Q. How can changing farm production practices increase
soil carbon levels?
A. When soils are disturbed
by cultivation or other field operations, or when
they are left exposed to the atmosphere, CO2
is released through the oxidation of carbon in soil's
organic material. For grassland and forest soils first
brought into production using conventional tillage, soil
carbon losses typically range from 30 to 50 percent over
the first 40 years of cultivation, after which soil carbon
levels tend to stabilize at a new equilibrium (Lal
et al., 1998). Because most U.S. cropland has been
in production for several decades, the large initial release
of carbon has already occurred.
Hence, in much of the United States, the potential exists to sequester
additional carbon by expanding the use of farm production practices
that build soil carbon levels. Conservation tillage methods—particularly
no-till—minimize soil disturbance by "drilling" seeds
into the soil rather than breaking up the surface layer and broadcasting
seeds. Other practices recognized to build soil carbon levels include
using winter cover crops, eliminating summer fallow, including forages
in rotations, adding organic matter, and irrigation (Bruce
et al., 1998).
While the above practices can build soil carbon levels, the degree
to which these practices can economically offset GHG emissions in
other sectors, such as power generation, is not clear. For any given
practice, say no-till, the addition to soil carbon associated with
adoption will vary significantly with region and site-specific conditions
(including location, soil type, climate, and past land use). Data
reflecting these variables are only now becoming available on a
scale useful for regional and national economic modeling.
Additionally, soil carbon enhancing practices are often associated
with other GHG emitting activities. For example, shifting from conventional
tillage to no-till or eliminating summer fallow generally requires
more nutrients and herbicides. In a program to reduce GHG emissions,
the emissions associated with these other activities will need to
be counted against the additional carbon stored in soils.
References
- Bruce, J.P., M. Frome, E. Haites, H. Janzen,
R. Lal, and K. Paustian. 1998. Carbon Sequestration in Soils.
Soil and Water Conservation Society, July.
- Lal, R., J.M. Kimble, R.F. Follet, and C.V.
Cole. 1998. The Potential of U.S. Cropland to Sequester Carbon
and Mitigate the Greenhouse Effect. Ann Arbor Press, Chelsea
MI.
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