lifetime of carbon in terrestrial systems can be expressed in several
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.