A watershed for all seasons

Background for Teachers

It is possible that the nitrogen cycle that the students drew in Unit 1 has little reference to watershed dynamics and seasonality. These two components- location and weather- are what takes the nitrogen cycle from an abstract diagram to a situated dynamic cycle. What happens to microbial activity when the temperature drops? Is the atmospheric nitrogen input constant throughout the year? Where does nitrogen go when it snows? This is a class discussion about what changes throughout the year that might affect N cycling.

Snow has been called the poor man’s fertilizer. In the early spring, as the snow melts, all of the nitrate stored in the snow goes into the soil and can be used to fertilize the local plants or is swept downstream to lower reaches of the watershed.

One potential effect of changing snowpack (built up snow on the ground) and snowmelt with a changing climate is a change in the timing or amount of nitrogen that flushes out of stream watersheds when snowpack melts in spring. If snowmelt is earlier, smaller, or absent, what would this mean for nitrogen budgets in watersheds? How would vegetation potentially be affected? Would this shift in nutrient delivery amount and/or timing affect biota, vegetation, or other watershed processes?

Goals

  • Students understand that there are seasonal changes to the N cycle

Outcomes

  • Students will add seasonality to their diagram of nitrogen cycling in their study site

Where does this lesson happen in the Project?

This activity is the first in Unit 2: Watersheds in Winter

Getting Ready

Using a topographic map (available from: http://nationalmap.gov/ustopo/index.html) outline your watershed and trace the path of waterflow from the tops of ridges to the final outflow. If possible, take pictures of your study site. Ask students, generally, to trace the flow of nitrogen through the picture (or ask for as many different paths as are possible). Ask, in general terms, how changing season might affect the cycling of nitrogen (refer back to the introductory video).

If you cannot visit your site prior to sampling then determine the slope and aspect (which way it faces… North? South? Northeast?) of your watershed or sampling area from topo maps, and determine the forest type and soil type from cover and soil maps or you can access general cover types and soil type from the following sources:

For forest cover type:

Use the National Atlas, select “Forest type” under the BIOLOGY heading on the right hand side of the screen. http://www.nationalatlas.gov/mapmaker

For soil type:

USDA’s web soil survey: http://websoilsurvey.nrcs.usda.gov/app/WebSoilSurvey.aspx

Materials

Student Prerequisites

Students need to understand that:

  • That living things need nitrogen to survive
  • That microbes make nitrogen available to other living things
  • What a watershed is
  • What an element cycle is

Time Needed

One to two class periods (one for general map and picture orientation, one for discussion and adding information to the diagram)

Doing the Activity

Using the maps, and/or your class’s own knowledge of your sampling location, determine the basic nature of your site.

This is a discussion, but can be augmented or initiated by reading Poor Man’s Fertilizer from Northern Woodlands http://northernwoodlands.org/outside_story/article/poor-mans-fertilizer.

The discussion is based on having students think about the seasons and rainfall from their own experience and applying those experiences to the N cycle.

  • When is it rainiest?
  • In terms of the nitrogen cycle what happens when the leaves fall?
  • In terms of the nitrogen cycle what happens when the snow melts? Where does the N end up if it gets in the stream?
  • Do microbes work all year long?
  • Do the different soil types have different microbe communities?
  • Does tree type affect nitrogen cycling?
  • What happens when the soil is saturated with water?

Add images to the diagram (from Unit 1, Activity 1) with up and down arrows, or other diagrammatic devices (fall=leaf fall means an increase in available N recycled from the breakdown of leaves…)

Assessment

Formative

Asking questions/speculate: Have your students ask questions about the N cycle that are specific to their study site (e.g. What kinds of tree are in/near my study site- do they have different N needs?)

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

Make some “If/Then, Because” statements about the N cycle in your study site. And defend the statements.

Have students answer: “Explain how nitrogen cycling changes through the seasons in your watershed.”

Summary

Add seasonal- and location-dependent conditions to the class N cycle diagram.

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 ESA issues paper. Students research nitrogen-fixing microbes- what conditions do they need to live, what kills them, etc.

Students design an experiment to investigate seasonal variation in stream nitrogen concentrations.

Ask students to alter a condition in their cycle diagram. For example, decrease the number of nitrifying bacteria or increase the input of nitrate, and discuss how this may change the cycle.

Watch Madeline Mineau’s nice Data Story video about her research on litterfall: https://vimeo.com/40567231

Lesson Resources