REDUCTION PROCESSES IN THE SOIL 551 



of the proper organic substances the bacteria may liberate practically 

 all the nitrogen present in the nitrate form as free nitrogen gas, while in 

 moderately moist soil only protein may be formed out of the nitrate. 

 But if the soil is very moist and nitrates are present, denitrifying bacteria 

 behave as in solution and liberate considerable quantities of free nitrogen 

 gas. The minimum moisture content at which the complete reduction 

 of nitrate may occur may depend upon the nature of the soil. Oelsner 32 

 found that it takes place in soils containing 40 per cent moisture even 

 when no additional carbon has been added. This is especially true of 

 rice fields 33 and of peat soils. 34 The loss of available nitrogen as a result 

 of liming acid peat soil was ascribed 35 to the reduction of nitrate to 

 nitrite and to atmospheric nitrogen. A decrease in aeration leads to 

 an increase in denitrification; cultivation alone could not prevent the 

 loss of nitrogen. The use of disinfectants for the destruction of the deni- 

 trifying bacteria in the soil is often recommended. 



Denitrification is found to be at an optimum at 27° to 30°. However, 

 it takes place abundantly in the soil even in the coldest seasons of the 

 year when the temperature of the soil at a depth of 0.2 to 0.3 meters 

 is about zero. 36 The optimum reaction for denitrification is at pH 7.0 

 to 8.2; the process is greatly retarded at pH 5.2 to 5.8 and 8.2 to 9.O. 37 



The denitrifying bacteria (except the forms which obtain their energy 

 from the oxidation of sulfur) require organic matter for their metabolism 

 and are, therefore, favored by an addition of available organic matter. 

 Various mono- and di-basic organic acids (except oxalic) can be utilized 

 as sources of carbon. 38 Glucose is one of the best sources of energy. 

 Fresh straw is utilized to a more limited extent and composted straw 



3J Oelsner, A. Uber Nitratereduktion in nassem Ackerboden ohne Zusatz 

 von Energiematerial. Centrbl. Bakt. II, 48: 210-221. 1918. 



33 Daikuhara, G., and Imaseki, T. Bui. Imp. Centrl. Agr. Exp. Sta. Japan, 

 1: 7. 1907. 



34 Ritter, 1912 (p. 790); see also Lemmermann, O., and Wichers, J. L. Verlauf 

 der Denitrifikation in Boden bei verschiedenem Wassergehalt. Centrbl. Bakt. 

 II, 41: 608-625. 1914. 



35 Arnd, 1914-1916 (p. 531). 



36 Barthel, 1909 (p. 537). 



37 Zacharova, T. M. The influence of soil reaction upon denitrification, in 

 connection with liming of soil (Russian). Institute of Fertilizers, Moskau. 

 No. 29. 1925. 



38 Jensen, 1897 (p. 180); Salzmann, P. Chemisch-physiologische Untersuchun- 

 gen liber die Lebendauer zweier denitrifizierender Bakterien und der Strepto- 

 thrix chromogena. Diss. Konigsberg. 1902. 



