264 MICROBIOLOGY OF SOIL. 



For more than a decade after that, data were rapidly accumulating in 

 support of Schoenbein's contention, until in 1882 Gayon 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, denitrificans a., and B. denitrificans /?., 

 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 increased the number of 

 known denitrifying bacteria, but added much to our knowledge con- 

 cerning 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 evolu- 

 tion of nitrogen gas or of nitrogen oxides. The following reactions illus- 

 trate diagrammatically the complete or partial reduction of nitrates. 



2HNO S =2HNO 2 +O 2 

 HNO 3 + H 2 O = NH 3 + 2 O 2 

 4 HNO 2 = 2H 2 O+ 2N 2 + 3O 2 



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

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

 ditions and transfer the oxygen of nitrates and nitrites to carbon com- 

 pounds. This is illustrated by the equations suggested by van Iterson: 



3 C + 4 KN0 2 +H 2 0=2KH CO 3 +K 2 CO 3 + 2 N 2 



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: 



C 2 H 5 NH 2 +HNO 2 =C 2 H 5 OH+N 2 + H 2 O 

 NH 4 C1+KNO 2 =KC1+2H 2 O + N 2 



An organism has been described by van Iterson that can decompose 

 nitrates in the presence of cellulose: 



5C 6 H 10 O 5 + 2 4 KNO 3 = 2 4 KHCO 3 + 6CO 2 + i2N 2 + i3H 2 O 

 Still more interesting is Thiobacillus denitrificans described by 

 Beyerinck as capable of reducing nitrates in inorganic media. The 

 nitrate oxygen is used to oxidize elementary sulphur: 



6 KNO s +5S+2CaCO s =3 K 2 SO 4 +2CaSO 4 +2CO 3 +3N 2 



