242 DENITRIFICATION 



An excessive alkaline reaction is also inimical to the growth and 

 activity of denitrifying organisms, as was early shown by Pfeiffer, 

 but the application of such large quantities of caustic lime to soil 

 as he found necessary to check denitrification tends to "burn out" 

 the nitrogenous organic matter, as has been amply demonstrated 

 by the work in Pennsylvania. 



Food Requirements. The food requirements of the denitrifiers 

 are quite similar to those of other soil organisms. They can, accord- 

 ing to Richards and Rolfo, survive in purely mineral media, but in 

 such media the reduction of nitrates takes place very slowly and 

 incompletely. Jensen found a certain relationship between the 

 nitrate destroyed and the carbon compounds used. No denitrifica- 

 tion takes place without a source of carbon. The optional relation 

 between the carbon and the nitrate used was found by von Caron 

 to be for two strong denitrifiers B. pyocyaneus and B. fluorescens 

 liquefaciens, a 1 per cent, dextrose to 1.6 per cent, potassium nitrate. 

 Reduction of the nitrate supply far below that of carbon greatly 

 reduces the intensity of the process. Furthermore, the destructive 

 fermentation of nitrates depends to a great extent on the character 

 of the organic substances in the nutritive medium, some being 

 much better adapted than others to furnish the necessary energy 

 for the breaking down of the nitrates. Stoklasa claims that most 

 of the denitrifying bacteria causes no reduction of nitrates in media 

 where chemically pure d. levulose and d. galactose are present. Nor 

 is denitrification favored by glucose in nutritive solutions (Stutzer) , 

 but is promoted by the presence of salts of organic acids, like potas- 

 sium lactate, or sodium citrate. The reason for this is that glucose 

 is not as suitable for furnishing the molecular energy required for 

 the breaking down of the nitrates as are the salts of organic acids. 

 Stutzer tried four different organisms and found that they possessed 

 the power of denitrification in a different degree. Their action on 

 the different meat extracts on the market is also variable. B. 

 hartlebii was the only organism tested by him which could destroy 

 nitrates in a medium containing Liebig's beef extract. He suggested 

 that this phenomenon may be due either to a difference in chemical 

 constitution of the compounds or a difference in ionization. The 

 knowledge we now possess concerning the specificity of enzymes 

 would lead us to believe the former to be the true explanation. 



Certain of the most widely distributed carbohydrates in soil and 

 manures, as for example xylose and arabinose, are not especially 

 good nutrients for denitrifying bacteria, according to Stoklasa and 

 Vitek. The quantitative relationship which they found to occur is 

 widely different with the various carbohydrates. Of the bacteria 

 which reduce nitrates to nitrites and finally to ammonia, B. mycoides 

 reduced 20.69 per cent, of the nitrate nitrogen present to ammonia 

 in the presence of glucose 1.9 per cent, in the presence of levulose, 

 1.72 per cent, in the presence of galactose, and 1.91 per cent, in the 



