EarthLabs > Climate and the Carbon Cycle: Unit Overview > Lab 2: The Carbon Cycle > 2A: A Forest Carbon Cycle

The Carbon Cycle: What Goes Around, Comes Around

Part A: A Forest Carbon Cycle

Plants and Food Webs: Passing the carbon on!

The global carbon cycle cannot exist without plants and the food webs they support. Plants are autotrophs and thus make their own food in the form of glucose sugar. Heterotrophs, like ourselves, do not photosynthesize and must instead find and eat food. This food is made of proteins, carbohydrates, fats and oils, and nucleic acids. Heterotrophs break these complex organic carbon compounds down into smaller molecules and biosynthesize new organic carbon compounds.

Soil microbes, the smallest organisms in the food web, have one of the most critical roles in transforming and moving carbon compounds through food webs and ecosystems. Their role is threefold:

Examine the terrestrial food web image below, taking take time to follow the carbon. Then, check your understanding of how carbon moves through food webs by answering the questions below.



Checking In

Check your understanding of how carbon moves through food webs by answering the questions below. Select all the answers that are correct and then click the Check Answers button at the bottom of the list.

  1. What process brings in carbon in from the air into the terrestrial food web?
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  2. Carbon compounds move from plants to animals via the process(es) of...
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  3. Which process(es) moves carbon from above-ground food webs to the food web in the soil?
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  4. Which process(es) releases carbon from food webs back into the air in the form of carbon dioxide?
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Stop and Think

1. Examine the Terrestrial Carbon Cycle food web diagram again. Describe how the carbon from carbon dioxide molecules in the atmosphere can end up in a coyote. Use a diagram to help you explain your answer if you need to.


Throughout this module, you will learn about subsets of the global carbon cycle that operate at many different spatial scales. For example, the terrestrial food web you just examined operates on a much smaller spatial scale than an ecosystem. In this next activity, you will take on the role of carbon atoms moving through a forest ecosystem carbon cycle.

The Forest Carbon Cycle Game

You can't see the carbon flowing through the image of the forest carbon cycle above, but taking on the role of a carbon atom moving through a forest carbon cycle will help you understand how carbon atoms move around the Earth' system. Before beginning the forest carbon cycle game, there are two carbon cycle terms you need to know before starting the game:

Carbon Reservoir:A carbon reservoir is a place in the Earth System where carbon is stored. These include bodies of water - oceans, lakes, streams- soil, rock, atmosphere, fossil fuels, living things such as animals and plants. A carbon reservoir is a place in the Earth System where carbon is stored. Carbon reservoirs can be large or small and somewhere in-between.

Carbon Process: A carbon cycle process (also called a flux) causes carbon to move from one reservoir to another. Examples of carbon processes are photosynthesis, respiration and combustion. A carbon cycle process (flux) causes carbon to move from one reservoir to another. Processes in the forest carbon cycle game include photosynthesis, respiration, decomposition, ingestion, excretion, combustion, exudation and diffusion

Instructions:
Your teacher will set up the stations and tickets for the Lodgepole Forest Carbon Cycle game in your classroom. As you play the Forest Carbon Cycle game developed by the University Corporation for Atmospheric Research, you will take on the role of carbon atoms traveling through the various carbon reservoirs of a forest. The stations that you see posted around the classroom represent reservoirs of carbon typically found in a forest ecosystem. For example, if you are at the "ATMOSPHERE" station, then you are a carbon compound currently stored in the atmosphere reservoir. Carbon can stay in these reservoirs sometimes for short time scales (minutes to years) and in some cases, for much longer time scales (thousands to millions of years).


Step 1: You will be given a Lodgepole Forest Carbon Cycle Passport Chart and assigned a carbon reservoir station to start at. This chart will be the record of your journey through the carbon reservoirs. Fill-in the name of reservoir and the Earth's sphere(s) in your Forest Carbon Cycle Passport Chart. Two examples have been provided for you on the chart.

Step 2: Draw a ticket. The ticket will tell you where you will be going next and how you will get there. The ticket may also have special instructions such as "count to 100 and then choose a different ticket."

Step 3: For each round, record the required information in the Lodgepole Pine Forest Carbon Cycle Chart.

Step 4: When your class has finished, compare your carbon cycle journeys. How were they the same? How were they different?


Discuss

As a class, discuss:
  • Why weren't all the carbon journeys through the forest the same?
  • Carbon does not spend the same amount of time in each of the carbon reservoirs. Why do you think that is?

Next watch a short animation Carbon Dioxide and The Carbon Cycle. Click on the carbon cycle button and spend some time following the carbon dioxide molecules as they cycle through the various carbon reservoirs. Note that the combustion of fossil fuels and wildfires also adds carbon to the forest carbon cycle. Finally, answer the Checking In questions below.

Checking In

  1. In a forest carbon cycle, which process(s) moves carbon out of the atmosphere to another reservoir? Check all that apply and then click the Check Answers button at the end of the list.
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  2. Which process(s) moves carbon into the atmosphere? Check all that apply and then click on the Check Answers button at the end of the list.
    [CORRECT]
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How much carbon moves? Gigatons, Carbon Sinks and Carbon Sources

As you were moving from carbon reservoir to carbon reservoir in the forest, you represented only a few carbon atoms. In reality, enormous amounts of carbon continually move between reservoirs in a carbon cycle. How enormous is enormous? So enormous that the amount of carbon moving between reservoirs per year is expressed in Gigatons(Gt). One gigaton is equivalent to 1,000,000,000 metric tons. That's one billion metric tons in just one Gigaton(Gt)! It is hard to visualize just how much carbon is in one gigaton so lets use an elephant to help us. Although elephants vary in size and weight, the average weight of an Asian elephant is about four metric tons. If the average weight of an elephant is 4 metric tons, simply divide 1,000,000,000 metric tons by four metric tons. Thus, one Gigaton of carbon is equivalent to 250,000,000 elephants!


Next, consider this simple representation of the global carbon cycle on the left. The arrows represent the movement of carbon between four reservoirs: the ocean, vegetation and land, fossil fuels, and the atmosphere. The numbers within these arrows represent gigatons of carbon moving per year. Note that more carbon is moving into the vegetation and land reservoir than is moving out to the atmosphere. This tells you that the vegetation and land reservoir is currently behaving as a carbon sink A carbon reservoir that absorbs more carbon than it releases. Some examples of carbon sinks are forests, soil and the ocean. taking in more CO2 from the atmosphere than it is releasing to the atmosphere. A carbon source is a carbon reservoir that releases more carbon than it absorbs. is just the opposite. A reservoir that behaves as a carbon source releases more carbon than it takes in.

Key ideas to remember about carbon sinks and sources are:.

Examine the gigatons of carbon moving into and out of the atmosphere and then answer the Checking In questions below.



Checking In

  1. Is the ocean behaving as a carbon sink or a carbon source?
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  2. Are fossil fuels and land use (deforesting forests for agriculture) currently behaving as a carbon sink or a carbon source?
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  3. Which biosphere process(s) are responsible for moving 450 GT of carbon from the atmosphere into the vegetation and land?
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  4. Which biosphere process is responsible for moving 439 GT of carbon from the vegetation and land into the atmosphere?
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  5. Which chemical process is responsible for moving gigatons of carbon from fossil fuels into the atmosphere?
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A carbon cycle out of balance?

Scientists consider the carbon cycle to be in balance when the total amount of carbon moving into vegetation and oceans is equal to or greater than the amount of carbon going into the atmosphere. Examine the "Gigatons and The Carbon Cycle" diagram again and then answer the Stop and Think questions below.

Stop and Think:


2. Does the carbon cycle in this diagram appear to be in balance or out of balance? Use specific evidence from the diagram to support your answer.

3. Imagine that we could go back to pre-industrial times before fossil fuels were being burned for energy. Would a pre-industrial carbon cycle be in balance or out of balance? What makes you think so?

Extensions (Optional]

Research the latest research! New research on the carbon cycle, climate and the environment is on-going. You can use ScienceDaily and Phys.org to research recent research on ecosystem carbon cycles by using combinations of the following tags: ecosystem, carbon cycle, forests, soil nutrients, soil microbes, gigatons. Here are two examples:

World's forests' role in carbon storage immense, research reveals

Predators affect the carbon cycle, study shows.

Video:

Watch The Carbon Cycle


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