Outcomes : Students better understand how carbon sequestration (in agriculture) can contribute to lower carbon dioxide concentrations in the atmosphere, what C sequestration is, and the biological processes that lead to organic C deposition and loss in soils. In general, they should be able to connect organic C deposition in soil with other aspects of the C cycle.
What students do : They describe and interpret a figure from the paper listed below that clearly shows higher organic C content in the upper layers of “no till” soil compared to
“plowed” soil. The questions ask them to describe in words and with a concept map why/how no till agriculture may increase C sequestration in soil plus why this is important in regard to climate change.
What to pay particular attention to : o This aspect of the C cycle is quite challenging for many beginning biology students and therefore you will need to devote sufficient time for discussion. In discussion you should take students stepwise from photosynthesis by corn plants, deposition of stalks and leaves onto/into the soil, and then decomposition of the organic matter in soil via respiration by soil bacteria and fungi. The process they will likely least understand is that tillage of soils with machines breaks up the soil and allows more air to mix in, which enhances microbial respiration of the organic matter. (The “Directions” gives students a hint about this.) o “Soil organic matter” is a grab-bag term that includes organic matter once part of a living organism or produced by an organism. It can also be defined as organic matter capable of decay. Obviously, this is a complicated subject. To make it more concrete for students you could use images showing soil with high/low OM content
(ttp://vasatwiki.icrisat.org/images/9/91/Biological_indicators1.jpg), corn residue on a field
( http://www.ars.usda.gov/SP2UserFiles/Place/66570000/images/Rye_in_cornresiduecw.JPG
), or components of OM soil ( http://www.gly.uga.edu/railsback/1121SoilOrganicMatter.jpeg
).
o Emphasize the process of respiration by microbes during decomposition of soil organic matter – what is going on, why bacteria and fungi “do it”, where the process occurs, etc. It is difficult for students to make the connection between the atomic/molecular/cellular scales and the ecosystem scale plus the context of
“what happens where” in the bacteria and in the ecosystem. o No till agriculture: The data clearly show larger concentration of organic C in the
“no-till” soils. No-till farming has long been of interest to organic farmers. Crops are grown from year to year without disturbing the soil through tillage. According to Wikipedia: “With this way of farming, crop residues or other organic amenities are retained on the soil surface and sowing/fertilizing is done with minimal soil disturbance. Research and farmers' experience indicate that with continuous notill, soil organic matter increases, soil structure improves, soil erosion is controlled, and in time crop yields increase substantially from what they were under tillage management, due to improved water relations and nutrient availability”.

Logistics : o The “Directions” ask students to make a concept map. If you have not worked with concept maps in this class, ask students instead to draw box-and-arrow diagrams with the processes written over the arrows. They will need experience with concept maps/box-and-arrow diagrams before doing this exercise. o You can do this exercise in a small/medium sized class with small groups working together on the two steps (interpretation of the figure and drawing the concept maps/box-and-arrow diagrams). Lead a discussion between these two steps. You can collect the maps/diagrams, comment on them, and go over them again in the next class. In a large class, the “turn-to-your neighbor” approach would work for the figure description/interpretation step leading to an all class discussion. Then students could work together or alone to sketch out the diagrams on index cards that you collect. This will give you a good idea of their comprehension of the core ideas.
Resources : The data are from R. Lal. 1997. Residue management, conservation, tillage and soil restoration for mitigating greenhouse effect by carbon dioxide enrichment . Soil and Tillage Research 43: 81-107 (PDF attached).
Information specific to exercise : No-till agriculture; C sequestration; C in soils.
Units & Quantitative Skills: Mass/distance/area measurements (mg. Cm, ha) – what these man; measurement of soil organic C; figures as “language” students must lean figure description and interpretation; translating information from a graph to a drawing.
HIDDEN CURRICULUM
• Principles : Conservation of Matter (tracing C through the C cycle, in this case in an agricultural system)
• Processes: Generation (photosynthesis); Transformation (e.g. building of biomolecules within an organism), Oxidation (respiration, decomposition)
• Scale & Time : Interconnections between molecular, cellular, organismal, and ecosystem processes,
• Forms & Representations: Movement of C through ecosystem

First, have someone in your group read the title of the paper, the general description, and the figure legend (below the figure). Then work together to address the questions.
The graph on the next page is from a study on the effects of two different agricultural methods on soil organic matter content in two corn fields. The researcher wanted to compare the soils under “no-till” (soil is not disturbed by plowing) and “plowing” (soil is plowed before planting) practices. This type of change in agricultural practice is of great interest to ecologists because “no-till” agriculture may help reduce CO
2
concentrations in the atmosphere and therefore lessen global warming.
The graph is set up somewhat differently from what you are used to. They Y axis shows the depth of the soil in cm, going down from the soil surface. Therefore 0 cm is the soil surface, -10 cm is 10 cm below the surface, and so on. The X axis is concentration of carbon (C) in organic form in the soil. Units are Mg (a measure of mass; megagrams or
1,000,000 grams) of C per ha (hectacre or 2.2. acres)
1. Interpret the Graph:
First, look at and then interpret the figure in the following two steps:
Step One: Make sure you understand all parts of the figure – such as, the X and
Y axes, the legends, all the words - plus the specific question being addressed and the experimental design. Then describe the findings – the data. Write your description below plus any questions you have (e.g. things you don’t understand)
Step Two: Interpret the figure. Using what you know about carbon dioxide and plants, why did the scientist see this difference between the two agricultural practices?
In regard to global warming and rising carbon dioxide concentrations is the atmosphere, what is the implication of this finding? (So what?) Write your interpretation, the implication, and any questions you may have.
). [HINT: Tillage of soils with farming machines breaks up the soil and allows more air to mix in, which enhances microbial activity.]

The data are from R. Lal. 1997. Residue management, conservation, tillage and soil restoration for mitigating greenhouse effect by carbon dioxide enrichment . Soil and
Tillage Research 43: 81-107

2. Draw a concept map: Based on your understanding of the graph draw a concept map showing possible effects of “no-till” agriculture on CO
2
concentrations in the atmosphere.