FIXATION OF ATMOSPHERIC NITROGEN 569 



On increasing the concentration of the energy source in the medium, 

 there is an increase in the amount of nitrogen fixed, but the process is 

 less economical, i.e., less nitrogen is fixed per unit of carbon utilized. 

 When the mannite contents of the medium are 0.5, 1 and 5 per cent, 

 the corresponding amounts of nitrogen fixed are 11.4, 8.2 and 7.45 mgm. 

 per gram of mannite oxidized. 28 The following amounts of nitrogen 

 were reported 29 fixed per gram of mannite in different concentrations of 

 the latter: 



Mannite, per cent 0.1 0.2 0.5 1.0 1.5 



Nitrogen fixed per gram of mannite, 

 mgm 10.5 8.3 6.4 4.68 3.22 



The more dilute the concentration of the energy source, the greater is 

 the nitrogen-fixing efficiency of the organisms. This probably accounts 

 for the very efficient energy utilization by the nitrogen-fixing bacteria in 

 the surface layers of the soil. 30 The amount of nitrogen fixed also 

 depends on the presence of minerals, especially phosphates, of certain 

 stimulating substances and upon the nature of the medium. Nearly 

 ten times as much nitrogen was fixed per unit of sugar consumed in sand 

 as in solution. 31 CI. yastorianum fixed 4.56 mgm. nitrogen per gram 

 of sugar consumed in a 0.5 per cent glucose solution and 1.93 mgm. in 

 a 3.0 per cent sugar solution. 32 



These considerations account for the great variability in the amounts 

 of nitrogen fixed (from 1.2 to 17 mgm.) per gram of sugar consumed by 

 crude and pure cultures of bacteria, as determined by various 

 investigators. 33 



The maximum amount of nitrogen fixed by non-symbiotic bacteria in 

 the soil was reported as 10 mgm. for one gram of nutrient consumed. 34 



28 Hoffmann and Hammer, 1910 (p. 563). 



29 Lipman, 1903 (p. 115). 



30 Omeliansky, 1923 (p. 559). 



31 Krainsky, A. V. Uber die Stickstoffanreicherung des Bodens. Centrbl. 

 Bakt. II, 26: 231-235. 1910. 



32 Omeliansky, 1923 (p. 559); Pringsheim, 1908 (p. 564); Truffaut, G., and 

 Bezssonoff, N. Sur les variations d'energie du Clostridium pastorianum comme 

 fixateur d'azote. Compt. Rend. Acad. Sci., 175: 868-870. 1921; Influence de la 

 concentration en Sucre des milieux sur 1'activite des bacteries fixatrices d'azote. 

 Ibid., 177: 649-652. 1923. 



33 Omeliansky, W. Sur la physiologie et la biologie des bacteries fixant 

 l'azote. Arch. Sci. Biol. Petrograd, 19: 162-208. 1915. 



34 Remy, Th. Untersuchungen liber die Stickstoffsammlungsvorgange in 

 ihrer Beziehung zum Bodenklima. Centrbl. Bakt. II, 22: 561-651. 1909. 



