PLANT METABOLISM 403 



Chemical 

 N fixation 



Soil ^ > Loss of N to sea by 



leachuig and erosion 



_ , . . / \ Nonsymbiotic 



u- , ^"'V xT°r .■ r \ biolosical N fixation 

 biological N fixation ^ -,. . ^ !! -.j 



V, >■ Plants < * iNIicroorganisms » JNj 



Denitrification 



Animals 



Loss of N to sea 

 as sewage 



Fig. 15-7. The nitrogen cycle in nature. 



land use and wasteful disposal of sewage, the land area of the earth 

 yearly increases its nitrogen debt, despite nitrogen fixation by plants 

 and bacteria. 



Agents Capable of Fixing No. Leguminous plants such as clover, al- 

 falfa, peas, and beans carry nodules on their roots. In these nodules, 

 No can be fixed into forms usable by the plant. The nodules are caused 

 by the invasion of the roots by bacteria, the rhizohia, which cause the 

 root tissue to grow at the site of invasion. Nodules have a characteristic, 

 well-organized structure, and certain of the plant cells are packed with 

 bacteria. It is interesting to note that a hemoglobin much like mam- 

 malian hemoglobin is present in nodules; this is the only reported occur- 

 rence of hemoglobin in the plant kingdom. Neither the leguminous plant 

 alone nor the rhizobia alone can fix N2, but in symbiotic association ^ 

 they can. Symbiotic nitrogen fixation is quantitatively the most impor- 

 tant means on the land surface of the earth for fixing No. 



The nonsymbiotic nitrogen fixers, that is, those organisms capable of 

 fixing No by themselves, include the aerobic Azotobacter, the anaerobic 

 Clostridium and Desulfovibrio, the photosynthetic bacteria Rhodospiril- 

 luni, Chromatium, Chlorobacterium, and Rhodomicrobium, and repre- 

 sentatives from 3 families of blue-green algae. It is very difficult to 

 appraise the quantitative importance of the No fixation by these organ- 



^ Symbiotic association, or si/mhiosls, is a relationship in whicli two different livinj; 

 organisms exist in close association with each other in such a manner that each 

 derives benefit fi'om the other's existence. 



