BACKGROUND

Agriculture and climate change are inextricably linked, as was shown in Figure Sets 1-4. Not only will climate change affect agricultural crop production, but agriculture is a primary source of several greenhouse gases. As shown in Figure Set 1, cultivation of undisturbed soils results in the loss of soil carbon. The production of nitrogen fertilizer, burning of fossil fuels by machinery and lime applications also emit carbon dioxide to the atmosphere. Fertilized agricultural soils contribute a substantial amount of nitrous oxide to the atmosphere. Methane oxidation in soil is lower in agricultural soils compared to adjacent forested areas. All of these factors must be examined simultaneously to understand the cumulative global warming potential of agroecosystems.

Many agricultural soils in temperate regions have been cultivated for many 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). Soil carbon loss eventually levels out and remains at a steady state under conventional crop management. However, soil carbon content can actually increase under certain crop management strategies, including conservation tillage, cover crop planting and perennial crop growth. Likewise, other management strategies such as reducing fertilizer applications can reduce the amount of greenhouse gases emitted during management activities. Taken together, the net global warming potential can be calculated for different agroecosystems. Negative global warming potential values indicate net decreases in atmospheric heat trapping potential and positive global warming potential values indicate net increases in atmospheric heat trapping potential.

The global warming potential (GWP) of five agroecosystems in the Long Term Ecological Research Experiment at the W.K. Kellogg Biological Station in SW Michigan were compared from 1989 - 1999 (Table 5). In this experiment, five ecosystems were compared from 1989 to 1999 for their total contribution to global warming. The first three ecosystems were cultivated with annual crops, in a corn-soybean-wheat rotation.

• The "Conventional Agriculture" ecosystem received both soil tillage and pesticides to control weeds and was fertilized to maximize crop yields.
• "No Till Agriculture" 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.
• "Organic Agriculture" 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 and organic agricultural practices reduced greenhouse gas emissions compared to conventional management, but most farmers still use conventional practices. There are several reasons why farmers may not switch to using management activities that reduce greenhouse gas emissions. Farmers need to maintain a steady income to continue farming. For example, farmers may not switch to no-till agriculture because soil compaction may occur, and tillage helps to breakdown surface plant litter that may reduce germination in future planting exercises. Farmers may not use organic agriculture practices because of slightly reduced crop yields and more labor involved in organic agriculture. Fertilizers ensure that plants will have enough nutrients to grow during the growing season. Economics and sociological pressures also play a big role in farmer decisions.

Successional communities are those that are left fallow and receive minimal 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.
• "Late Successional Forest" ecosystems had not been disturbed for over 100 years and were dominated by large, hardwood trees such as oaks and maples.

Three primary gases contribute to the global warming potential (GWP) of the different agroecosystems shown in Table 5. GWP refers to the relative radiative forcing (heat trapping) ability of each source. Nitrous Oxide (N₂O), methane (CH₄) and carbon dioxide (CO₂) molecules do not have the same ability to trap heat. A molecule of N₂O traps the most heat over its lifetime in the atmosphere while a molecule of CO₂ traps the least amount of heat over its lifetime. Therefore, a molecule of N₂O is given more weight in terms of GWP than the other two gases. The table already reflects this change. See IPCC (2007) for further information on greenhouse gas concentrations and relative radiative forcing.

Carbon dioxide is produced through several agricultural processes. Tillage often leads to the loss of soil carbon due to enhanced decomposition of organic matter. Nitrogen fertilizer production is an energy intensive process. Currently, fossil fuels are used to "fix" nitrogen from the atmosphere and transport it to fields. Lime is often applied to fields to increase the soil pH, which can lead to net emissions of carbon dioxide in certain circumstances. Fuel is needed to power tractors and other equipment used to complete various agricultural activities, such as planting, tilling, spraying pesticides and harvesting.

Nitrous oxide (N₂O) is a gas that is produced during nitrification and denitrification processes. Nitrification is the process by which certain types of bacteria convert ammonium (NH4+) to nitrate (NO3-). Denitrification occurs when no oxygen is available. Anaerobic bacteria utilize nitrate as an electron donor for the oxidation of organic matter, which leads to the production of N₂ and N₂O gases. High levels of soil nitrogen due to fertilization can lead to increased levels of N₂O production (McSwiney and Robertson 2005).

Methane (CH₄) is also produced under anaerobic conditions. Microbes that thrive in these oxygen poor environments produce methane as a byproduct of carbon mineralization (Segers 1998). However, some soil organisms are able to oxidize methane, effectively removing it from the atmosphere and producing carbon dioxide (Roslev et al. 1997).