Carbon Cycle and Carbon Storage. The loss of soil carbon during the westward expansion and cultivation of the U.S. appears to have stabilized, and gains in soil carbon are occurring in some soils. Conversion from conventional tillage to reduced or no-till causes soils to act as repositories for carbon, rather than sources of atmospheric carbon dioxide. Perennial grasses established on the more than 34 million acres of marginal croplands enrolled in the Conservation Reserve Program have deposited organic carbon in the soil at an annual rate of more than 0.5 ton per acre. Replacing carbon in carbon-depleted cultivated soils thus may represent a potential mitigation against projected increases in atmospheric carbon dioxide. Research is needed to determine if management practices that favor carbon storage also affect yield or profitability through their interaction with other soil factors including temperature, water status, and nutrient cycling. Goals of this component are focused to describe the current and potential roles of agriculture in the global carbon cycle with sufficient accuracy to inform policy and aid producers in making decisions that are both economically and environmentally sound.

Trace Gases. Several types of processes and activities produce gases that are implicated in global change. For example, methane is produced as ruminant animals digest feed and as animal manure is stored. Nitrogen fertilizers can stimulate emissions of nitrous oxide from soil. Research is necessary to eliminate conflicts between environmental and economic goals, such as developing the most economical and environmentally sound methods of manure storage, developing feeds and feed additives that minimize methane production during digestion, and optimizing the use of nitrogen-fixing leguminous plants as a way to replace some applications of manufactured fertilizers. Additional research in this area also will focus on developing systems for crop and animal agriculture that minimize trace gas emissions while preserving productivity. The synthesis of information to provide larger scale estimates of agricultural emissions will be enhanced by developing models and obtaining date to test them at the appropriate spatial scales.

Agricultural Ecosystem Impacts. The concentration of carbon dioxide in the atmosphere continues to increase. Based on experimental evidence, plant growth, yield, and water-use efficiency have increased and will continue to do so as carbon dioxide levels rise. However, these effects have not been studied extensively under field conditions in interaction with other expected changes such as higher temperature, higher ozone concentrations, and higher UV-B radiation levels. The effects of increasing carbon dioxide in the atmosphere and possible climate change on pests (weeds, pathogens, insects and other arthropods) are largely unknown. The interactive effects of multiple stressors in cropping systems and grazinglands will be the primary research topic in this component. As in other components, development and improvement of models and testing them against field data also will receive emphasis to improve confidence in prediction of future impacts of global change on agricultural productivity.

Changes in Weather and the Water Cycle at Farm, Ranch, and Regional Scales. General circulation models (GCMs) used to simulate climate responses to rising greenhouse gas concentrations project changes in precipitation will accompany rising temperatures, but will vary regionally. Some GCMs also predict that weather variability will increase with global warming, introducing yet more uncertainty and risk into agricultural production. Droughts, floods, storms, and periods of excessive heat or cold may occur more frequently, with impacts on agricultural operations, alterations in agricultural water supplies, and increased crop insurance costs and disaster payments. Much of the research required to address these issues involves projection of climate and weather changes through models at many different spatial scales, which requires special attention to improve the predictive capability that will enable decision makers to manage resources on the farm and across the nation.