STUDENT INSTRUCTIONS

The cultivation of agricultural fields has resulted in the loss of soil carbon, which was part of the soil organic matter pool. Cultivation results in higher rates of decomposition relative to photosynthesis, causing this decline in soil carbon over time (See Figure Set 1 for more details). Consequently, soils depleted in organic matter have the potential to store carbon - and therefore remove carbon dioxide from the atmosphere - if agricultural practices cause organic matter concentrations to increase. In this case net primary productivity by plants would be greater than decomposition of the dead plant material. Scientists are studying ways to remove carbon from the atmosphere and store it in soil, thus increasing soil carbon amounts back to their original levels.

Take a look at Figure 4, including the legend, and interpret the graph. Make sure to read the labels carefully. Your instructor will describe each of the six agroecosystems to you, which are also described in the Figure Set 4 Background. Once you understand the graph and what the different agroecosystems are, work with a partner to answer the questions below. The student with the earlier birthday will write down the answers while the person with the later birthday will answer the questions out loud, telling the recorder what to write down.

• Which experimental ecosystems appear to have had the lowest soil carbon content in 1999?
• Why might some agricultural practices (such as growing perennial plants) be more effective at sequestering carbon in the soil? To address this question, consider various sources of carbon to the soil
• Did any agroecosystems decline in soil carbon content from 1989 to 1999?
• What is the best management strategy to build soil organic matter among annual agroecosystems (conventional, no-till, organic)?
• In addition to removing carbon dioxide from the atmosphere, what benefits do you think a farmer might gain from having higher organic matter content in his/her soil?