Working Group I: The Scientific Basis |
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3.2.2 Terrestrial Carbon Processes
3.2.2.1 BackgroundHigher plants acquire CO2 by diffusion through tiny pores (stomata)
into leaves and thus to the sites of photosynthesis. The total amount of CO2
that dissolves in leaf water amounts to about 270 PgC/yr, i.e., more than one-third
of all the CO2 in the atmosphere (Farquhar et al., 1993; Ciais et
al., 1997). This quantity is measurable because this CO2 has time
to exchange oxygen atoms with the leaf water and is imprinted with the corresponding
18O “signature” (Francey and Tans, 1987; Farquhar et al.,
1993). Most of this CO2 diffuses out again without participating
in photosynthesis. The amount that is “fixed” from the atmosphere,
i.e., converted from CO2 to carbohydrate during photosynthesis, is
known as gross primary production (GPP). Terrestrial GPP has been estimated
as about 120 PgC/yr based on 18O measurements of atmospheric CO2
(Ciais et al., 1997). This is also the approximate value necessary to support
observed plant growth, assuming that about half of GPP is incorporated into
new plant tissues such as leaves, roots and wood, and the other half is converted
back to atmospheric CO2 by autotrophic respiration (respiration by
plant tissues) (Lloyd and Farquhar, 1996; Waring et al., 1998).
Most dead biomass enters the detritus and soil organic matter pools where it
is respired at a rate that depends on the chemical composition of the dead tissues
and on environmental conditions (for example, low temperatures, dry conditions
and flooding slow down decomposition). Conceptually, several soil carbon pools
are distinguished. Detritus and microbial biomass have a short turnover time
(<10 yr). Modified soil organic carbon has decadal to centennial turnover
time. Inert (stable or recalcitrant) soil organic carbon is composed of molecules
more or less resistant to further decomposition. A very small fraction of soil
organic matter, and a small fraction of burnt biomass, are converted into inert
forms (Schlesinger, 1990; Kuhlbusch et al., 1996). Natural processes and management
regimes may reduce or increase the amount of carbon stored in pools with turnover
times on the order of tens to hundreds of years (living wood, wood products
and modified soil organic matter) and thus influence the time evolution of atmospheric
CO2 over the century. When other losses of carbon are accounted for, including fires, harvesting/removals (eventually combusted or decomposed), erosion and export of dissolved or suspended organic carbon (DOC) by rivers to the oceans (Schlesinger and Melack, 1981; Sarmiento and Sundquist; 1992), what remains is the net biome production (NBP), i.e., the carbon accumulated by the terrestrial biosphere (Schulze and Heimann, 1998). This is what the atmosphere ultimately “sees” as the net land uptake on a global scale over periods of a year or more. NBP is estimated in this chapter to have averaged -0.2 ± 0.7 PgC/yr during the 1980s and -1.4 ± 0.7 PgC/yr during the 1990s, based on atmospheric measurements of CO2 and O2 (Section 3.5.1 and Table 3.1). By definition, for an ecosystem in steady state, Rh and other carbon losses
would just balance NPP, and NBP would be zero. In reality, human activities,
natural disturbances and climate variability alter NPP and Rh, causing transient
changes in the terrestrial carbon pool and thus non-zero NBP. If the rate of
carbon input (NPP) changes, the rate of carbon output (Rh) also changes, in
proportion to the altered carbon content; but there is a time lag between changes
in NPP and changes in the slower responding carbon pools. For a step increase
in NPP, NBP is expected to increase at first but to relax towards zero over
a period of years to decades as the respiring pool “catches up”. The
globally averaged lag required for Rh to catch up with a change in NPP has been
estimated to be of the order of 10 to 30 years (Raich and Schlesinger, 1992).
A continuous increase in NPP is expected to produce a sustained positive NBP,
so long as NPP is still increasing, so that the increased terrestrial carbon
has not been processed through the respiring carbon pools (Taylor and Lloyd,
1992; Friedlingstein et al., 1995a; Thompson et al., 1996; Kicklighter et al.,
1999), and provided that the increase is not outweighed by compensating increases
in mortality or disturbance. |
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