396 MICROBIOLOGY OF SOIL 



number of organisms are already known that can cause the complete 

 destruction of nitrates with the evolution of nitrogen gas or nitrogen 

 oxides. The following reactions illustrate diagrammatically the com- 

 plete or partial reduction of nitrates. 



2HNO 3 = 2HNO 2 + O 2 

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



4HNO 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 

 conditions and transfer the oxygen of nitrates and nitrites to carbon 

 compounds. This is illustrated by the equations suggested by van 

 Iterson: 



5 C + 4 KN0 3 + 2H 2 = 4 KH C0 3 + 2 N 2 + CO 2 

 3 C + 4KNO 2 + H 2 O = 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 + 2 H 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 + i 3 H 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: 



6KNO 3 + 58 + 2CaCO 3 = 3K 2 SO 4 + 2CaSO 4 + 2CO 2 + 3 N 2 



The Actinomyces reduce nitrates to nitrites, but they do not cause 

 any loss of free nitrogen, for the nitrites are utilized by the organisms, 

 and complete denitrification does not take place. Thus these organ- 

 isms may prevent the leaching out of nitrates and nitrites in the soil, 

 or the active denitrification by other organisms. 



Relation to Environment. Nitrate reduction is favored by insuffi- 

 cient aeration, as well as by an abundance of readily decomposable 

 organic matter. In fine-grained, compact soils nitrate formation and 

 nitrate reduction may alternate, depending upon the more or less 



