SOME ASPECTS OF NITRATE REDUCTION 



In most textbooks it is stated that even low concentrations of oxygen 

 are able to suppress dissimilatory nitrate reduction, a conclusion to 

 which much importance is attached, since it tends to imply that not- 

 withstanding the ubiquity of denitrifying bacteria in various soils 

 nitrogen losses from nitrate do not occur, if a sufficient aeration of the 

 soil is warranted. 



In later years several authors, Korochkina [1936], Meiklejohn 

 [1940], Korsakova [1941], Sacks and Barker [1949], Skerman et al. 

 [1951] and Verhoeven [1952] have convincingly shown for quite 

 divergent types of denitrifying bacteria that only a very thorough 

 aeration of the cultures is capable of suppressing denitrification. With 

 the aid of the polarographic method Skerman et al. have clearly demon- 

 strated that in growing cultures nitrate reduction does not commence 

 before the oxygen concentration in the medium is reduced to 'zero', 

 and in addition they have shown that only an energetic stirring of the 

 culture in contact with air suffices to maintain a measurable concen- 

 tration of oxygen in the medium. 



We must conclude from their experiments that even very small 

 amounts of oxygen, indeed, at once suppress denitrification. Whether 

 this is due to a direct inactivation of the nitrate reducing enzyme by 

 the oxygen, or whether the respiration process as such in some way 

 interferes with the nitrate reduction must be left undecided. 



Although the extreme sensitivity of nitrate reduction towards free 

 oxygen is in itself very reassuring from the point of view of nitrogen 

 losses in arable soils, on the other hand the demonstration of the dif- 

 ficulty of maintaining oxygen tensions at places of active bacterial metab- 

 olism leaves no doubt that not seldom in micro-elements of soils oxygen 

 tension will be zero, thus opening the possibility of denitrification. 



A most remarkable result of Verhoeven's investigation was that 

 oxygen had a most unexpected effect on the metabolism of various 

 strains of Denitrobacillus licheniformis . In studying the disappearance 

 of nitrate in shallow layer cultures with surface aeration, it was found 

 that a surprisingly high percentage (sometimes 90%) of the nitrogen 

 of the nitrate consumed was under these conditions converted into 

 its ultimate reduction stage: ammonia. Also in this case only the 

 vigorous aeration obtained in shake cultures led to a preservation of 

 the nitrate. 



Taking into account that under anaerobic conditions Denitrobacillus 



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