STUDENT INSTRUCTIONS

Warm Up Exercise - Turn to your neighbor and take five minutes to answer the following question. You are allowed to use a calculator if you have one.

One morning, two old men are drinking coffee and are debating who contributed more carbon dioxide to the atmosphere during their lifetime. The first man (Fred) was a farmer and converted 50 hectares of prairie to corn fields 40 years ago (50,000 kg soil carbon per hectare), growing corn ever since while using draft horses for power. The amount of soil carbon in his fields declined by 50% during this period. The second man (Joe) was a carpenter, and drove an average of 130 kilometers round trip to work in his pickup truck 200 days per year for 40 years, getting an average of 6 kilometers per liter of gasoline. There are 0.72 kg of carbon per liter of gasoline.

• Who was responsible for more carbon dioxide emissions during their lifetime, Fred or Joe? If you feel stuck, start here: what is the source of carbon dioxide for Joe's and Fred's activities? Then, how would you start doing this calculation? What do you need to know?

Discuss the following questions with a neighbor. After each set of questions, your instructor will talk about the question set with the entire class before moving on to the next question set.

Question 1. Look at Figure 1a and make sure that you understand the X and Y axes. Next, describe the pattern displayed by the two lines.

• During what time period was the rate of change of C loss the highest? 
• What happens to the rate of loss over time? 
• How would you describe the pattern of change over the 40 years?
• Why do you think the decline in soil C levels off with time?

Question 2. If carbon is lost from the agricultural soil over time, this carbon has a source (comes from somewhere) and goes somewhere (sink). Prior to cultivation, one hectare (about two football fields) of agricultural land in the Midwest could have contained more than 50,000 kg of carbon, which is equivalent to the amount of carbon in about 20,000 gallons of gasoline.

• Where did that much carbon come from and how did it get into the top soil?
• During the cultivation of crop plants, do you think organic carbon was still entering the top soil?
• Since the grain (seeds of crop plants) is removed from the field for food, is there more or less carbon left on the soil surface compared to the original prairie?
• What plant parts other than the grain might remain in the soil as organic carbon?

Question 3. Soil contains many different types of microorganisms such as bacteria and fungi. There is a very common microbial process in which these microorganisms use organic carbon in the soil.

• What it is this process called?
• Why to microorganisms engage in this process?
• What ultimately happens to the organic carbon?

Question 4.

• How could tillage increase the rates of decomposition by soil microorganisms?
• What gas do these microorganisms require for cellular respiration?
• Would tilling the soil change the size of particles composed of organic carbon (e.g. dead plant material)?
• In agriculture, there are often periods when the soil is bare and no plants are growing. Does decomposition of soil organic matter by microorganisms stop when plants stop growing?

Question 5. Cellular respiration by microorganisms results in the conversion of soil organic carbon to carbon dioxide.

• Why is it important to monitor the amount of carbon dioxide in the atmosphere?