334 MICROBIOLOGY OF SOIL 



and Dupetit made it certain that nitrate reduction with the evolution 

 of nitrogen gas may be caused by a "ferment." Finally, in 1886, the 

 same investigators described two organisms, B . denitriUcans a, and B. 

 denitrificans /3, capable of completely reducing nitrates. Subsequently 

 the studies of Giltay and Aberson, Burri and Stutzer, Severin, van 

 Iterson, Jensen, Beyerinck and of many others not only greatly in- 

 creased the number of known denitrifying bacteria, but added much to 

 our knowledge concerning the development and activities of these 

 organisms. It has been shown that a very large number of species 

 can reduce nitrates to nitrites and ammonia; moreover, a considerable 

 number of organisms are already known that can cause the complete 

 destruction of nitrates with the evolution of nitrogen gas or nitrogen 

 oxides. Ihe following reactions illustrate diagrammatically the com- 

 plete or partial reduction of nitrates. 



2HNO3 = 2HNO2 + O2 

 HNO3 -f H2O = NH3 + aOi! 



4HNO2 = 2H2O + 2N2 + 3O2 



In the soil, manure or other culture media the denitrifying bacteria 

 which are, for the most part, aerobic develop also under anaerobic 

 conditions and transfer the Oxygen of nitrates and nitrites to carbon 

 compounds. This is illustrated by the equations suggested by van 

 Iterson: 



SC + 4KNO3 + 2H2O = 4KH CO3 + 2N2 + CO2 

 3C + 4KNO2 + H2O = 2KH CO3 + K2CO3 + 2N2 



/ When nitrates are reduced to nitrites in the presence of amino- 

 compounds, or even of ammonium compounds, elementary nitrogen 

 may escape as shown by the following reactions: 



C2H6NH2 + HNO2 = CaHjOH + N2 -I- H2O 

 NH4CI -f KNO2 = KCl + 2H2O + N2 



An organism has been described by van Iterson that can decompose 

 nitrates in the presence of cellulose: 



SCeHioOs + 24KNO3 = 24KHCO3 + 6CO2 + 12N2 + 13H2O. 



Still more interesting is Tkiobacillus denitrificans described by 

 Beyerinck as capable of reducing nitrates in inorganic media. The 

 nitrate oxygen is used to oxidize elementary sulphur: 



6KNO3 + sS + 2CaC03 = 3K2S04 + 2CaS04 + 2CO2 + 3N2 



