Presentations

The 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 construction.

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.

References

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.