Report Contents

Report#:EIA/DOE-0573(98)

November 5, 1999 
(Next Release: November,  2000)

Executive Summary

Preface

U.S. Emissions of Greenhouse Gases in Perspective

Carbon Dioxide Emissions

Methane Emissions

Nitrous Oxide Emissions

Halocarbons and Other Gases

Land Use Issues

Appendix A: Estimation Methods

Appendix B: Carbon Coefficients Used in this Report

Appendix C: Uncertainty in Emissions Estimates

Appendix D: Emissions Sources Excluded

Appendix E: Emissions of Energy-Related Carbon Dioxide in the United States, 1949-1997

Appendix F: Common Conversion Factors

References

Glossary

 Completed Report in PDF Format 921 KB)

Related Links

Greenhouse Gas Page

Environment Page

Carbon Sequestered in Agricultural Soils

In previous years, research on the effects of land use on carbon emissions and sequestration and their incorporation into emissions inventories focused on forests, which were generally believed to be the most important potential biological carbon source or sink. Recent research has tended to suggest that agricultural soils can also be a potentially important carbon source or sink. Natural soils suitable for agriculture, whether grasslands or forests, tend to contain large amounts of mineral or biological carbon, the product of atmospheric carbon dioxide absorbed by plants in the past and left in the soil when they decay.a

Farming practices before the 20th century tended to deplete soil carbon. Deep plowing (partly to discourage weeds) tended to expose soil carbon to natural weathering and oxidation, returning it to the atmosphere, and burning or removing crop residues prevented the soil from regaining the lost carbon. Before the introduction of chemical fertilizers, farming also tended to deplete soil nutrients, requiring farmers periodically to leave a portion of their lands fallow or temporarily abandon them.

Researchers believe that a combination of shallow plowing (also known as conservation tillage) and leaving crop residues to decay in the fields tends to replace soil carbon deleted in the 19th and early 20th centuries. Further, abandoned croplands, even if they revert to grasslands rather than forest, will also sequester carbon in the soil, so long as the abandoned lands are not permitted to erode.

Researcher J.S. Kern has estimated that the soil carbon pool in "major field cropland" in the lower 48 United States is in the range of 5.3 to 8.7 billion metric tons of carbon, and that total carbon losses from this pool since the settlement of the United States total 1.0 to 1.7 billion metric tons of carbon. Extrapolating from this estimate to the total United States may imply a total of 5 billion metric tons of carbon lost to the atmosphere from U.S. agricultural soils over the past 200 years.b

Estimating current carbon emissions and sequestration from agricultural soils is difficult, because it requires matching cultivation and crop residue handling practices with land use, soils, and cultivation practices. Work by the U.S. Department of Agriculture may yield additional information at the national level in the future. Department of Agriculture researchers believe that there are an array of low-cost methods to sequester carbon in agricultural soils, including increasing the incidence of conservation tillage, encouraging spreading agricultural residues on soils, controlling erosion, and an array of "improved cropping systems." They assert that the carbon sequestration potential of agricultural lands is on the order of 75 to 208 million metric tons of carbon per year.c

aThe following discussion draws on: R. Lal, J.M. Kimble, R.F. Follet, and C.V. Cole, The Potential of U.S. Cropland to Sequester Carbon and Mitigate the Greenhouse Effect (Chelsea, MI: Sleeping Bear Press, 1998).
bR. Lal et al., p. 20.
cR. Lal et al., p. 81.