184 THE NITROGEN CYCLE 



nitrification, and finally to denitrification and the liberation 

 of gaseous nitrogen again. 



Conditions affecting the nitrogen cycle 



Whenever we have a thoroughly mixed bacterial popula- 

 tion, we have the possibility that organisms and enzymes 

 for the entire nitrogen cycle will be present. Which particular 

 part of the cycle will predominate at any moment will depend 

 on conditions holding in the immediate environment. The 

 various reactions in the cycle will be conditioned as follows : — ■ 



ORGANIC N < a Ng 



t 

 b e 



d d I 



NH3 ^ _^ NO2 < ^ NO3 



c c 



Reaction (a) can occur only in the absence of fijxed-nitrogen. 



(6) Breakdown to ammonia will occur as long as there is 

 excess of organic-nitrogen ; if ammonia is the more abundant 

 and a source of carbon is present, then organic-nitrogen will 

 be synthesised at the expense of ammonia, 



(c) Cannot occur in the presence of organic matter. 



(d) Can occur only in the presence of organic matter or 

 hydrogen as H-donator. 



(e) Occurs in presence of nitrate or nitrite as sole source 

 of nitrogen. 



Sewage purification 



The nitrogen cycle can function in whole or in part in the 

 soil and is put to use in sewage purification. Raw sewage 

 as it comes to the sewage farm contains much organic material 

 and is heavily inoculated with heterotrophic organisms. In 

 the settling tanks proteolytic organisms digest the solid 

 material forming utilisable organic-nitrogen, which is broken 

 down by heterotrophic action to ammonia. The fluid sewage 



