Carbon can be sequestered in terrestrial
Increasing the amount of aboveground biomass in an
ecosystem. Biomass is matter originally created by living organisms.
For example trees, dead leaves, bacteria and people are all biomass.
The ultimate origin of the carbon in virtually all biomass is
atmospheric CO2. So storing biomass is storing atmospheric carbon. Dry
biomass is roughly 50% carbon by weight. Forest ecosystems contain more
living biomass than any other ecosystem so converting grasslands or
croplands to forest is one way of sequestering carbon.
Increasing the amount of carbon held in soils. Soil carbon
originates primarily from plant and fungal material which is then
processed by other fungi and bacteria. Soil carbon can also originate
from charcoal or char created when an ecosystem burns. Many factors
control how much carbon goes into soil and how long the carbon stays in
the soil. CSiTE is focused on understanding soil carbon and enhancement
of soil carbon storage.. both the retention of existing carbon and
enhancement of storage.
Both approaches can be addressed simultaneously on the same piece of
land. In general croplands store less carbon than grasslands which
store less carbon than forests. Grasslands are particularly good at
storing carbon in soils because they often have extensive and deep
roots. Soil carbon is less vulnerable to rapid loss than aboveground
biomass which can be quickly lost to the atmosphere in a fire.
carbon sequestration reduces global warming by slowing down the
build-up of carbon dioxide in the atmosphere. Because the organic
carbon in biomass and soils originated from carbon dioxide in the
atmosphere, the more carbon is stored in biomass and in soils, the less
carbon is stored in the atmosphere as carbon dioxide. Carbon dioxide is
the most significant greenhouse gas. Roughly speaking, terrestrial
carbon sequestration is the only technology which could actually reduce
the concentration of CO2 in the atmosphere. Other technologies
energy efficiency, fossil fuel carbon capture and storage, renewable
energy such as wind, solar, bioenergy simply reduce the amount of CO2
going into the atmosphere; they don’t actively remove the
of carbon in terrestrial systems can be expressed in
different ways, but a commonly used measure, and the one that we will
use here, is mean residence time or the average time that a carbon (C)
atom resides in a particular place.
In aboveground biomass, the mean residence time of C can vary widely
depending on the type of vegetation and its management. For example, C
captured and stored in annual vegetation is not generally regarded as C
sequestration. On the other hand, many regard C captured and stored by
forests as a form of C sequestration because trees can live for decades
or centuries depending on the species and climate. Forest management
and the end-use of forest products is an important determinant of the
mean residence time of C incorporated into trees. The lifetime of C is
generally extended when lumber from forests is used for building
The lifetime of C stored in aboveground biomass can be both
unpredictable and ephemeral because of both natural and anthropogenic
disturbances like fire, plant diseases, and land-use change. For this
reason, C stored in soils is generally regarded as a more permanent
form of terrestrial C sequestration.
The lifetime of soil C depends on both chemical form and
physicochemical soil properties. Inorganic C, in the form of carbonate,
can have a very long lifetime in non-acidic soils. The lifetime of soil
organic C (i.e., C derived from dead plant debris) varies widely
depending on the form of organic matter and soil management practices.
Calculated mean residence times for total soil organic C are generally
on the order of decades to centuries (Six and Jastrow, 2002). Some
strategies for C sequestration seek to promote C storage in deeper
soils because the lifetime of soil C generally increases with
increasing soil depth.
Like aboveground C in terrestrial vegetation, stored organic soil C can
be lost. Soil disturbance, through practices such as tillage, exposes
soil organic C to decomposition by soil microorganisms and this process
returns C to the atmosphere. For example, the average mean residence
time of total soil organic C under no-till agriculture is about 1.5
times longer than under conventional tillage (Six and Jastrow, 2002),
and losses (on average about 30%) of total soil organic C following
cultivation of previously untilled soils have been widely reported
(Davidson and Ackerman, 1993).
In summary, C stored in long-lived perennial vegetation, like forests,
as well as C stored in soils can last from decades to centuries and
thereby help with short-term mitigation of increasing atmospheric CO2
concentrations, however the lifetime of C sequestration in these
reservoirs is highly dependent on land and soil management practices.
Davidson, E.A., and I.L. Ackerman. 1993. Changes in soil carbon
inventories following cultivation of previously untilled soils.
Biogeochemistry 20: 161-193.
Six, J., and J.D. Jastrow. 2002. Organic matter turnover. Pp. 936-942.
In R. Lal (ed.), Encyclopedia of Soil Science. Marcel Dekker, New York.
a global perspective, the biological potential for carbon sequestration
depends mainly on climatic factors. Climates with high temperatures,
sufficient rainfall, and long day lengths are most adequate for plant
growth, whether it is forest growth or grassland production (Alexandrov
et al., 2000, 2002). Therefore, equatorial regions, where both rainfall
and temperatures are high, have the highest biological potential for C
sequestration. The map below shows the potential for C sequestration
specifically for planting forests.
Figure: Biological potential
C-sequestration rates over the world.
Alexandrov et al., 2000 (http://www-cger.nies.go.jp/carbon/pannel.htm)
However, the true potential for carbon sequestration is also dependent
on the costs of these practices, and the land costs in a specific
region. By combining the biological potential for carbon sequestration
with estimates of costs to establish a forest and the costs of using
land, it is possible to calculate a world map showing a geographic
distribution of the costs involved in sequestering a ton of carbon. The
figure below gives an example for planting forests. Areas where the
cost to sequester a ton of C is smallest are most interesting for C
Figure: Geographical distribution of costs
to sequester carbon (From
Benitez et al., 2007)
Alexandrov, G.A., Yamagata, Y. and Oikawa, T., 1999. Towards a model
for projecting Net Ecosystem Production of the world forests.
Ecological Modelling, 123(2-3): 183-191.
Alexandrov, G.A., Oikawa, T. and Yamagata, Y., 2002. The scheme for
globalization of a process-based model explaining gradations in
terrestrial NPP and its application. Ecological Modelling, 148(3):
Benitez, P.C., McCallum, I., Obersteiner, M. and Yamagata, Y., 2007.
Global potential for carbon sequestration: Geographical distribution,
country risk and policy implications. Ecological Economics, 60(3):
amount of carbon that can be sequestered by planting a forest depends
on climate factors, the specific species used, the management of the
forest, and economic costs. The tree species selected will depend on
the local climate. One can opt to establish a forest which is native
for that specific region, or one can establish a commercial plantation
with fast growing tree species. In the first case, the focus is on
regeneration of native tree species, by cutting invasive species and
planting some native species. In the second case, a large number of
fast growing trees are planted and harvested in short rotation cycles
by industrial means. Typical species used in the latter case are
Loblolly Pine in the temperature region, or Eucalyptus in tropical
The amount of carbon sequestered in a forest depends on the age of a
forest. The graph below shows the evolution over time for three forest
types in the U.S. The annual carbon uptake is high for a young forest,
but decreases as the forest grows older (after 25-50 years, depending
on the tree species), and eventually becomes zero for a completely
mature forest. In the latter case, it is said that the forest is carbon
neutral. The graph shows that this occurs after about 75 years for a
Loblolly pine forest in the Plains States.
Figure from Pew center on global climate change, 2005. Note that 1
ton/acre/year equals 2.24 metric tons/hectare/year.
If one wants to keep sequestering carbon in a forest, it is necessary
to periodically harvest the forest by cutting some trees in the forest.
After a cutting, the forest will take up carbon again at a high rate
(see Figure below). Obviously, the use of wood from the forest has to
be long lived in order to have true carbon sequestration. This
illustrates that an adequate forest management is necessary to maintain
or optimize the sequestration potential of a forest.
Figure from Pew center on global climate change, 2005.
that sequester carbon in the soil or
plants often require a change in how the land is managed. Changes in
land management can increase or decrease emissions, and these changes
can be compared to benefits from sequestration activities to estimate
the net impact on greenhouse gas emissions. With respect to
agricultural lands, management may include planting, soil tillage,
fertilizer and pesticide applications, harvesting, crop drying, and a
number of alternative soil amendments. These production inputs can
produce direct emissions from the cropland area (e.g., tractor
emissions from tillage machinery, N2O emissions from nitrogen
fertilizer use) and indirect emissions from the production of
management inputs (e.g., CO2 from natural gas in the production of
nitrogen fertilizers). As the use of management inputs change with
sequestration strategies, these changes together can influence the net
greenhouse gas emissions from a given plot of land.
Figure Caption: Total US average carbon dioxide emissions for three
crop types using three different tillage practices. CT, RT, and NT are
conventional tillage, reduced tillage, and no-till, respectively. The
graph is for non-irrigated areas, which comprise 85% (by area) of US
corn crops, 95% of soybean crops, and 93% of wheat crops. Carbon
dioxide emissions from agricultural inputs (fertilizers, pesticides,
seeds, etc.) and machinery are based on data from 1995.
Source: West, T.O. and G. Marland. 2002. A synthesis of carbon
sequestration, carbon emissions, and net carbon flux in agriculture:
Comparing tillage practices in the United States. Agriculture,
Ecosystems, and Environment 91:217-232.