CHAPTER XXII 



Fixation of Atmospheric Nitrogen by Microorganisms 



Attention has been called to the fact that fixation of nitrogen in the 

 soil is carried on largely by bacteria. The most active representatives 

 of the non-symbiotic nitrogen-fixing bacteria are Azotobacter and Clos- 

 tridium (Bac. amylobacter) groups, while the symbiotic nitrogen fixing 

 bacteria are so far represented by one group, the Bad. radicicola. The 

 great majority of investigations on the activities of the non-symbiotic 

 bacteria, particularly on the energy utilization, mechanism of nitrogen 

 fixation, and influence of various environmental conditions upon this 

 process have been carried out with species of Azotobacter. The same 

 principles may or may not apply to the CI. pastorianum, Bac. astcro- 

 sporus and the other bacteria capable of fixing nitrogen non-symbio- 

 tically. Special emphasis should be laid on the difference in the mechan- 

 ism of energy utilization, nitrogen fixation and general principles of 

 growth between the aerobic and anaerobic bacteria. The first decom- 

 pose most of the sugar to carbon dioxide and water and utilize, there- 

 fore, a large amount of the energy that can be made available from the 

 decomposition of carbohydrates ; the anaerobic organisms break down 

 the sugar largely to organic (butyric) acids and gases, and utilize only 

 a small part of the energy (see p. 412). The difference in growth 

 and nitrogen fixation between the aerobic and anaerobic bacteria is 

 further emphasized by the difference in the influence of oxygen tension 

 and resistance towards acidity and alkalinity. 



Non-symbiotic fixation of nitrogen. Berthelot 1 was the first to recog- 

 nize that the fixation of atmospheric nitrogen leading to an increase in 

 the supply of combined nitrogen in the soil is due to the action of micro- 

 organisms. When the soil was heated to 100°, the property was lost. 

 By placing 50 kgm. of soil in pots, having a surface of 1500 sq. cm. and 

 a perforated bottom, and allowing the pots to remain exposed to weather 

 for seven months, he found an increase of 12.73 grams of nitrogen, tak- 



1 Berthelot, M. Nouvelles recherches sur la fixation de l'azote atmospherinue 

 par les microbes. Compt. Rend. Acad. Sci., 101: 775. 1885; 115: 569-574. 

 637, 737. 1892; Chimie vegetale et agricole. Paris, Masson et Cie. 1899. 



558 



