Which reservoirs and processes in the biosphere are part of the carbon cycle?

Carbon on the Move!

In Lab 1, you learned about the molecular nature of carbon compounds and the carbon cycle, and its relationship to other biogeochemical cycles such as the nitrogen cycle. In this Lab section, you will learn how carbon compounds move throughout a terrestrial and aquatic food webs. Then, you will take on the role of a carbon atom moving throughout a Lodgepole Pine forest ecosystem.

Organisms in food webs pass the carbon on

The global carbon cycle cannot exist without plants and the food webs they support. As autotrophs organisms capable of synthesizing their own food from inorganic substances using light or chemical energy; green plants, algae, and certain bacteria are autotrophs. , plants make their own food in the form of glucose sugar. Heterotrophs organisms that cannot manufacture their own food and instead obtain their food and energy by taking in organic substances, usually plant or animal matter; animals, protozoans, fungi, and most bacteria are heterotrophs., like ourselves, do not photosynthesize and so must find and eat food made of carbon compounds such as proteins, carbohydrates and lipids. Heterotrophs break these complex organic carbon compounds down into smaller molecules and use the carbon atoms to 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:

• When soil microbes (bacteria and fungi) decompose dead material, they break down larger carbon compounds into smaller compounds. This process releases CO2 to the surrounding soil and to the atmosphere in a process called soil respiration carbon dioxide produced when organisms living in soil carry out cell respiration; includes respiration by soil bacteria, fungi and fauna, and the cells of plant roots.
• Soil microbes move carbon down into the soil where it can be stored for hundreds of years.
• Soil microbes change nitrogen compounds into forms that can be used by plants.

1. Examine the terrestrial food web image below, taking time to follow the carbon. Remember that carbon atoms move as part of a carbon compound, not as single atoms.
2. Then, check your understanding of how carbon moves through food webs by answering the Checking In and Stop and Think questions below.

A Terrestrial Carbon Cycle

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.

Stop and Think

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

A Forest Carbon Cycle. Source: NOAA

Carbon atoms move through a Lodgepole Pine Forest ecosystem

The food web carbon cycle you just investigated operates on a much smaller spatial scale than a larger, more complex forest ecosystem carbon cycle. In the activity below, you will take on the role of carbon atoms moving through various carbon reservoirs of a Lodgepole Pine forest ecosystem. Remember that carbon atoms move as part of a carbon compound, not as single atoms. Before beginning, there are two carbon cycle terms you need to know:

Carbon Reservoir:a place in the Earth System where carbon atoms are stored; examples include organisms, bodies of water, soil, rock, atmosphere, and fossil fuels. A carbon reservoir is a place in the Earth System where carbon atoms are stored. Carbon reservoirs can be large like an ocean, microscopic like bacteria and somewhere in-between.

Carbon Cycle Process causes carbon to move from one reservoir to another; examples are photosynthesis, respiration, and combustion. (also called a flux): A carbon cycle process 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.

Activity instructions: 
Your teacher will set up the stations and tickets for the Lodgepole Pine Forest carbon cycle game. The stations 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 atom in a carbon compound currently stored in the atmosphere reservoir. Carbon can stay in these reservoirs for shorter time scales (minutes, days, months, decades) up to longer timescales of hundreds, thousands and even millions of years.

Key questions for this activity are:

• What carbon reservoirs do carbon atoms pass through as they move through a Lodgepole forest carbon cycle?
• What processes drive the movement of carbon atoms from one reservoir to another?
• Why do you think carbon atoms can stay for longer periods of time in some reservoirs but shorter times in others?

1. Begin the Lodgepole Pine Forest carbon cycle game by going to any forest carbon reservoir or to one assigned by your teacher.
2. Draw a ticket from the first station. The ticket describes the carbon reservoir you are currently in, the carbon reservoir you will move to next and the process that moves you.
3. Fill-in your PASSPORT CHART with the information from the ticket, including any time scale information (example, 10 days, 100 years) if included. You are free to take an educated guess if no time scale information is given;
4. Identify whether the carbon reservoir is part of the BIOSPHERE (B) or the GEOSPHERE(G).
5. Follow any special instructions on the ticket such as "count to 100 and then go to......"
6. For each round, record all required information in the Lodgepole Pine Forest Carbon Cycle PASSPORT CHART. 
    
7. Do as many rounds as time allows and then share your carbon journey with your peers in the discussion below.

Discuss

As a class, discuss:

• Compare your carbon journey with the carbon journeys of your classmates. How were they the same? How were they different? What do you think accounts for these similarities and differences?
• In the next section (LAB 2B), you will be investigating the Global Carbon Cycle which operates on a much larger spatial scale than a forest ecosystem. New reservoirs such as oceans, sediments and volcanoes are included. Identify any carbon cycle processes you would expect to see operating on a global spatial scale that also operates in a forest carbon cycle.

Next watch the short animation Carbon Dioxide and The Carbon Cycle

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 Lodgepole Pine forest carbon cycle. Finally, answer the Checking In questions below.

Checking In

Large amounts of carbon can move between reservoirs each year

The average mass of an Asian elephant is 4 metric tons. Source: Wikicommons.

As you were moving from carbon reservoir to carbon reservoir in the Lodgepole Pine forest, you represented only one carbon atom. In reality, the mass of carbon atoms that continually moves between reservoirs is enormous! How enormous is enormous? So enormous that the mass of carbon moving between reservoirs per year is expressed in Gigatons one gigaton(Gt) is equivalent to 1,000,000,000 (one billion) metric tons; also written as gigatonnes; one gigaton is equivalent to one petagram. (Gt/y). One gigaton (Gt) of carbon is equivalent to 1,000,000,000 (one billion) metric tons of carbon. Lets use an elephant to help us visualize just how much carbon is in one gigaton. The average mass of an Asian elephant is about four metric tons. If the average mass 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 the mass of 250,000,000 elephants!

Carbon reservoirs can be carbon sinks or carbon sources

Gigatons of Carbon Moving per Year (Gt/y) Source: IPCC 2004

Next, consider this simple representation of the global carbon cycle pictured 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 (Gt) 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 behaving as a carbon sink a carbon reservoir that absorbs more carbon than it releases; examples include forests, soil and the ocean. . A carbon source a carbon reservoir that releases more carbon than it absorbs; fossil fuels extracted from deep within the ground are a carbon source. is just the opposite. A reservoir that behaves as a carbon source releases more carbon to the atmosphere than it takes in and stores.

Examine the amount of gigatons of carbon moving into and out of the atmosphere in the global carbon cycle image above and then answer the Checking In questions below.

Checking In

A carbon cycle out of balance?

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

Stop and Think:

Gigatons of Carbon Moving per Year (Gt/y) Source: IPCC 2004

1: 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.

2: 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?

Optional Extensions

Want to learn more about ecosystem carbon cycles, carbon sinks and sources? Check out this resource below:

• 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 and carbon sinks and sources by using combinations of the following tags: food webs, ecosystem, carbon cycle, carbon sinks and carbon sources. Here are some examples of the types of recent research you can find:
  • Read World's forests' role in carbon storage immense, research reveals
  • Read about a Carbon sink found underneath world's deserts

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