing microbes- what conditions do they need to live, what kills them, etc.
  • Ask students to alter a conditions in their cycle diagram… decrease the number of nitrifying bacteria, increase the input of urea- discuss how this may change the cycle.
  • Students can calculate their “Nitrogen Footprint” (

Lesson Resources

  • Student Nitrogen Additions packet
  • Graphs
  • TIEE data set
  • TIEE Teacher data