DcMiitrification in Soil 185 



hydrogou acceptors). In i1k> last process oxygen is ntilized by the 

 organism lor the oxidation ol earhon eoniponnds or inorganic sub- 

 stances, such as sulfur. The energy thus derived is used for the 

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

 nitrogen, or to ammonia. The formation of nitrogen gas from nitrate 

 may be so rapid under fa\ orable 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 proc- 

 esses of nitrate reduction and nitrate assimilation has often been 

 referred to as "denitrification." However, the reduction of nitrates to 

 nitrites and annnonia, as well as their assimilation by microorganisms, 

 involves no losses of nitrogen, but merely indicates that the nitrates 

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

 from which nitrate can be again produced. The nitrates may com- 

 pletely disappear without in\olving any loss of nitrogen, as in their 

 assimilation by fungi and various bacteria in the presence of avail- 

 able energy. The term "denitrification" ( or complete denitrification ) 

 should designate the complete reduction of nitiates to atmospheric 

 nitrogen and oxides of nitrogen, whereas the other processes involv- 

 ing disappearance of nitrates may be referred to as "nitrate reduc- 

 tion" and "nitrate assimilation." 



Certain bacteria are capable of reducing nitrates to nitrites, am- 

 monia, and atmosplieric nitrogen or oxides of nitrogen. Goppels- 

 roder was the first to observe that nitrates are reduced in the soil 

 to nitrites. He ascribed this property to the organic matter of the 

 soil. Schoenbein in 1868 and Meusel in 1875 recognized the bac- 

 terial nature of the process. This idea was developed further by 

 Gayon and Dupetit and others, as shown previously. 



In absence of free oxygen but in presence of nitrate, various aerobic 

 bacteria are capable of existing anaerobically. Some organisms bring 

 about complete denitrification; others reduce the nitrate to the nitrite 

 stage only, a smaller amount of oxygen thereby becoming available: 



2HNO3 - 2HNO2 + O2 

 When the nitrite is reduced to atmospheric nitrogen, 



2HNO2 = N2 + 1^02 + H2O 



In the reduction of nitrate to ammonia, the following reaction takes 

 place: 



HNO3 + H2O = NH3 -f 2O2 



