38 NITROGEN METABOLISM 



nature. Most natural organic compounds, such as amino- 

 acids, are not inhibitory, methionine being a notable excep- 

 tion. Although potassium chlorate in low concentrations 

 (e.g. io~^ M.) prevents the proliferation of Nitrohacter, it 

 did not affect either the growth of Nitrosomonas or the 

 oxidation of NO 2^ by an established culture of Nitrohacter. 

 Chloromycetin, an antibiotic containing a nitro group, is 

 especially active against organisms oxidizing NO 2" [22]. 



The rate of nitrification was found to be a function of the 

 amount of NH 4 adsorbed by the base exchange complexes 

 in the soil and could be increased by increasing the soil's 

 base exchange capacity. No such effect was produced by the 

 addition of materials (e.g. sand) whose only effect was to 

 increase the available surface area. Moreover, the presence 

 of ions such as Ca++, which can displace NHt, depressed 

 the rate of nitrification. Few bacteria were found in the per- 

 colating fluid itself, and Lees and Quastel deduced that the 

 nitrifying organisms grow on the surface of soil particles 

 [cf. 16] around receptor areas which combine with or adsorb 

 NHt- All these areas are occupied in a 'saturated soil' and 

 further growth of the bacteria is restricted to replacing dead 

 cells [16]. Quastel and Scholefield have developed a tech- 

 nique whereby the Warburg manometer can be used in 

 studies of soil metabolism and they showed that following 

 the addition of NHt or NO 7 to soil saturated with nitri- 

 fying bacteria, the Og uptake was equal to that required for 

 complete oxidation to NO 7 [22]. 



Nothing is known about the mechanisms which enable 

 the organisms to utilize the energy made available by the 

 oxidation of NH3 and NO^. Ammonia is oxidized even 

 when the cells are unable to grow, e.g. in the absence of 

 CO 2 [21c], and the suggestion has been made that a metal- 

 activated enzyme may be involved in this process [15]. 

 With regard to energy relationships and the efficiency of 

 energy utilization, the best data are those of Baas-Beck- 

 ing and Parks, who calculated the changes in free energy 

 (AF) taking place at 25° C. in conditions shown by Meyer- 



