DcMiitiificatioii in Soil 



187 



amoiiuts ol c'iu'iij;y aw libtMatcd, siiico tlic rctliictioii of nitratos to 

 atmospheric nitrogen does not consume 'a large amount of energy. 

 Most of tlie denitriUing bacteria reduce nitrate to nitrogen gas 

 and NjO in varying proportions, B. nifroxits being particularly active 

 in the process. A 5-12 per cent solution of nitrate inoculated with 

 soil gives, at 20-37'"C, a current of gas which is 80 per cent NoO. 



16 



•Nitrate 



30 40 50 



Time, days 



Fig. 80. Reductiun of nitrate by niicrobos (from Korsakowa). 



Various other denitrifying bacteria, like Ps. aeruginosa and B. stutzeri, 

 give in solutions of nitrate ( particularly NH4NO3 ) a gas rich in NoO. 

 Of 100 cultures of bacteria tested by Maassen, 31 were found capable 

 of reducing nitrate to nitrite; the latter is then reduced to atmospheric 

 nitrogen and various oxides of nitrogen. This process was rather 

 slow and independent of the oxygen supply. Tacke found that 38 

 per cent of the gas mixture formed during the process of nitrate re- 

 duction by bacteria may consist of NoO. The formation of nitric 

 oxide in the reduction of nitrates has also been demonstrated by- 

 other investigators. 



In general, the liberation of atmospheric nitrogen by reduction of 

 nitrate depends upon changes in oxidation-reduction potential of 

 medium, pH value, presence of nitrite, and nature of available car- 

 bon sources. The presence of certain growth-inhibiting substances 

 is also of importance. The addition of KCN to a culture of Micro- 

 coccus dcnitrificans inhibited the formation of elementary nitrogen; 

 the last stage in the reduction process, that of hyponitrite to gaseous 



