Box 121

Terrestrial photosynthesis and decomposition are closely balanced. On a geological time scale photosynthesis has been slightly larger than decomposition, leading to reserves of fossil fuels that are four times as great as the C stored in the soil. Oceanic photosynthesis and decay are approximately equal to that of terrestrial, but this is carried out by biota of only 45 petagrams (Pg), resulting in a much faster turnover rate than in the terrestrial environment. A petagram is equivalent to 1015g or 109 metric tons. The oceans have large reserves of dissolved C, both inorganic and organic, especially in the thermocline and in the deep sea, representing a pool of 38,000 Pg C, substantially larger than soil C. Woody components of the land biota account for 75% of stored terrestrial plant C. Subtracting the values for respiration from gross primary production gives net primary production values of 50-60 Pg, indicating that it takes approximately 10 years for the terrestrial plants to recycle their C. This represents, on average, fast turnover rates for leachates and easily decomposable residues relative to the tens, hundreds, and even thousands of years necessary for the turnover of woody components and humified material and hundreds of thousands of years for deep ocean and fossil C (Houghton et al., 2001).

Autotrophic respiration & fires (60)

Gross primary production (120)

Atmosphere (790)

Fossil fuel combustion & cement production (5.4)

Gross primary production (120)

Fossil fuel combustion & cement production (5.4)

(Robinson, 1990). This may be why the Carboniferous and Permian coal reserves outweigh those from any other period in the earth's history.

Combined, the carbonate-Si and organic matter cycles dominate controls on the long-term levels of CO2 and O2 over the millions of years of the Phanerozoic eon. These dominant C cycles were probably in play during the Precambrian, but the fossil record, geology, and climate data are not as abundant as for the Phanerozoic, making the interpretation of the Precambrian C cycle more difficult. Organic matter deposition today occurs more slowly than during the Devonian, presumably because of less primary production and the evolution of plant-debris-degrading organisms. This, coupled with reduced volcanic activity, represents the new equilibrium of atmospheric CO2 controlled by these cycles. During the past 500,000 years (Box 12.2), the atmospheric CO2 level has been closely related to the advance and retreat of glaciers and was lower than the present-day level of 370ppmV (Petit et al., 1999). In the past 30,000 to 40,000 years, CO2 levels of 200ppmV or less were common. Carbon dioxide levels exceeding 1000 ppmV were common prior to this period up to 4 million years before present (Retallack, 2001). Though these cycles are robust, it is evident that the burning of fossils fuels can have an immediate impact on the C cycle that has led to accelerated changes in the cycling of C at the global scale.

the short-term c cycle

The short-term C cycle is dominated by the interplay of terrestrial and marine photosynthesis, respiration, and organic matter formation (Fig. 12.2). The short-term C cycle is dependent on two principle gases, CO2 and methane (CH4). Perturbations of the short-term C cycle causing changes in the concentration of these two gases in the atmosphere cause potential changes in climate because both are greenhouse gases. These gases absorb outgoing infrared radiation from the earth's surface, trapping heat. Over time, these gases have regulated the temperature of the planet. Other greenhouse gases produced by microbes such as nitrous oxide also play an important role in climate change (see Chap. 13). Variations in the sun's energy output and changes in earth's distance from the sun have also contributed to climate change over the Quaternary.

It has been estimated that in the year 1860 the atmosphere contained about 260 ppm CO2; in 2006 it contained approximately 375 ppmV or 765 Pg C (Box 12.2). Since the late l800s, fossil fuel use, forest clearing, and the conversion of extensive areas of virgin land to agriculture have led to a net transfer of terrestrial C to the atmosphere (Falkowski et al., 2000). This has been partially offset by the continuous net uptake of carbonates as sedimentary rocks in the oceans. Approximately up to 20% of the C evolved in the last 100 years cannot be accounted for in present estimates of the global C budget. Suggestions have been made that terrestrial C sequestration potential of terrestrial ecosystems has been underestimated. This may be a result of higher photosynthetic rates and plant water use efficiency under

Marine Photosynthesis

Marine Photosynthesis

Marine Respiration FIGURE 12.2 The short-term C cycle (adapted from Berner, 2004).
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