THE BIOLOGICAL ACTIVITIES OF BACTERIA 53 



85 out of 109 investigated by Maassen ' being found to possess this power. 

 This, however, is not nearly so harmful a source of nitrogen waste as 

 the process technically spoken of as true denitrification, in which 

 nitrates are reduced, through nitric and nitrous oxides, to elementary 

 nitrogen. 



This phenomenon, more widely spread among bacteria than at first 

 beheved, depends essentially upon simple oxygen extraction from the 

 nitrates by the bacteria, and for this reason goes on most actively when 

 the supply of atmospheric oxygen is low. The first bacteria described 

 as possessing this power of denitrification were the so-called B. denitri- 

 ficans I and II, the first an obligatory anaerobe, the other a facultative 

 aerobe. Since then numerous other bacteria, among them B. coli and 

 B. pyocyaneus, have been shown to exhibit similar activities. It is 

 important agriculturally, therefore, to know that many species which 

 are able to utilize atmospheric oxygen when supplied with it, will get 

 their oxygen by the reduction of nitrates and nitrites when free oxygen 

 is withheld. It is thus clear that a loss of nitrogen is much more apt 

 to proceed rapidly in manure heaps which are piled high and poorly 

 aerated. There are other factors, however, in regard to the physi- 

 ology of these microorganisms, which must be considered for practical 

 purposes. 



In order that these bacteria may develop their denitrifying powers 

 to the best advantage, it is necessary to supply them with some carbon 

 compound which is easily absorbed by them. This, ia decomposing 

 material, is furnished by the products of the carbohydrate cleavage 

 going on side by side with the proteolytic processes. It is still more 

 or less an open question whether the facilitation of denitrification 

 brought about in manure heaps by the presence of hay and straw is due 

 to the carbon furnished by these materials, or whether it is due to the 

 fact that bacilli of this group are apt to adhere to the straw which acts 

 in that case as a means of inoculation. 



The actual danger of nitrogen depletion of the soil by denitrifying 

 processes is probably much less threatening than was formerly supposed ; 

 for, in the first place, the conditions for complete denitrification are 

 much more perfect in the experiment than they ever can be ia nature, 

 and the nitrifying processes going on side by side with denitrification 

 make up for much of the loss sustained. 



1 Maassen, Arb. a. d. kais. Gesundheitsamt, 1, xxviii, 1901. 



