PRODUCTS or METABOLISM 



185 



Nitrogen Cycle. — Nitrogen shows the same continuous change 

 as carbon. Plants take up nitrogen in mineral form usually as nitrates. 

 The plants change this mineral nitrogen to the most complex bodies, 

 proteins, where it is combined with the other elements of organic nature. 

 The plants may be eaten by animals; part of the protein is then digested 

 to urea or hippuric or uric acid, which in turn are readily decomposed 

 by microorganisms to ammonia (Fig. 103). Part of the protein will be 

 stored in the growing animals, and if the animal dies, the body will 

 decay or putrefy, and the nitrogenous compounds of that body will 

 pass through the various stages of decomposition to the final product. 



Dead 

 Organisms 



rates 



Fig. 103. — Nitrogen cycle. 



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. 

 More nitrates would be produced from decaying organic matter and 

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



