CARBON AND NITROGEN CYCLES IN THE SOIL 89 



and some calcium or magnesium carbonate, but no other carbon com- 

 pound 1 After three or four weeks at 25 the ammonia has all gone 

 and its place is taken by nitrates. The conversion is almost quantitative, 

 only an insignificant quantity of nitrogen being retained by the organisms. 



The course of the oxidation is unknown, and nothing intermediate 

 between ammonia and nitrous acid has been detected. Omelianski 

 could obtain no evidence of an oxidase in Nitrosomonas (222). The 

 action of both organisms seems to be entirely specific. Nitrosomonas 

 oxidises ammonium carbonate and nothing else; it will not touch 

 nitrates, urea, or the substituted ammonias. Even ammonium salts are 

 only nitrified in presence of a carbonate that can change them into 

 ammonium carbonate (296). Nitrobacter 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, glu- 

 cose and peptone being particularly harmful (312). Carbon dioxide 

 suffices as the source of carbon for the growth of the organism. God- 

 lewski 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 (311) 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 : 



1 Omelianski (221) used 2 grams each (NH^SO 4 and NaCl, i gram KH 2 PO 4 , 

 5 MgSO 4 , -4 FeSO 4 in i litre of water, and added -5 gram MgCO 3 for each 50 c.c. of so- 

 lution used. Nitrite formation goes on in this solution. For nitrate production he used 

 i gram each NaNO 2 and Na 2 CO 3 , -5 each KH 2 PO 4 and NaCl, -4 FeSO 4 and -3 MgSO 4 in i litre 

 of water. Ashby (5) found that both processes went on simultaneously when he diluted the 

 first of these solutions to one quarter the strength. 



7 



