PLANT METABOLISM 



227 



of plants of the alfalfa, clover, pea, and bean groups. The plants 

 supply something to the fixing system that has not been successfully 

 replaced by plant extracts, metabolic intermediates, coenzymes, metal 

 ions, or biological energy sources like ATP. The plant itself, of course, 

 fixes no nitrogen. Ultimately, it will be possible to replace the plant 

 with suitable chemicals, and the accomplishment will contribute 

 greatly to an understanding of nitrogen fixation in general. 



The importance of nitrogen fixation and denitrification is illustrated 

 schematically in Figure 9-7. In view of the problems of feeding ever- 

 increasing human populations, it is important that the available 

 nitrogen of the soil be kept in balance to provide for the growth of 

 our plant and animal foods. Indeed, the rapidly increasing number 

 of human beings demands that soil nitrogen be increased to provide 

 an adequate food supply. Likewise an increased use of the sea is 

 indicated. 



Unfortunately many people feel that losses of nitrogen from the 

 land surface by nitrification and denitrification, leaching and erosion, 

 and sewage exceed the gains from marine fertilizers (a limited factor 

 at present) and fixation. If so, then the reservoirs are being depleted, 

 especially in Asia, where the demands are greatest and biological fixa- 

 tion is the only significant means of replacement. In Europe and North 

 America chemical fixation of nitrogen for fertilizers is becoming quite 

 significant to the overall nitrogen supply of the soil. The United 

 States in 1956 used chemical fertilizers, principally in the form of 

 ammonia, ammonium nitrate and sulfate, calcium and sodium nitrates. 



Biological 

 fixation 



Atmosphere 

 foods 



Denitrification 



Chemical fixation 

 (fertilizers) 



Denitrification 



Fixation 



FIGURE 9-7. Some of the large-scale transfers and reservoirs of nitrogen. The 

 critical lole of nitrogen fixation in the maintenance of life is evident. 



