NITROGEN ASSIMILATION; THE NITROGEN CYCLE 91 



through a soil, the reason for the loss of nitrates is apparent. The 

 ammonium compounds are found to be absorbed by the soil 

 particles and do not leach through, while the nitrates are held 

 only slightly by the soil and are easily carried away by the water. 

 The organic nitrogen compounds and the ammonium salts thus 

 behave as a kind of reservoir of nitrogen. They are broken down 

 or transformed into nitrates slowly and thus furnish a gradual sup- 

 ply of available nitrogen for the plants growing in the soil. 



Nitrification. — How does this transformation from ammonia 

 to nitrates occur? Schlosing and Miintz (1877) found that, if 

 water containing ammonia was allowed to percolate slowly through 

 sand, the filtrate contained more nitrate than the original! If 

 chloroform vapor was allowed to permeate the sand, however, 

 the nitrification was slowed up and ultimately stopped. Now 

 chloroform probably had no chemical action, and, since it was 

 known to have a direct effect upon living organisms, it was con- 

 cluded that nitrification was a process depending upon micro- 

 organisms. This conclusion has been verified many times since. 

 Winogradski (1890) has been given the credit for the original iso- 

 lation in pure culture of the nitrifying organisms, although there 

 has been some doubt thrown on the purity of his cultures and the 

 reliability of his culture methods. 



Winogradski further showed that nitrification is not a simple 

 process in which only one genus of bacteria is involved but that 

 there are two distinct steps in the process. One group of bacteria 

 (Nitrosomonas and Nitrosococcus) oxidizes the ammonia as far 

 as nitrites only, while a second genus (Nitrobacter) continues the 

 process and oxidizes the nitrites to nitrates. 



Nitrosomonas and Nitrosococcus: 2NH 3 +302 = 2HN02+2H 2 

 Nitrobacter : 2HN0 2 + 2 = 2HN0 3 . 



Nitrosomonas is a small, oval/ motile form, characteristic of 

 the soils of Europe and Asia; while Nitrosococcus is a round, disk 

 form (as the ending coccus implies), found in the soils of the New 

 World. Nitrobacter is a short rod-shaped form that is not motile. 

 All three forms are checked in their activity by the presence 

 of organic compounds and, in pure cultures, are grown on a purely 

 inorganic medium. This peculiarity in their physiology was dis- 

 cussed in the preceding chapter and, since organic materials acl 

 like poisons to them, it is obvious that they cannot produce ni- 



