BACKGROUND

Many agricultural fields in temperate regions have been cultivated for hundreds of years. In these fields, much of the carbon stored in soil has been lost to the atmosphere due to enhanced decomposition during cultivation (See Figure Set 1). Under conventional crop management, soil carbon loss eventually levels out & remains at a steady state at approximately 50% of original carbon levels. However, certain management strategies can slowly increase soil carbon content back towards original levels, which is called soil carbon sequestration. This can occur when net primary productivity due to plant growth exceeds respiration of organic carbon by soil biota. See Schlesinger (1999 - pdf included) or Post and Kwon (2000) for more information about carbon sequestration on agricultural soils. A simple explanation of carbon sequestration can be found at the U.S. EPA website: http://www.epa.gov/sequestration/local_scale.html.

The data on soil carbon sequestration in Figure 4 were collected from a Long Term Ecological Research (LTER) Experiment at the W.K. Kellogg Biological Station in southwest Michigan. In this experiment, six ecosystems were established in 1989 and compared from 1989 to 1999 to characterize their ability to sequester carbon in the soil. These systems were compared to conventionally tilled (physically turned over) agricultural fields in which soil carbon concentrations were hypothesized to remain relatively unchanged over time. The first three ecosystems were cultivated with annual crops, in a corn-soybean-wheat rotation.

• The "Conventional" ecosystem received both soil tillage and pesticides to control weeds and was fertilized to maximize crop yields.
• "Organic" refers to an ecosystem that received no fertilizer or pesticides, but tillage was used to control weeds and legume (nitrogen fixing) cover crops were used as nitrogen fertilizer sources.
• "No-Till" refers to an ecosystem in which the soil was not disturbed after the start of the experiment in 1989. Instead, weeds were controlled using pesticides.

The last three agroecosystems contained perennial plants and no tillage.

• "Alfalfa" is a perennial nitrogen fixing plant that is grown for animal feed. Alfalfa was planted in 1989 and the above ground growth was cut and removed from the fields 3-4 times per year. Although perennial, alfalfa was replanted every 5-7 years to maintain vigorous growth, as older plants died and weeds invaded the fields.
• Successional communities are those that are left fallow and receive no human induced disturbances. "Early Successional" ecosystems were last tilled in 1988, but were then left undisturbed, except for occasional burning to prevent trees from growing in the experimental plots.
• "Poplar trees" were first planted in 1989, and were harvested after 10 years of growth. Trees were cut and used as biofuel for electricity generation. After harvest, the trees re-sprouted and will be harvested a second time for the same purposes.