CARBON AND NITROGEN CYCLES IN THE SOIL 189 



ammonium salts are nitrified only in presence of a carbonate 

 that can change them into ammonium carbonate (296). Nitro- 

 bacter is equally specific, oxidising nitrites only and not 

 ammonia. 



Addition to the solution of almost any carbon compound 

 other than calcium or magnesium carbonates retards the rate 

 of nitrification, glucose and peptone being particularly harm- 

 ful (31 1(^). Carbon dioxide suffices as the source of carbon 

 for the growth of the organism. Godlewski^ showed that 

 nitrification proceeds in solutions free from organic matter so 

 long as the air supplied contained carbon dioxide, but stops 

 as soon as the carbon dioxide is removed by passage over 

 caustic potash. But the synthesis of complex cell substances 

 from carbon dioxide is an endothermic process requiring a 

 supply of energy. In the case of the green plant — the only 

 other living thing known to utilise carbon dioxide — the energy 

 comes from light, the transformer being chlorophyll. Here, 

 however, light is out of the question, and is even fatal to the 

 organism. Winogradsky (3li<2) suggested that the necessary 

 energy is afforded by the oxidation of ammonia and of the 

 nitrite, and he traced a definite relationship between the 

 amount of ammonia oxidised and the carbon assimilated.- 



In these experiments mixed cultures were used, the nitrate 

 producers predominating. More recently Coleman (68), using 

 pure cultures of nitrate producers, obtained ratios varying 

 from 40 to 44 for the second stage of the process. 



No useful hypothesis has yet been put forward to account 



^ Quoted in Lafar, Tech. Mykologie, 1906, Bd. 3, 165. 



