270 Oxygen and Carbon Dioxide 



subsequent generations have gradually reduced the pH in the ad- 

 jacent area. Thus we see that in mud and soils, as well as in the free 

 water of the aquatic environment, the pH may act as a specific limit- 

 ing factor for some species, but that it is most generally useful as an 

 index of the overall conditions. 



Calcium Carbonate 



Another way in which carbon dioxide exerts an effect upon environ- 

 mental conditions is through its influence on the formation and dis- 

 solution of calcium carbonate ( lime ) . Carbon dioxide in water forms 

 carbonic acid which dissociates to form bicarbonate and carbonate 

 ions, and the latter reacts with calcium ions to form CaCOg: 



CO2 + H2O ^ H.2CO3 ^ H+ + HCO3- ^ H+ + C03= 

 €03= + Ca++ ^ CaCOa (ppt) 



The amount of these ions that will remain in solution is greater at 

 higher hydrogen ion concentrations (lower pH) and, unlike most 

 salts, at lower temperatures. Since a change in amount of carbon 

 dioxide affects the equilibrium between the carbonate and bicarbonate 

 ions, it also influences the equilibrium between the precipitated 

 CaCOa and the Ca++ and COa^ ions in solution. If CO2 is added (as 

 by respiration), more H2CO3 is produced, and this will dissociate to 

 form hydrogen ions, lowering the pH and reacting with the COg^ ions 

 to form more HCOs" ions. In nearly neutral solutions the equilibrium 

 is favorable to this reaction (Fig. 7.9). The reverse situation occurs 

 when CO2 is withdrawn (as by photosynthesis), with the result that 

 more CaCOg is formed. In this way lime is often caused to deposit 

 on or in the tissues of aquatic plants. 



As mentioned in Chapter 3, fundamental differences in the nature 

 of soils depend in part upon the abundance of CaCOa. The amount 

 of lime present is a result of the nature of the parent rock material as 

 modified by the combined action of climatic and biological agents. 

 In the pedocal soil-group division the ecological conditions are such 

 that CaCOa tends to be retained in the upper soil horizons but in the 

 pedalfer soil-group division carbonic acid and other acids, produced 

 by the metabolism of roots and of soil microorganisms and as by- 

 products of decomposition, tend to bring about the dissolution of any 

 CaCOa present and thus to allow it to be leached away. Ca++ and 

 other cations in the soil tend to be replaced by hydrogen ions, and 

 this causes the soil to become progressively more acid. As soil acidity 

 increases, ion replacement is accelerated and often results in a serious 



