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Sources of Carbon Dioxide -- Summary
In answering a couple of questions relative to sources of atmospheric CO2 - How much CO2 is produced by cars and people? and What percent of CO2 in the air derives from volcano exhaust? - we note that (1) an average person exhales approximately 2.2 pounds of CO2 per day, (2) the burning of one gallon of gasoline produces about 20 pounds of CO2, and (3) the world's volcanoes, on average, only emit about 3% as much CO2 as what is produced by anthropogenic activity.  With respect to the first of these items, we further note that exhaled breath can be a good source of CO2 to use in atmospheric CO2 enrichment experiments of the Poor Man's Biosphere type (Idso, 1997), variations of which are described in the Experiments section of our web site.  We also further note, with respect to the second item, that vehicular exhaust is the primary source of the Urban CO2 Dome, which is described in more detail in its own separate section of our site.  Our greatest interest in this topic, however, is related to natural environmental sources of CO2 that may be influencing nearby vegetation.

With respect to this last topic, we note in our book review of Plant Responses to Elevated CO2: Evidence from Natural Springs that there are a number of sources of localized atmospheric CO2 enrichment - including burning coal seams in southern Utah, and CO2 springs in Italy and Florida - that have been used to assess various impacts of elevated CO2 on vegetation growing close to these CO2 sources.  In several studies described in the book it was found that (1) a mere 70 ppm increase in the air's CO2 content is doubling the water use efficiency of affected plants in Utah, (2) naturally-CO2-enriched plants in both Florida and Italy have much reduced leaf stomatal densities, (3) some of the naturally-CO2-enriched plants in Italy are growing on soils that have a soluble aluminum content that is normally well above the plants' toxicity threshold, and (4) plants grown from seed collected from plants growing in the vicinity of naturally high atmospheric CO2 concentrations in Italy respond better to atmospheric CO2 enrichment in the laboratory than do plants grown from seed collected from plants growing in ambient air.

In other studies that have utilized natural sources of high atmospheric CO2 concentration to study various plant responses, Tognetti et al. (1998) observed oak trees growing near high-CO2 springs in central Italy to transpire less water and thus maintain a better internal leaf water status in the face of drought than similar trees growing in ambient air.  Likewise, Schwanz and Polle (1998) observed that some of these same naturally-CO2-enriched trees, as well as others, apparently experience less stress of all kinds than trees growing just outside the influence of the high-CO2 springs.  And in a study of an herb and a tree growing near a high-CO2 spring in Venezuela, Fernandez et al. (1998) observed that plants exposed to the extra CO2 continue to sequester carbon during the local dry season of the year, while plants exposed to normal air just tens of meters away actually lose carbon to the air.

References
Fernandez, M.D., Pieters, A., Donoso, C., Tezara, W., Azuke, M., Herrera, C., Rengifo, E. and Herrera, A.  1998.  Effects of a natural source of very high CO2 concentration on the leaf gas exchange, xylem water potential and stomatal characteristics of plants of Spatiphylum cannifolium and Bauhinia multinerviaNew Phytologist 138: 689-697.

Idso, S.B.  1997.  THE POOR MAN'S BIOSPHERE, including simple techniques for conducting CO2 enrichment and depletion experiments on aquatic and terrestrial plants.  Environmental and Experimental Botany 38: 15-38.

Schwanz, P. and Polle, A.  1998.  Antioxidative systems, pigment and protein contents in leaves of adult mediterranean oak species (Quercus pubescens and Q. ilex) with lifetime exposure to elevated CO2New Phytologist 140: 411-423.

Tognetti, R., Longobucco, A., Miglietta, F. and Raschi, A.  1998.  Transpiration and stomatal behavior of Quercus ilex plants during the summer in a Mediterranean carbon dioxide spring.  Plant, Cell and Environment 21: 613-622.