Lab 5: Soil and the Carbon Cycle

In Part A, students use videos and animations to investigate the relationship between soil and the carbon cycle by focusing on soil carbon storage and the role of microbes in decomposition and soil respiration. To better understand the impact of climate change on the carbon cycle, they investigate the effects of temperature, an important abiotic climate variable, on soil microbial respiration using a simple soil microbial respiration chamber and limewater. Finally, students investigate what ranchers are doing to create carbon-rich healthy soil, a process that has the potential to mitigate climate change.

Time estimate: 2 to 4 50-minute class periods

In Part B, students use videos to investigate permafrost, a carbon-rich frozen soil with the potential to further unbalance the carbon cycle if it thaws.

Time estimate: 1 to 2 50-minute class periods

Equipment and materials and for the the temperature and soil respiration rates experiment in Lab 5A:

Equipment:

Each lab team will have 1-2 soil respiration bottle set-ups depending on the experimental design.

• Soil Microbial Respiration Apparatus (SMRA) is comprised of the following:
• Soil microbial respiration chambers (20 fluid oz plastic soda bottles )
• CO₂ gas delivery apparatus- one per bottle.MAKE THESE AHEAD OF TIME
  • #4 one-hole stopper
  • glass chemistry tubing - hollow (5mm) diameter
  • amber rubber tubing - 3/16th inch diameter
• Collecting vessels (small jars, flasks, beakers, etc.) with a square of aluminum foil.
• 50-100 ml graduated cylinders (to measure limewater and sugar water)
• Celsius thermometers (to test ambient temperatures)
• Vernier CO₂ probe (optional) This probe will give you quantitative data of respired CO₂ over time. If using a probe, you will need to make sure that you have a soil respiration chamber bottle with a wide enough mouth for the probe. Bottles like VitaminWater work well. Expect to see a lot of variability in the CO2 data but with an upward trend. Because there are so many different poplulations of bacteria in the soil, not all will respire at the same rate or at the same time.
• N-P-K nutrient soil testing kit (optional)
• Equipment for cooling/freezing/warming the soil bottles. This would include what you have available such as refrigerator, freezer, ice chest, heat lamp, crockpot, warming oven, sunlit window etc.)

Materials:

• Limewater (calcium hydroxide- Ca(OH)₂ in solution ) — 50 ml per collecting vessel. Plan on 1 liter of limewater per class. You can purchase limewater already prepared or you can make your own from Ca(OH)₂ and distilled water. Safety Alert: Do not get in eyes, on skin or on clothing. Wear safety goggles and a lab apron. Wash hands after handling.
• Materials Safety Data Sheet for Limewater
• Duct tape and scissors (to cut and tape the plastic soda bottles for the respiration chamber bottle) NOTE: The teacher may want to cut the bottles ahead of time.
• Soil which is carbon-rich and very dry. Top soil purchased at gardening store works well. Do NOT use potting soil. Make sure you have enough to completely fill all of the soda bottles you will be using in your class.
• Sugar water — 60-70 ml from a class stock solution of 5g of sugar dissolved in 500ml of distilled or boiled water (you want to eliminate any microbes that might be in the water)
• Square of aluminum foil — one per collecting vessel.

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• Lab 5 Stop and Think Questions:
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General Recommendations:

• If unfamiliar with the temperature and soil respiration experiment in Lab 5A, consider a practice run before implementation.
• Print out any paper-based materials before starting the lab.
• Have students keep a lab notebook or journal to record important notes, questions, data and findings.
• Consider FLIPPING parts of the lessons. This will save you class time and reduce the need to have computer access in your classroom. For example, students could do the soil carbon basics section in Lab 5A for homework. The next day, they can come in and do the soil microbial respiration experiment.
• Discussion questions, Checking In questions and Stop and Think questions can be adapted and used in a variety of ways based on teachers' needs. For example, some questions might make great "DO Now" activities as students enter the classroom or great "exit quizzes" as students leave.
• You may want to spend time projecting graphs and important images on the board and going over the elements (e.g. units of measure, variables on axis, trends, color schemes etc) with them.
• In many of the Optional Extensions sections throughout the carbon cycle module, students are prompted to "research the latest research" on important carbon cycle topics pertinent to the lab section they are working in. Rich conversations can ensue when students go to ScienceDaily and/or Phys.org - News and Articles on Science and Technology to find abstracts of new research that supports, contradicts or enhances current understanding on how the carbon cycle works.

Specific recommendations for the the experiment "Temperature's Effect on Soil Microbial Respiration Rates" in Lab 5A.

• There are multiple ways to adapt this experiment to fit teacher's specific needs. For example, you may decide to fill test tubes with soil instead of bottles. A change like this would also require you to reduce the amount of sugar water and use a different size rubber stopper.
• In Lab 2C, students did a short investigation on blowing into a flask of limewater and observing a chalky white precipitate appear. You may want to review this with students.
• As written, the class decides together how to use the soil microbe respiration apparatus (SMRA) to investigate the effect of temperature on soil microbial respiration. For example, some students may want to expose the soil microbes in the bottle to freezing or very cold temperatures. Others may want to expose the soil microbes to temperatures that are warm or even hot. How they do this is part of an important class discussion and will depend on students' ideas and supplies available to the teacher. For example, if a refrigerator (cold), a freezer and a crockpot is available, students can but the SMRAs into three very different temperatures in addition to being exposed to room temperature as a control.
• Students need to understand that it is the microbes living in the soil that are doing the respiring and not the "soil" which is made of minerals, partially decomposed remains of living things (organic matter)water and air. To make sure that students understand the above, teachers should consider setting-up a class control using soil that has been sterilized in a microwave. To do this, spread moist soil on a flat dish, and then heat it on high for two minutes or until it's steaming. Then set up one bottle with sterile soil and one with non-sterile soil. Prepare and set-up these controls a few days before beginning the experiment and let students observe. Safety tips: It is best not to let students do this.
• Most soil respiration experiments typically require harsh chemicals (ex. sodium hydroxide, HCL) and chemical titration to get quantitative measurements of respired microbial CO₂. This lab is a qualitative lab - i.e. students will need to compare the cloudiness and amount of calcium carbonate precipitate in the limewater in their experimental jars/test tubes. It is OK if some results are inconclusive. NOTE: Teachers and students may come up with creative ways to quantify the amount of calcium carbonate that forms when CO₂ is bubbled through the limewater.
• Based on their experimental design, some students may see discernible results the next day whereas others will not see results for 2-4 days. These differences should be temperature dependent.
• Show students how to set up the SMRA bottles as described below.
• Cutting the bottles: (NOTE: Teachers may want to cut the bottles ahead of time) 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.
• Filling the bottles with soil and sugar water:

1. Fill the bottles approximately 1/3 to the top with soil, add 20 ml of sugar water and then gently tap the bottle on the counter once to tamp down the soil.
2. Add more soil to 2/3 full and add 20 ml of sugar water. Tamp down the soil again.
3. Fill bottle with soil up to the cut rim. Add 20 ml of water. Close and seal the bottle with duct tape.

• Connecting the CO₂ gas delivery apparatus:

1. Insert the #4 one-hole stopper/tubing gas delivery apparatus into the bottle top.
2. Next, insert the tubing into a container (small jar, flask etc) with the 50 ml of lime water. Make sure the end of the tubing is immersed in the limewater.
3. 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.

• Observing and Drawing a Conclusion: Make observations every day for the next few days. Students should look for tiny "flakes" of white calcium carbonate and/or cloudiness forming in the limewater. Stop the experiment when you feel there is enough experimental evidence for students to be able to draw a conclusion on the effect of temperature on the rates of soil microbial respiration. Plan for 2-5 days for results. Students should be able to talk about this experiment when they move on to Lab 5B which is about permafrost, a carbon-rich, perennially frozen soil.

• Have students write down the learning objectives for Lab 5.
• Drawings and notes from videos, animations, discussions and activities
• Have students record answers to all Stop and Think questions.
• Have students record answers to Discussion questions.
• Have students record diagrams they have drawn, with labels and a short description of what the diagram represents.
• Have students record important hands-on or minds-on activity components. This could include research questions, data, tables, observations, drawings, graphs, and conclusions.
• Have students write down any questions they still have about the content covered in this lab.

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.

1. Assess student understanding of topics addressed in this investigation by grading their written responses to the Stop and Think questions or by using Stop and Think questions as part of whole-class or small group discussions.
2. Written Test for Lab 5:
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3. Many lab sections have a "learning assessment" component. While carrying out the learning assessment activity, students are learning important concepts and demonstrating their understanding in a performance assessment. Lab 5A has a performance assessment which could be assessed with a formal lab report.
4. Anything that students create such as graphs, diagrams, and conclusions would serve as good assessments.

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

Examples of how students engage with the standards:

Go to Next Generation Science Standards.

Background Information

Basic soil science:

• Berkeley Lab has a wide range of articles and videos on soil science. Here is an article about their soil research: The Great Escape- How soil microbes protect us from carbon emissions
• Nature Scitable article on the basics of soil carbon storage
• Understanding Soil Microbes and Nutrient Recycling
• Soil Science Society of America

Permafrost: 

• National Snow and Ice Data Center - Frozen Ground
• Permafrost Carbon Network
• Secrets of the Soil from Berkeley Lab
• A Simpler Way to Estimate the Feedback Between Permafrost Carbon and Climate
• Permafrost Carbon and Climate Feedbacks in a Warmer World and Soils, Microbes, and Landscape Changes NOTE: There are many other videos like this on the Association of Polar Early Career Scientists (APECS) website.

Content Extension

• Investigate the science of "soil meta-genomics," which is revealing new understanding of how soil microbial populations change with changing environmental conditions. Here is an example: Preparing for a Thaw: How Arctic Microbes Respond to a Warming World and Metagenomics and Our Microbial Planet provides excellent background on the science of metagenomics.
• If you live in a climate with seasonally frozen soil, try this Frost Tube Experiment activity withour students. Your students can share data collected with other students at the GLOBE network.