Problem Statement

Rationale. To determine the soil's capacity to store carbon, it is critical to know the amount and rate of carbon exchanges between soil and the atmosphere. Carbon dioxide uptake by photosynthesis has been extensively researched. In contrast, the major limitation to understanding carbon exchanges, within the context of soil carbon storage, is the rate of carbon return to the atmosphere. Long-term conventional tillage of soils is known to deplete soil organic carbon below pre-cultivation levels. However, it is difficult to quantify and chart the time courses involved and to measure total carbon exchanges that result in soil carbon storage generated by land use and management practices.

What is known. A variety of meteorological, gas chamber, and carbon isotope techniques has been developed for measuring atmospheric and soil carbon interchanges and fluxes. Daily carbon dioxide exchanges can vary widely, with large uptakes of carbon by growing plants during the summer, and large emissions of carbon to the atmosphere in the fall as vegetation dies. Several models of carbon cycling in soil are available, although validation of key components has been hampered by lack of data and shortcomings in measurement technology.

Gaps. Most important to determining the amount of carbon stored in soils is the ability to measure soil carbon content and validate changes to that content over time. Adequate measurement of changes in soil carbon must include evaluation of the physical, biological, and chemical characteristics of soil organic matter and soil inorganic carbon. In addition, stabilities of various physical and chemical components of soil organic matter need to be evaluated. Measurement of changes in soil carbon storage must include sampling schemes that address the spatial and temporal variability of soil carbon; soil bulk density (weight per volume); and chemical, physical, and biological soil properties. We also need rapid analytical and field surveillance methods to extend our predictive capability of soil carbon storage and changes.

Thus far, concerns about changes in soil carbon have focused mainly on those resulting from biological processes; however, changes may result from soil inorganic carbon gains and losses caused by soil erosion, downward movement through the soil profile as dissolved organic or inorganic carbon, and burning. We need methods to estimate these abiotic losses.

Systematic methodologies to determine the impact of land use and management practices on soil carbon storage are needed to predict actual and potential soil carbon storage at local, regional, national, and global scales. Techniques and models are needed to estimate and predict soil carbon storage and storage potentials over similar land management areas from field to regional and national scales. Finally, models must be able to predict the concurrent impact of agricultural practices on both carbon dioxide exchange and the exchange of other greenhouse gases to assess the integrated effect on climate change.

Goals

• Develop tools and techniques to measure carbon exchange processes and to quantify soil organic matter, soil carbon, and soil nutrient (e.g., nitrogen) storage or loss for major agricultural ecosystems and
• Develop predictive tools (models) to understand, integrate, and predict the impacts of land use and management decisions and global change on soil carbon storage in agricultural ecosystems from the local to the national scale.

Approach

Field techniques, including micrometeorological methods and destructive sampling, will be used to measure carbon dioxide balances (increase vs. decrease) over representative landscapes for the long term. Soil sampling and chamber techniques will be used to determine land use and management-induced soil carbon losses. Emphasis will be placed on the development of new techniques to measure the physical, chemical, and biological changes in soil organic matter over time. Subsurface soil water sampling will be used to estimate convective losses of soluble carbon. Models will be developed and used to estimate soil carbon storage and storage potentials over similar land use and management areas and for scaling up from field to regional and national level estimates. This effort will include the use of remote sensing and geographical information systems.

Outcomes

• Tools will be developed to determine carbon budgets on short- and long-term time scales and on field-to-regional spatial scales.
• Standard techniques will be available to sample soils, determine bulk density, and analyze soil carbon.
• Methodologies will be developed to quantify the contribution of agricultural land use and management practices to soil carbon storage
• Precision and accuracy of soil carbon storage estimates will be improved.
• Techniques will be improved to assess changes in soil carbon pools resulting from abiotic processes such as erosion.
• Inventories of soil carbon for U.S. agricultural lands will be improved.
• Models and decision support systems will help to determine the specific amount, quality, and value of carbon storage for various agricultural land use and management practices.

Linkages to Other ARS National Programs

• Integrated Agricultural Systems
• Soil Resource Management