Problem Statement

Rationale. Agriculture can both impact and be impacted by global change. One of the major impacts is associated with greenhouse gas emissions. Cultivated agricultural ecosystems are a carbon dioxide emission source, with estimates suggesting that perhaps 40% of the pre-settlement soil organic carbon has been lost to the atmosphere. Agricultural soils are important in the global context not only because of the annual carbon dioxide exchange with the atmosphere but also because carbon storage in these soils is sensitive to management practices such as cropping systems and tillage. With improved conservation management practices, crop production systems also can become soil carbon repositories. However, the roles of tillage and soil erosion in carbon loss are not clearly understood because the erosion mechanics for transport of carbon and tillage-induced carbon loss have not been clearly identified. Agricultural conservation systems accomplish carbon storage relatively quickly and inexpensively and also buy time to develop new technologies to solve larger, long-term greenhouse gas emission problems.

What is known. Carbon can be stored in agricultural soils in organic or inorganic form, with the organic the most dynamic and complex. Higher amounts of carbon in soil enhance soil productivity, fertility, water-holding capacity, and other soil conditions that reduce erosion and control nutrient and pesticide availability and the release of these chemicals into the environment. Among practices that aid organic carbon storage are increased cropping intensity, conservation tillage, cover crops, crop rotations, and manure or other organic amendments. Estimates indicate that U.S. agriculture removes about 200 million tons of carbon as carbon dioxide from the atmosphere each year. A portion of the carbon within plants ultimately enters the soil, where a fraction of it may reside for hundreds of years, but the ability of the soil to store carbon over long periods of time and the rate at which it can be accomplished is a highly debated scientific question. Furthermore, recent measurements indicate that soil carbon can be rapidly lost following certain types of tillage.

Gaps. Key issues for the fate of global carbon dioxide are how the management of agronomic inputs impacts soil carbon storage; how the maximum potential carbon storage of a soil can be estimated; how long it takes to attain the storage potential; how long it resides; what the regional differences are; and how changes in the global environment, such as increased atmospheric carbon dioxide levels and weather patterns, impact soil carbon cycling. The role of plants with genetically modified physical and chemical attributes in soil carbon storage is unknown. The role of the carbon-to-nitrogen ratio of crop residue in greenhouse gas emissions and the dynamics associated with the equilibrium carbon-to-nitrogen value in a specific soil are not clearly understood.

Goals

• Define animal and cropping system effects, including tillage and residue management, on soil carbon storage, rates of soil carbon change, and carbon quality in different soils and climatic zones, including analysis of long-term experiments;
• Quantify inorganic fertilizer and organic byproduct effects on plant growth and soil carbon storage, rates of soil carbon change, and carbon quality in different soils and climatic zones;
• Quantify impacts of global change on soil carbon storage, rates of soil carbon change, and carbon quality in different soils and climatic zones;
• Define environmental and economic co-benefits of agricultural practices that reduce production risks, promote soil carbon storage, achieve agricultural profitability and sustainability, and improve soil productivity; and
• Develop monitoring protocols, sampling frameworks, and verification schemes to evaluate impacts of land use changes and management practices on greenhouse gas emissions and carbon storage and to document potential carbon credits in an emissions trading system.

Approach

Emphasis will be placed on measuring soil carbon storage and understanding carbon dynamics using an interdisciplinary, multidimensional approach that brings together physical, chemical, and biological processes and properties. As a baseline, existing experimental carbon data on crop production systems can be analyzed to determine the state of current knowledge on soil carbon storage in major U.S. agricultural lands. Methods must be developed to estimate upper limits of soil carbon storage by soil type, farming systems, eco-regions, and nationally to quantify potential reduction in greenhouse gas emissions attributed to agriculture. Existing carbon flux methods and networks (systems to measure exchanges of carbon dioxide between soil and the atmosphere) should be expanded to regions not currently covered and to other ecosystems and management strategies. Resulting data will be used to develop and test models that predict effects of shifts in cropping and tillage systems and land use changes on soil carbon input and storage.

Outcomes

• New and improved systems of management practices will promote and preserve stored soil carbon.
• Improved management will increase the amount and rate of soil carbon storage in grazed and cultivated lands.
• Conservation crop production will remain economically viable, meeting the food and fiber needs of a growing population, and will help reduce fossil fuel use and atmospheric carbon dioxide..
• Scientifically based information will be communicated to provide a solid foundation for national natural resource stewardship and energy conservation policies.
• Soil resources and air and surface and ground water quality will be enhanced.
• New tools for a wide range of spatial and temporal scales will lead to regional assessments of carbon and nitrogen fluxes which, in turn, will be used to quantify multiple environmental benefits of soil carbon storage and conservation agriculture.

Impact

Improved cropping and tillage systems that supply high quality food and fiber while reducing agriculture's impact on the environment through reduced greenhouse gas emissions and enhanced soil carbon storage

Linkages to Other ARS National Programs

• Integrated Agricultural Systems
• Manure and Byproduct Utilization
• Soil Resource Management