DENITRIFICATION. 67 



are comprised under this head: i. The reduction of nitrates into 

 nitrites. 2. The reduction of nitrates to give off free nitrogen. 3. 

 The reduction of nitrites into free nitrogen. The term denitrifica- 

 tion is sometimes used to cover all of these types of reduction, and 

 sometimes more particularly to refer only to the reduction of the 

 nitrates and nitrites, so as to liberate free nitrogen. In its strict 

 use it should be confined to the latter process. 



It is evident that these different types of reduction will have 

 different effects upon soil fertility. Those portions of the nitrogen 

 that are reduced to nitrates or ammonia, under proper conditions 

 may be built up again into nitrates by the nitrifying bacteria. But 

 those that are reduced to a condition of free nitrogen pass off 

 into the air and out of the reach of plant life. This nitrogen, 

 therefore, represents an actual loss to the soil. Denitrification 

 is a process very different from the general type of decomposition 

 which we have described. Decomposition begins with proteids and 

 reduces them to ammonia compounds. Denitrification begins with 

 nitrates and nitrites, and liberates free nitrogen. 



The Denitrifying Bacteria. Denitrification is the result 

 of the action of a class of bacteria known as the denitrifiers. Very 

 many bacteria have the power of extracting the oxygen from nitrates, 

 reducing them to nitrites, but the list of those that can liberate free 

 nitrogen is shorter. Some of them act in aerobic conditions, and 

 others in anaerobic conditions. The names B. denitrificans^ 

 I and II have been given to two of them, but others have been found 

 with similar properties. They are very widely distributed; they 

 are found not only in soil and water, but in the air and all organic 

 decomposing refuse. They are very abundant in the manure heap, 

 especially if it contains much hay and straw, and they are likely 

 to cause a considerable loss of nitrogenous matter by liberating 

 the nitrogen as free nitrogen gas. Excrement always contains 

 them, but they are more abundant in the excrement of herbivorous 

 animals than of carnivorous animals. These bacteria, in order to 

 grow vigorously, require some carbon-holding food, and they cause 

 the largest amount of denitrification when abundantly supplied 

 with carbohydrates. Sugars, starches, glycerin, or organic acids 



