REDUCTION PROCESSES IN THE SOIL 543 



under anaerobic conditions, with the formation of pyruvic acid and 

 hydrogen. 5 



C 6 Hu 6 = 2 CH 3 -CO-COOH + 2 H 2 + 12 Cal. 



Nitrates and sulfates can act as the hydrogen acceptors and are reduced 

 to nitrites, sulfides, etc. 



Reducing conditions in the soil have usually been recognized either 

 by the absence of oxidation or by the presence of specific reducing sub- 

 stances, as ferrous carbonate; soils acquire these conditions when water- 

 logged for a few days. The H 2 S formed from the reduction of sulfates 

 combines with iron to form insoluble iron sulfide. The ferrous com- 

 pounds themselves act as reducing agents. The very presence of these 

 compounds indicates the intense reducing power of the soil. 



Bacterial cultures themselves are normally reducing. 6 Processes of 

 reduction require sources of energy to enable the organisms to carry on 

 their activities. In most cases, however, these are obtained from various 

 organic compounds. 7 The autotrophic bacteria, for example, use the 

 energy obtained by chemosynthetic processes for the reduction of CO2. 

 Various organic compounds may be reduced under anaerobic conditions, 

 especially in connection with oxidation of other substances which result 

 in the liberation of energy. The reducing power of bacteria has 

 commonly been determined by the use of certain organic substances, 

 especially dyes, and of certain inorganic substances, such as nitrates 

 and sulfates, acting as hydrogen acceptors. The hydrogen obtained 

 from the decomposition of organic matter is used by the bacteria for 

 the reduction of the dye 8 or the nitrate. In some cases aldehydes or 

 purine bases (hypoxanthine, xanthine, adenine) are required as hydro- 

 gen donators. 9 



Transformation of nitrates by microorganisms. The disappearance of 

 nitrates in the soil as a result of activities of microorganisms may be 



5 Aubel, E., and Salabartan, J. Mecanisme de la production d'hydrogene 

 aux depens du glucose par le bacille coli. Compt. Rend. Acad. Sci., 180: 1183- 

 1186, 1784-1787. 1925. 



8 Rubner, M. Reduktionswirkungen bei Bakterien. Arch. Hyg., 16: 62. 

 1893. 



7 Beijerinck, M. W. Phenomenes de reduction produits par les microbes. 

 Arch. Sci. Ex. Nat. Neerl. (II), 9: 131-157. 1904. 



8 Carapelle, E. tlber die Reduktionerscheinungen bei Bakterien. Centrbl. 

 Bakt. I, Orig., 47: 545-559. 1908. 



9 Dixon, M., and Thurlow, S. Studies on xanthine oxidase. III. The reduc- 

 tion of nitrates. Biochem. Jour., 18: 989-992. 1924. 



