NUTRITION OF MICROORGANISMS 261 



ammonia. Ammonia is then oxidized to nitrites and nitrates, when 

 the nitrogen cycle is completed. 



There is, however, one discrepancy in this cycle. It has been 

 mentioned already that some organisms are able to reduce nitrates to 

 nitrogen gas. This is one of the " leaks" in the rotation of elements 

 which would be disastrous to organic life on earth if there were no means 

 to compensate for the loss of nitrogen in circulation. Imagine what 

 would happen if there were no such compensation. Part of the nitrate 

 in the soil is destroyed, the nitrogen gas escapes into the air and is as 

 indifferent as the nitrogen of the atmosphere lost to organic life forever. 



nyolroaen -*Julpn/de 



-/ ^^HBrifeK^^ta_ / 



Dead 

 Urwan/sms 



FIG. 116. Sulphur cycle. 



More nitrates would be produced from decaying organic matter and 

 would eventually be destroyed. After a certain time, this continuous 

 loss of nitrogen would become quite noticeable in the growth of plants; 

 there would be a scarcity of nitrogen in soil, since part of it is lost continu- 

 ously. Finally, the plants would cease to grow because the nitrogen in 

 the soil would be exhausted. 



The compensation for this destruction of available nitrogen is found 

 in the nitrogen-fixing bacteria, which, either living in symbiosis with 

 leguminous plants or growing independently in the soil, have the power 

 to use the atmospheric nitrogen for the formation of their own proto- 



