454 



NITROGEN METABOLISM 



The Origin of Nitrogenous Compounds in the Soil. — The entire nitro- 

 gen supply of the higher plants is obtained from compounds in the soil that 

 contain this element. Nitrogen compounds are continually being lost from 

 the soil by the leaching action of rains and by the removal of the plant cover 

 through fire or other agencies. Large quantities of nitrogen are lost every 

 year from cultivated soils vi^hen crops are harvested. The soils, however, 

 show no equivalent depletion of their nitrogen supply. Since there is no 

 nitrogen in the rocks from which the soils are derived it is obvious that the 



supply of nitrogenous compounds in the 

 soil must constantly be replenished in 

 some way. Maintenance of a supply of 

 nitrogenous compounds in the soil is ac- 

 complished principally by the activities 

 of certain soil organisms, the nitrogen 

 fixing bacteria. The process of nitro- 

 gen fixation and other important phe- 

 nomena influencing the soil nitrogen 

 supply will be briefly discussed in the 

 following paragraphs. 



I. Nitrog en Fixation. — Two 

 groups of bacteria are able to fix appre- 

 ciable quantities of atmospheric nitro- 

 gen in organic combination : ( i ) cer- 

 tain saprophytic bacteria which obtain 

 their energy from dead organic matter 

 in the soil and (2) symbiotic nitrogen 

 fixing bacteria which live in the roots 

 of members of the legume family. 

 Other bacteria and some fungi may also 

 be able to combine atmospheric nitro- 

 gen with organic compounds but the 

 amount of the nitrogen fixed by these organisms is probably not important. 



Symbiotic nitrogen fixation is the result of the activities of species of 

 Rhizobiuju which are rod-shaped bacteria that enter the roots of legumes by 

 way of the root hairs and cause the formation of nodules on the young roots 

 (Fig. 99). These bacteria live inside the nodules and there synthesize organic 

 nitrogen compounds from the carbohydrates of the host and gaseous nitrogen 

 of the air. 



Some of the nitrogenous compounds synthesized by these organisms are 

 utilized by the legume plants in their protein metabolism, some may move 



Fig. 99. Nodules containing nitrogen- 

 fixing bacteria on roots of soy bean. 

 Photograph courtesy of Dr. H. W. 

 Batchelor. 



