544 PRINCIPLES OF SOIL MICROBIOLOGY 



due to three groups of phenomena: (1) direct utilization of nitrates 

 by microorganisms as sources of nitrogen, in the presence of sufficient 

 energy material, (2) reduction of nitrogen to nitrites and ammonia in the 

 process of the nitrate assimilation, (3) utilization of nitrates as sources 

 of oxygen (nitrates as hydrogen acceptors) . In the last process oxygen 

 is used by the organism for the oxidation of carbon compounds or inor- 

 ganic substances, such as sulfur. The energy thus derived is used for 

 the reduction of the nitrate to the nitrite, free nitrogen gas, oxides of 

 nitrogen or the ammonia stage. The formation of nitrogen gas from 

 nitrate may be so rapid under favorable conditions that the gas can 

 actually serve as a measure of the amount of nitrate reduced. 



The disappearance of nitrates in the soil due to the various processes 

 of nitrate reduction and nitrate assimilation has often been referred to 

 as "denitrification." However, the reduction of nitrates to nitrites and 

 ammonia as well as their assimilation by microorganisms does not 

 involve any losses of nitrogen, but merely indicates that the nitrates 

 are for the moment taken out of circulation and transferred into forms 

 from which nitrate can be again produced. The nitrates may even 

 completely disappear without involving any loss of nitrogen, as in the 

 case of their assimilation by fungi and various bacteria in the presence 

 of available energy. 10,11 The term denitrification (or complete denitri- 

 fication) should designate the complete reduction of nitrates to atmos- 

 pheric nitrogen and oxides of nitrogen, while the other processes 

 involving disappearance of nitrates may be referred to as nitrate 

 reduction and nitrate assimilation. 



Nitrate assimilation. Large numbers of microorganisms, including 

 bacteria, actinomyces, fungi and algae are capable of utilizing .nitrates 

 as a source of nitrogen. In the presence of a sufficient source of avail- 

 able energy, the microorganisms rapidly assimilate the nitrate nitro- 

 gen and transform it into proteins. The nature of the organism, the 

 amount and nature of energy source, as well as the environmental con- 

 ditions, influence the amount of nitrate thus assimilated. 



The fungi readily utilize nitrate-nitrogen, although often not to such 

 an extent as ammonia nitrogen. For every 30 to 40 units of carbohy- 

 drate decomposed, certain fungi assimilate one part of nitrogen. 

 The nitrate is usually first reduced to ammonia before it is assimilated. 

 Only certain groups of bacteria (so-called "nitrate" bacteria) are 

 capable of utilizing this source of nitrogen. The actinomyces assimi- 



10 Vogel, J. Ammoniak und Salpeterassimilation durch Mikroorganismen. 

 Centrbl. Bakt. 11,^32: 169-179. 1912. 

 "Bierema, 1909 (p. 486). A 



