148 TRANSFORMATION OF NITROGEN BY SOIL MICROBES 



hydroxylamine generally do not accumulate but are formed as 

 intermediary products, but the others may appear in appre- 

 ciable amounts where active reduction occurs. The relation- 

 ships of these compounds to each other are schematically 

 represented in Fig. 64. 



These reduction products are the result of either direct reduc- 

 tion reactions or secondary reactions. The reducing power of the 

 organism determines the primary products which are formed 

 (either nitrous acid, hyponitrous acid, hydroxylamine, or am- 

 monia), these leading to the formation of the gaseous sub- 

 stances. 



It is apparent from Fig. 65 that the reaction takes place in sev- 

 eral stages, nitrous acid first appearing as the nitrate is reduced. 

 In turn, the nitrous acid is reduced to other more completely 

 reduced compounds, and eventually they also disappear coincident 

 with the formation of elementary nitrogen. 



The organisms concerned in the various reductions of nitrate are 

 quite numerous. Bacteria play by far the most important role in 

 these transformations. The formation of nitrite from nitrate is 

 common to actinomyces as well as to numerous bacteria. The 

 formation of ammonia, nitrous oxide, and nitrogen are reactions 

 confined to certain species of bacteria. In the absence of uncom- 

 bined oxygen, the oxygen contained in the nitrate is utilized by 

 these organisms to perform various oxidation processes. Sulfate 

 serves a similar purpose under anaerobic conditions, where it is 

 reduced to sulfide. 



Energy is required to effect the reductions, and the reactions do 

 not occur in the absence of food materials from which micro- 

 organisms may obtain the necessary energy. Various soluble 

 carbohydrates, cellulose, organic acids, and alcohols suit the 

 requirements of the organisms. Some bacteria even use sulfur 

 as follows: 



6KNO3 + 5S + 2CaC03 = 3K2SO4 + 2CaS04 + 2CO2 + 3N2 



Table 34 indicates that the amounts of ammonia or gaseous nitro- 

 gen formed from nitrate differ with the organisms active in the 

 reaction and the carbohydrates utihzed as foods. 



The formation of the gaseous products is the most undesirable 

 change of all the nitrate reductions, since the volatilization of the 



