Carbon reservoirs terrestrial biosphere

The content of the material in a carbon reservoir is a measure of that reservoir s direct or indirect exchange rate with the atmosphere, although variations in solar also create variations in atmospheric content activity (Stuiver and Quay, 1980, 1981). Geologically important reservoirs (i.e., carbonate rocks and fossil carbon) contain no radiocarbon because the turnover times of these reservoirs are much longer than the isotope s half-life. The distribution of is used in studies of ocean circulation, soil sciences, and studies of the terrestrial biosphere. 

A number of other interesting possibilities for utilizing the excess 14C in the atmosphere as a tracer of natural processes come easily to mind. Not much is known about the rate of turnover of humus in the soil. Measurements of 14C in soil humus over the next several years, while the terrestrial biosphere continues to fix carbon with significant amounts of excess 14C, should help to determine the rate of turnover of carbon in the reservoir of humus. Some work along these lines is already in progress (41). 

The place of the biological pump in the global carbon cycle is illustrated in Figure 2. The atmosphere exchanges carbon with essentially three reservoirs the ocean, the terrestrial biosphere, and the geosphere. The ocean holds —50 times as much carbon as does the atmosphere, and... 

Fig. 11-6. Four-reservoir model of organic carbon in the terrestrial biosphere. Reservoir contents in units of PgC ( = lO,2kgC) and fluxes in units of PgC/yr follow from the data assembled in Section 11.2.4. The flux due to biomass burning (Seiler and Crutzen, 1980) excludes deforestation. Emanuel et al (1981) have employed a similar compartmental subdivision but somewhat different fluxes.

Table 11-10. Carbon Reservoirs of the Terrestrial Biosphere, Fluxes and Residence Times ...

Carbon is exchanged among the atmosphere, oceans, the terrestrial biosphere, and sediments and sedimentary rocks, the latter two more slowly than the other reservoirs. The more rapid components of the cycle are shown in Figure 21.11. During the preindustrial period, when the carbon cycle can be assumed to have been in balance, 98.1% of the CO2... 

Figure 1 The global carbon cycle. Arrows indicate fluxes of carbon befween fhe various reservoirs of the atmosphere, lithosphere, terrestrial biosphere, and the ocean. All stocks are expressed as 10 g C. All fluxes are decadal means and expressed as lO g C (Adapfed wifh permission from Sigenthaler and Sarmienfo, 1993), copyrighf 1993, Macmillan Magazines Ltd.). Data used to construct this figure came from Sigenthaler and Sarmiento (1993), Hansell and Carlson (1998), and Sarmiento and Wofsy (1999).

Among the carbon reservoirs of the biosphere, a large proportion is stored in soil organic matter and marine sediments (Bolin, 1977). The accumulation of carbon in soils and sediments is a function of the organic carbon balance between net primary production (carbon fixation) and heterotro-phic metabolism (decomposition). The fixation of atmospheric carbon through photosynthesis is the major sonrce of carbon to terrestrial, wetland, and aquatic ecosystem. 

The portion of Earth and its atmosphere that can support life. The part (reservoir) of the global carbon cycle that includes living organisms (plants and animals) and life-derived organic matter (litter, detritus). The terrestrial biosphere includes the living biota (plants and animals) and the litter and soil organic matter on land, and the marine biosphere includes the biota and detritus in the oceans. 

Insight into the controls of atmospheric CO2 levels is provided by the sizes and turnover times of the different carbon reservoirs. Lithospheric processes control atmospheric CO2 variations on time scales of millions of years. With its large carbon inventory and air-sea fluxes, the oceanic processes dominate atmospheric CO2 variations on millennial time scales. On seasonal to interannual time scales, the terrestrial biosphere must play the leading role. 

First I want to review some relevant information regarding the global carbon cycle and the processes that affect atmospheric concentrations of carbon dioxide. There are vast reservoirs of carbon in the system (see Figure 3.1) that can exchange fairly rapidly with the atmosphere, which contains about 750 gigatons (1 gigaton = 10 tons) of carbon (GtC). The terrestrial biosphere and soils contain about 2,000 GtC the mixed layer of the ocean contains about 1,000 GtC and the deep oceans, 38,000 GtC. 

Figure 1.5 extends this notion to the geochemical level which shows an estimate of the influence of the global biosphere carbon and oxygen cycles on the fluxes of major elements through the terrestrial reservoirs, and includes the effects of both primary and secondary biogeochemical processes. 

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