Capacity for Soil
Storage of Carbon
There is tremendous capacity for
storage of carbon in soil, both in the United States and globally. In fact, it
is estimated that about 100 billion tons of carbon that was originally in the
soil has been lost due to human activity, especially agriculture. Most of this
carbon could be returned to the soil, improving the soil in the process. With
mankind’s annual carbon emissions estimated to be just 6 to 8 billion tons
this signifies huge storage potential.
World soils contain about 3.2
trillion tons of carbon within the top six feet. An estimated 2.5 trillion
tons is in the form of soil organic carbon. This is the organic matter
in the soil that makes it fertile. The remaining 0.7 trillion tons is soil
inorganic carbon.
These are very large numbers. In fact
the soil carbon pool is 4.2 times the entire atmospheric pool, and 5.7 times
the biotic pool. Thus, even a relatively small increase in soil carbon, if it
is taken from the air, could provide a significant reduction in atmospheric
carbon. Moreover, because plants feed on carbon dioxide (CO2) in
the air, the primary way to store carbon in soil is to grow plants. Improved
agriculture is the key to soil storage of carbon.
Soil organic matter is concentrated
in the upper 12 inches of the soil. So it is readily depleted by anthropogenic
(human-induced) disturbances such as land use changes and cultivation. The
magnitude of soil carbon depletion is increased by soil degradation,
especially due to erosion. (See figure
– Soil Carbon Storage Replaces Lost Carbon)
Land use changes in forests,
grasslands and wetlands have transformed large areas of the earth from
relatively stable ecosystems to agro-ecosystems under extensive and intensive
use. The introduction of agriculture involves land clearing, draining, sod
breaking, cultivation, replacing perennial vegetation with annual crops, and
fertilizing. These changes have had major impacts on carbon pools and fluxes
around the globe. In the initial phases of these transformations, major losses
of CO2 to the atmosphere occurred as soil carbon levels adjusted to
reduced carbon inputs and increased soil disturbance.
Intense pressure for production also
has led to serious soil degradation through erosion and nutrient losses. These
trends continue in many areas of the world. In the U.S. and other
industrialized nations with available energy and technology, agricultural
productivity has steadily increased, land degradation has slowed or reversed,
and soil carbon pools have stabilized or increased. However, soil carbon
levels are still well below pre-agricultural levels.
It is estimated that a large part (75
to 80%) of the lost carbon can be re-sequestered in the soils of the earth. Of
course the soil carbon storage capacity is finite. Moreover, ecological
factors and management practices limit the rate of storage. Nevertheless, it
is thought possible to achieve this storage over the next 25 to 50 years.
Since most of the original productive capacity of the earth would be restored
in the process, this is indeed the ultimate "no-regrets" climate
policy.
The following table shows estimates
of annual global soil storage of carbon that might be sustained over the next
25 to 50 years. [Source: Carbon Sequestration – State of the Science,
U.S. Department of Energy, Office of Science and the Office of Fossil Energy,
February 1999 available on-line at http://www.fe.doe.gov/coal_power/sequestration/index.html.
|
(carbon in billion tons
per year) |
Agricultural
lands |
0.85 –
0.90 |
Biofuel
croplands |
0.50 –
0.80 |
Grasslands |
0.50 |
Rangelands |
1.20 |
Forests |
1.00 –
3.00 |
Deserts and degraded
lands |
0.80 –
1.30 |
Terrestrial
sediments |
0.70 –
1.70 |
Boreal peatlands and
other wetlands |
0.10 –
0.70 |
Total |
5.65 –
10.1 |
These numbers compare favorably with
total human carbon emissions due to fossil fuel combustion, which are
presently estimated to be six billion tons per year.
One of the key research questions is
how long these rates of carbon storage could be maintained. Also, there
clearly is some maximum capacity for soil storage, but that capacity is far
from certain. Refining such estimates should be one of the central R&D
tasks in any soil-carbon-storage program.
While perhaps surprisingly large,
these relatively high levels of potential carbon storage are not unreasonable.
For example, a five percent increase in total carbon contained in global
terrestrial ecosystems and agro-ecosystems over a 25-year period would store
over 100 billion tons of carbon. Storing 100 billion tons over 25 years
requires increasing the rate of storage by an average of only 0.2 percent per
year – roughly one half of the amount estimated
above.