Terrestrial carbon dioxide levels

D. M. Hunten, Atmospheric evolution of the terrestrial planets. Science 259, 915-920 (1993) J. F. Kasting, Earth s early atmosphere. Science 259, 920-926 (1993) R. A. Berner, Atmospheric carbon dioxide levels over phanerozoic time. Science 249, 1382-1386 (1990) R. A. Berner, Paleozoic atmospheric CO2 importance of solar radiation and plant evolution. Science 261, 68-70 (1993). 

Aquatic ecosystems account for approximately the same amount of primary productivity as do terrestrial ecosystems. However, the turnover of carbon in aquatic ecosystems is much faster (0.06 to 0.1 y ) than that of terrestrial ecosystems (27-59 y ) (1). Hence, short term disturbance of aquatic primary productivity has the potential to have a major impact on planetary atmospheric carbon dioxide levels. It follows that accurate prediction of aquatic primary productivity requires the development of rapid methods to assess phytoplankton... 

Two bioassays are employed to evaluate the effect of samples on terrestrial life forms. For gas samples, the plant stress ethylene test is presently recommended. This test is based on the well-known plant response to environmental stress release of elevated levels of ethylene (under normal conditions plants produce low levels of ethylene). The test is designed to expose plants to various levels of gaseous effluents under controlled conditions. The ethylene released during a set time period is then measured by gas chromatography to determine toxicity of the effluent. For liquid and solid samples, a soil microcosm test is employed. The sample is introduced on the surface of a 5 cm diameter by 5 cm deep plug of soil obtained from a representative ecosystem. Evolution of carbon dioxide, transport of calcium, and dissolved oxygen content of the leachate are the primary quantifying parameters. 

Responses of terrestrial ecosystems to increased atmospheric carbon dioxide concentration are of great current interest. Part of this debate relates to the role of limitation by various nutrients, like nitrogen, phosphorus and other vital nutrients on growth and biomass accumulation in terrestrial vegetation. Because vegetation must maintain a nutritional balance, it is expected that nutrient limitation will constrain responses to increased CO2 level. 

The residence time of CO2 in the atmosphere is about two years, which makes the atmospheric air quite well mixed with respect to this gas. However, a more recent analysis shows that the terrestrial ecosystems have much stronger sinks of carbon dioxide uptake. The details of major ecosystem-level CO2 experiments have been shown recently (Koch and Mooney, 1996). 

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