CHANGES BROUGHT ABOUT BY NON-PATHOGENIC ORGANISMS 73 



The Nitrogen Cycle. The relationship of microorganisms to 

 nitrogen and its compounds has been noted in the preceding pages. 

 These changes may be summarized as follows: Certain bacteria 

 break down organic compounds containing nitrogen with the ulti- 

 mate liberation of ammonia. Other species change the ammonia 

 to nitrous acid and nitrites. Still other species transform the 

 nitrites to nitrates, and these the higher plants take up from the 

 soil and transform again into complex organic substances. These 

 eventually again decay or are eaten by animals and converted into 

 "animal tissues. The nitrogen of both plant and animal tissues 



OnimoJ 



Qm*ion i TI car/on 



Fig. 40. Nitrogen cycle. Changes brought about by bacteria indicated by 

 solid lines, other changes by dotted lines. 



ultimately undergoes the change first noted and the nitrogen again 

 appears as ammonia. This series of changes is called the nitrogen 

 cycle. It is to be noted in addition that some bacteria are found 

 which decompose nitrates with the formation of nitrites and 

 liberate free nitrogen in the so-called process of denitrification. 

 Other species, either alone or in symbiosis with higher plants, 

 take up and fix free nitrogen from the air and eventually convert it 

 into a form available for higher plants. These changes may be 

 better understood by reference to the accompanying diagram. 



