ECOLOGr 



757 



temporarily, but eventually geologic changes or man's mining and drill- 

 ing bring the coal and oil to the surface to be burned to carbon dioxide 

 and restored to the cycle. 



Much of the earth's carbon is present in rocks as carbonates— lime- 

 stone and marble. These rocks are gradually worn down and the car- 

 bonates are in time added to the carbon cycle, but other rocks are 

 forming at the bottom of the sea from the sediments of dead animals 

 and plants, so that the amount of carbon in the carbon cycle remains 

 about the same. 



330. The Nitrogen Cycle 



The nitrates of the soil and water are taken up by plants and are 

 the source of nitrogen for the synthesis of amino acids and proteins. 

 The plants may then be eaten by animals that in turn use the amino 

 acids from the plant proteins in synthesizing their own amino acids, 

 proteins, nucleic acids, and other nitrogenous compounds. \V'hen animals 

 and plants die, the decay bacteria convert these nitrogenous compounds 

 into ammonia. Animals excrete several kinds of nitrogenous wastes- 



Nitrogen Fixing 

 Bacteria 



Figure 37.3. 



The nitrogen cycle in nature. See text for discussion. 



