186 



Transformation of Nitrogen 



The more nearly complete the reduction of the nitrate, the more 

 oxygen becomes available, and, therefore, the greater is the amount 

 of carbohydrate that can be oxidized and the greater is the gain in 

 energy. In the case of many aerobic microorganisms, nitrate can 

 act as the hydrogen acceptor, whereby it is first reduced to the NO2 

 ion, and this, through the hypothetical dioxyammonia (HON -HON), 



7.0 6.5 6.0 5.5 5.0 4.5 



Reaction of soil, pH 



Fig. 79. Influence of reaction upon nitrogen liberation in forest soil (from 



Aaltonen ) . 



to NHo-OH ( hydroxylamine ) and then to NH3. 

 the reduction can be presented as follows: 



The first stage of 



HCOOH + HNO3 -^ CO2 + HNO2 + H2O 



Nitrate reduction can be brought about readily by a number of 

 soil bacteria, under anaerobic conditions, when carbon complexes 

 are available as sources of energy. Nitrates enable many facultative 

 anaerobes to develop under anaerobic conditions, using sources of 

 carbon which would otherwise not be available. 



The reduction of nitrates to atmospheric nitrogen always goes 

 through the nitrite stage. The following reaction was at first sug- 

 gested to explain the complete reduction of the nitrate molecule: 



SCeHiaOe + 24HNO3 = -24H2CO3 + GCOo + ISHoO + I2N2 



The carbohydrates or organic acids of the media are decomposed 

 with the formation of carbon dioxide and nascent hydrogen; the 

 nitrate is then used by the organism as the hydrogen acceptor, which 

 results in the reduction of the nitrate. When tartaric acid is oxidized 

 by atmospheric oxygen or by reduction of nitrates, nearly equal 



