342 FACTORS AFFECTING PHOTOSYNTHESIS 



and subterranean species of higher animals. An acre of forest soil may re- 

 lease as much as 20 lb. (9 kg.) of carbon dioxide gas per hour. Rates in 

 agricultural or meadow soils are lower, but are nevertheless very appreciable. 

 The respiration of the soil bacteria alone probably results in a greater return 

 of carbon dioxide to the atmosphere than the respiration of all animals. 



Carbon dioxide is also released into the atmosphere from volcanos, mineral 

 springs, and in the combustion of coal, oil, gasoline, wood, and other fuel 

 materials, Spoehr (1926) has estimated that the combustion of the world's 

 annual output of coal in 1920 — some 1,317,000,000 metric tons — would pro- 

 duce 338 X lOiO kg. of carbon dioxide or only about O.16 per cent of the 

 existing supply. Such a small annual increment in the carbon dioxide supply 

 could not be detected. On the other hand the weathering of certain igneous 

 rocks (feldspars) combines carbon dioxide and thus tends to reduce the 

 quantity of this gas in the atmosphere. 



Oceans and bodies of fresh water are even more important reservoirs of 

 carbon dioxide than the atmosphere. The former are far more important 

 than the latter as storehouses of carbon dioxide. The oceans occupy nearly 

 three-fourths of the earth's surface, and ocean water contains nearly 50 cc. 

 of carbon dioxide including both dissolved and combined forms (carbonates 

 and bicarbonates) per liter. The absolute carbon dioxide content of the oceans 

 is estimated at from thirty to forty times as much as is present in the atmos- 

 phere. The carbon dioxide in ocean waters is involved in a complex series 

 of chemical and biological cycles which have never been fully evaluated. 

 Marine plants consume carbon dioxide in photosynthesis and release it in 

 respiration. Marine animals feed either upon marine plants or other animals, 

 but ultimately, as with land animals, all of their food comes from the process 

 of photosynthesis. A part of the carbon in the food consumed by such or- 

 ganisms is released into the water as carbon dioxide in the process of respira- 

 tion. Aquatic micro-organisms accomplish the decay of dead plants and 

 animals, releasing most of the carbon in these organic remains in the form 

 of carbon dioxide. Complex equilibria between the dissolved carbon dioxide, 

 carbonates, and bicarbonates also exist. Some marine animals precipitate large 

 quantities of carbon dioxide in chemically combined form as the calcium 

 carbonate of calcareous rocks. The conversion of bicarbonates into carbonates 

 results in the release of carbonic acid and thus increases the available carbon 

 dioxide content of the water. Eventually such rocks (limestones, etc.) may 

 be raised above sea level and the carbon dioxide tied up in the form of car- 

 bonates again released to the atmosphere or dissolved in running water dur- 

 ing dissolution of the rock. Similar although not quite such complex cycles 

 of carbon dioxide exist in the bodies of fresh water. 



