Q. What are the agricultural sources of greenhouse gas
emissions?
 A.
The major greenhouse gases (GHG's) associated with U.S.
crop and livestock production are carbon dioxide (CO2),
methane (CH4), and
nitrous oxide (N2O).
Agriculture contributes greatly to both methane and nitrous
oxide emissions. Annual U.S. emissions of these gases
are currently
estimated at 180.9 million metric tons carbon equivalent
(mmtce) for methane and 119.4 mmtce for nitrous oxide
(U.S. EPA, 2000).
Agriculture accounts for about a
third of national methane emissions—mostly from enteric fermentation
(the digestive process in ruminant animals) and manure management—and
nearly three-quarters of nitrous oxide emissions, mainly from the
use of nitrogen fertilizers. Rice cultivation and crop residue burning
also contribute to GHG emissions.
Farm production emits carbon dioxide mostly through the use of
fossil fuels in farm operations, the manufacture and distribution
of nitrogen fertilizers and other farm chemicals, and the generation
of electricity used on farms. Carbon
emissions associated with these activities for 1990-95 have
been estimated at between 33.0 and 38.1 million metric tons (mmt)
per year (Lewandrowski et al., 2000).
Agricultural soils can be either a net source or a net sink of
atmospheric carbon, depending on how they are managed. Carbon is
released from soils when land is tilled or otherwise exposed to
the elements and accumulated in soils when land is managed to minimize
disturbances and exposure to the atmosphere. Using conservative
estimates of carbon accumulation in soils under no-till and under
longrun land retirement programs, Eve et al. (2000)
estimate carbon emissions from U.S. agricultural soils to be about
7.5 mmt annually. Using more generous carbon accumulation estimates,
they find U.S. agricultural soils to be a net carbon sink of 37.0
mmt annually.
The conservative accumulation estimates suggest annual carbon emissions
from U.S. agriculture of 40-46 mmt per year. The more generous estimates
suggest U.S. agriculture is probably a marginal carbon sink.
References
- Lewandrowski, J., H. McDowell, R.
House, and M. Peters. 2000. “Mitigating Greenhouse Gas Emissions:
Implications of the Kyoto Protocol for U.S. Agriculture and U.S.
Agricultural Policy,” World Resources Review. Vol.
12, No. 1, pp. 126-148.
- U.S. Environmental Protection Agency. 2000.
Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1998.
EPA 236-R-00-001, (April).
-
Eve, M.D., K. Paustian, R. Follet, and E.T.
Elliot. 2000. Table IIIa, page 46, in U.S. Department
of State, United States Submission on Land-Use, Land-Use
Change, and Forestry, August 1. U.S. submission to the United
Nations Framework Convention on Climate Change, Aug. 1.
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