DENITRIFICATION 421 



denitrification are: B. ramosus and B. pestifer, which 

 reduce nitrates to nitrites; B. mycoides, B. subtilis, B. 

 mesenteric us vulgatus and many other ammonification 

 bacteria which are capable of converting nitrates into 

 ammonia. 



Bacterium denitrificans alpha and Bacterium deni- 

 trificans beta reduce nitrates with the evolution of 

 gaseous nitrogen. 



In addition to these nitrate-destroying bacteria, 

 there are other bacteria which also utilize nitrates; 

 but, like higher plants, they convert the nitrogen into 

 organic nitrogenous substances. However, as they 

 operate in the dark and cannot obtain energy from 

 sunlight, they must have organic acids or carbohy- 

 drates as a source of energy. While these bacteria 

 cannot be considered to be denitrifiers, they help to 

 deplete the supply of nitrates when conditions are 

 favorable for their development. What these condi- 

 tions are is not well understood, nor can any estimate 

 be made as to the extent of their operations. 



Most of the nitrifying bacteria perform their func- 

 tions only under a limited access of oxygen, while 

 others can operate in the presence of a more liberal 

 supply; but, in general, thorough aeration of the soil 

 practically prevents denitrification. Straw and dung 

 apparently carry an abundant supply of denitrifying 

 organisms, and also furnish a supply of carbohydrates 

 which favors their action, so that stable manure is 

 very likely to undergo denitrification, and straw or 

 coarse stable manure are conducive to the growth of 

 denitrifying bacteria in the soil. 



