REDUCTION PROCESSES IN THE SOIL 545 



late nitrate readily, but usually reduce it first to nitrites; carbon 

 sources favoring growth also favor nitrate reduction; when nitrite 

 is the sole source of nitrogen, particularly in low concentrations (0.01 to 

 0.05 per cent), it is assimilated very readily as such, but when present 

 in the nitrate form it is first reduced to nitrites and then assimilated. 

 The reduction of nitrate by microorganisms is usually accompanied by 

 an increase in alkalinity of the medium, due to the fact that the reduced 

 anion is assimilated and the cation is left. The amount of nitrate- 

 nitrogen converted into microbial protoplasm will thus depend upon the 

 nature of the organisms active in the process as well as upon the environ- 

 mental conditions. Seiser and Walz 12 demonstrated that a pure culture 

 of a bacterium (Bad. putidum) assimilated, under anaerobic conditions, 

 about ten per cent of the nitrate nitrogen in the medium, but, under 

 aerobic conditions, nearly thirty-three per cent of the nitrogen was 

 assimilated, due to the greater utilization of the energy (citric acid), 

 under aerobic conditions. 



Utilization of nitrates by microorganisms as sources of oxygen. Certain 

 bacteria are capable of reducing nitrates to nitrites, ammonia, and atmos- 

 pheric nitrogen or oxides of nitrogen. Goppelsroder 13 was the first to 

 observe that nitrates are reduced in the soil to nitrites. This was at- 

 tributed by Meusel 14 to the action of bacteria. As already mentioned, 

 microorganisms can utilize nitrates as sources of oxygen in the presence 

 of inorganic or organic substances which serve as sources of energy (or 

 as hydrogen donators). In the absence of free oxygen but in the 

 presence of nitrate, various aerobic organisms are capable of existing 

 anaerobically. Some organisms bring about complete denitrification; 

 others reduce the nitrate to the nitrite stage only, with a smaller 

 amount of oxygen becoming thereby available. 



2 HN0 3 = 2 HN0 2 + 2 (- 36.6 Cal.) (1) 



If we assume that one molecule of oxygen can liberate 112 Calories, when 

 carbohydrates are used as a source of energy (with complete oxidation to 

 H 2 and C0 2 ) a net gain in the above reaction is obtained: 



112 - 36.6 = 75.4 Cal. 



12 Seiser, A., and Walz, L. Stickstoffumsatz bei der Denitrifikation. Arch. 

 Hyg., 95: 189-208. 1925. 



13 Goppelsroder, F. Beitrage zum Studium der Salpeterbildungen. Poggens- 

 dorf's Annallen., 115: 125. 1862. ■ 



14 Meusel, E. Nitritbildung durch Bakterien. Ber. deut. chem. Gesell., 8: 

 1214, 1653. 



