EarthLabs for Educators > Climate and the Carbon Cycle > Lab 5: Soil and the Carbon Cycle

Lab 5: Soil and the Carbon Cycle

The lab activity described here was developed by Candace Dunlap of TERC for the EarthLabs project.

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Investigation Summary and Learning Objectives

Students use animations and short videos to learn the basics of decomposition and microbial soil respiration and their relationship to the carbon cycle. They design and carry out a laboratory experiment on how soil respiration rates might be influenced by one of the following climate-related variables - temperature, soil moisture, soil nutrients, soil fertility, and soil type. Following that, they learn about permafrost, a frozen soil covering vast areas of the the Arctic and sub-arctic regions, that is beginning to thaw. They will consider possible carbon cycle feedback loops to a warming atmosphere if the permafrost continues to thaw.

After completing this investigation, students will be able to:

For more information about the TOPIC, read the section titled Background Information under Additional Resources below

Activity Overview

In Part A: Students learn the basics of decomposition and microbial soil respiration and how these processes influence the carbon cycle. To better understand the impact of climate change on the carbon cycle, they investigate the effects of a change in an environmental variable (e.g. temperature, soil moisture, soil nutrients, etc) on microbial soil respiration using a simple soil respiration chamber and lime-water.

In Part B: Students learn the basics about permafrost soil and what might happen as this permanently frozen soil begins to thaw due to a warming atmosphere.

Printable Materials

To download one of the PDF or Word files below, right-click (control-click on a Mac) the link and choose "Save File As" or "Save Link As."

Teaching Notes and Tips

In Part A:

1. Think about having students do the first section on decomposition etc at home. After a brief class discussion, you can begin the microbial soil respiration experiment.

2. The inquiry-based "soil microbe" respiration experiment - important things to consider.

  • Soil temperature
  • Soil moisture (dry, moist, water-logged)
  • Soil fertility and organic carbon (how much or type of organic matter decomposing in the soil)
  • Soil nutrient availability* (N-P-K : nitrogen, phosphorus and sulfur)
  • Soil oxygen availability (aerated soil versus compacted soil, moist soil versus water-logged)

Special Materials to order from school science catalogs such as Carolina Biological, Science Kit, Flynn etc.

  • Here is a recipe for limewater.
NOTE: There is a company making very safe and easy-to-use quantitative soil respiration kits. However, they are very expensive. Here is the link to their store if your school is interested in purchasing a product like this. The CO2 basal respiration test is cheaper than the CO2 burst test, which requires an expensive digital color reader. Here is the link

You will find more specific teaching tips and explanations below. Tips are in bold italics.

Experimental Variables:

  • Soil temperature (Students may suggest refrigerators, lamps, freezers etc as part of their design)
  • Soil moisture (dry, moist, water-logged) Water-logged soil and oxygen availability are linked. Water-logged or flooded soil environments include rice paddies, wetlands (swamps, marshes, and bogs); compacted soils; and post-rain soils. Water-logged soils often lead to anoxic (without oxygen) conditions because oxygen is only slightly soluble in water and diffuses much more slowly through water than through air. In lab 5-B, students will read about permafrost soil which is often water-logged. Anoxic conditions there favor a special group of bacteria (Methanogens) that produce methane instead of CO2 when they respire.
  • Soil fertility and organic carbon (how much or type of organic matter decomposing in the soil) Students may suggest adding "compost" food such as banana and apple peels. They could eliminate the sugar and use plain water. As PI, you can bring this up and discuss.
  • Soil nutrient availability* ( N-P-K: nitrogen, phosphorus and sulfur) *Note: Use a soil nutrient test kit to find soils outside that are low in nutrients. Fertilizer can be easily added to half of the soil to create high nutrient soil. Another possibility is to use Venus Fly-trap soil(low in nutrients- especially nitrogen). Students should test the low and high nutrient soils they will be using to ensure that the nutrient levels are indeed different.
  • Soil type (sandy, clay, rich with humus, etc.)
  • Soil oxygen availability (aerated soil versus compacted soil, moist soil versus water-logged) (Refer to soil moisture explanation above) Students can add perlite to aerate the soil and/or not compact it by tamping it down.
  • Soil pH (acidic, alkaline, neutral) To change the pH, they will need to add chemicals to the water before they add it to the soil in the bottle. Baking soda, lime water etc will make the water (and thus the soil) alkaline. Diluted vinegar will make water and soil acidic. Students should definitely use a soil pH meter or if unavailable, pH papers.
  • Other (for example, adding important soil fungi like Mycorrhiza) Mycorrhiza are fungi that are essential to plant nutrition.

  • 2-4 20 fl oz(or approximate) plastic water or soda bottles per lab group based on their design.
  • Duct tape + scissors
  • Well-dried soil (top soil and/ or compost) – enough to fill 2-4 bottles to the top - per group
  • DISTILLED or BOILED filtered water that has sugar dissolved in it.
  • Two – four collecting vessels for the limewater: A variety of vessels will work: plastic bottles, small transparent collecting jars, flasks or large tests tubes.
  • 50ml – 100 ml graduated cylinder
  • 50 ml Limewater per collecting vessel. Safety Alert: Do not get in eyes, on skin or on clothing. Wear safety goggles and a lab apron. Wash hands after handling.
  • You can purchase limewater in liter bottles from any science supply company or make your own. Plan on at least a liter per class. Limewater has a limited shelf life of 1-2 years.
    • Here is a recipe for limewater.
    • Safety alert! Lime water should be treated as a eye irritant. Safety goggles should be worn.
    • Materials Safety Data Sheet for Limewater
  • Gas delivery mechanism: Number 4 one-hole stopper, glass tube and rubber tubing. One per bottle. These should be put together before class begins. Make sure that the glass and rubber tubing are clean. Any residual chemicals from other experiments may bias the results.
  • A bottle of distilled water or boiled filtered water with sugar added (5 grams sugar per 500 ml water) for the class. The sugar provides an immediate food source for the microbes. It takes microbes a long time to digest the lignan and cellulose in dead plant material in the soil.
  • Square of aluminum foil – one per collecting vessel.
  • Soil testing kits and meters (optional but good to have) You will want to have a variety available(pH, moisture, soil temperature, soil nutrient N-P-K etc.) Some experiments (such as the pH experiment) definitely need pH papers or meters.
  • Experimental protocol:

    Note: Don't forget to make the sterile soil versus non-sterile soil control ahead of time.

    1. Make a cut about 5 cm from the top of the bottle. Using scissors, cut approximately 8/10 of the way around the bottle. DO NOT cut all the way around. You want to be able to fold the top of the bottle back as you fill the bottle with soil and sugar water. Then, use duct tape to tape the bottle closed.

    2. Fill the bottles approximately 1/3 to the top, add 20 ml of sugar water and then gently tap the bottle on the counter once to tamp down the soil.

    3. Add more soil to 2/3 full and add 20 ml of sugar water. Tamp down the soil again.

    4. Fill bottle with soil up to the cut rim. Add 20 ml of water. Close and seal the bottle with duct tape.

    5. Insert the #4 one-hole stopper gas delivery apparatus into the bottle top. Insert the tubing into the test tube (or small jar) with the 50 ml of lime water. Make sure the end of the tubing is in the lime water. Use the aluminum foil to cover the test tube/jar.The collecting jar/test tube must be covered so CO2 from the air cannot dissolve in the limewater and the water in the limewater cannot evaporate. Both would bias the results and be sources of error.

    6. Observing and Drawing a Conclusion: Make observations every day for the next few days. Look for tiny "flakes" of white calcium carbonate and/or cloudiness forming in the lime water. Stop the experiment when you feel you have enough experimental evidence to answer your research question. Plan for at least 2-5 days for results.

    In Part B:

    This section is fairly straight forward. Consider letting students examine samples of frozen soil, semi-frozen soil and thawed soil. If any students chose temperature's effect on soil respiration rates to carry out in the Lab 5A soil respiration experiment, they can become "experts" for the class and share their results.

    Student Notebooks

    Student notebooks are optional. Here are just a few suggestions for what to include in student notebooks for Lab 5:

    Note: You may want to separate actual data collection and analysis into a Lab Notebook.


    There are several options for assessments of students understanding of material produced in this Lab. Teachers can choose from the following list or create their own assessments.

    Assessment Options:

    1. Assess student understanding of topics addressed in this investigation by grading their responses to the Stop and Think questions.
    2. Assess lab reports for the soil respiration lab.
    3. Assess permafrost feedback loop drawings in Lab5B
    4. Written Test for Lab 5 (Microsoft Word 2007 (.docx) 76kB Feb11 14) (Answer Key (Microsoft Word 2007 (.docx) 140kB Feb11 14)

    National Science Teaching Standards

    Lab 5 supports the following Next Generation Science Standards (NGSS):

    Science and Engineering Practices

    2. Developing and Using Models

    3. Planning and carrying Out Investigations

    4. Analyzing and Interpreting Data

    Disciplinary Core Ideas

    ESS2.A Earth's Materials and Systems

    ESS3.D Global Climate Change

    LS2.B Cycles of Matter and Energy transfer in Ecosystems

    Cross-Cutting Concepts

    2. Cause and effect

    4. Systems and System Models

    5. Energy and Matter

    Additional Resources

    Background Information

    Basics: Nature Scitable article on the basics of soil carbon storage

    Understanding Soil Microbes and Nutrient Recycling

    Soil Quality and Respiration


    If you would like to learn more about soil and permafrost, here are several excellent on-line lectures:

    Secrets of the Soil from Berkeley Lab

    Permafrost Carbon and Climate Feedbacks in a Warmer World

    Biogeochemistry of the Arctic System

    Soils, Microbes, and Landscape Changes

    Content Extension

    These resources review how the new science of "metagenomics" is revealing new understanding of how microbial populations change with changing environmental conditions.

    Metagenomics and Our Microbial Planet

    Preparing for a Thaw: How Arctic Microbes Respond to a Warming World

    These resources are on changing agricultural practices to keep carbon in the soil. How the Soil Carbon Coalition is changing agricultural practices that will build carbon-rich soil This site has background information, videos, and links on initiatives to build carbon-rich soil.

    If you live in a climate with seasonally frozen soil, try this Frost Tube Experiment. You can share data collected with other students at the GLOBE network.

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