108 TRANSFORMATION OF NITROGEN BY SOIL MICROBES 



As shown by Fig. 51, the largest amount of nitrogen is fixed per 

 unit of food material used during the early periods of cell develop- 

 ment of a culture. This decreases rapidly as the culture ages. 

 In the young culture most of the cells are using the food materials 

 for growth and multiplication, and nitrogen fixation is propor- 

 tionately efiicient. In the later stages much of the energy is con- 

 sumed merely to support respiration of many of the cells, and few 

 cells are multiplying. Eventually a stage is reached where multi- 



-Nitrogen Fixed per Gm. Sugar 

 -Nitrogen Fixed 

 "Sugar Decomposed 



/ 



/ 



/ 



/.. 



/• 



15 20 25 



Time (Days) 



30 



35 



40 



Fig. 



51. — Relationship between carbohydrate decomposed and nitrogen fixed 

 (after Omeliansky). 



plication ceases entirely and no nitrogen is fixed, even though food 

 is being consumed to keep alive the cells which already exist. 



For similar reasons, the lower the initial concentration of the 

 energy source in the absence of fixed forms of nitrogen, the greater 

 are the amounts of nitrogen fixed per unit of energy material 

 decomposed. This is apparent from Table 24. 



Since under soil conditions there is generally a comparatively 

 small amount of energy materials present at any one time, it is likely 

 that a relatively large amount of nitrogen is fixed per unit of carbo- 

 hydrate consumed during the process. In fact, where nitrogen 



