36 NITROGEN METABOLISM 



in very high concentrations [22]. But it must also be 

 emphasized that the inhibitory action of any substance is 

 determined by the organism concerned and also by the 

 physical properties of the environment, e.g. peptone is far 

 less inhibitory in the presence of sand than in ordinary 

 liquid cultures [32]. 



Owing to their slow growth and the difficulty of isolating 

 pure cultures, there have been few comprehensive investi- 

 gations of the metabolism of Nitrosomonas and Nitrohacter. 

 Meyerhof has dealt with the effect of substrate concentra- 

 tion, inorganic ions and various inhibitors on the rate of 

 nitrification in liquid cultures [20]. Nitrification under con- 

 trolled conditions in soil has been studied more recently by 

 Lees and Quastel using a technique based on the continuous 

 repercolation of fluid through a column of soil [16]. This 

 was accomplished by a simple apparatus which ensured that 

 the soil was nearly saturated with water and was adequately 

 aerated (cf. Fig. 3.1). 



When NHt was added to the percolating fluid there was 

 a lag period after which the rate of nitrate formation 

 gradually increased and at the end of five days became 

 linear. During the first two days small quantities of NO "2 

 were detected. A graph of the progress of nitrification is 

 reminiscent of that expressing the rate of growth of a 

 bacterial culture and it appears that the soil ultimately 

 becomes saturated with nitrifying bacteria. At this stage the 

 behaviour of the system is analogous to that of a washed 

 cell suspension, i.e. it is metabolically active but the organ- 

 isms are not actively dividing. When a fresh solution con- 

 taining NH| was percolated through such a 'saturated soil', 

 nitrification occurred at a linear rate from the beginning 

 (Fig. 3.2). Consequently when the 'saturated soil' is treated 

 with a substance which is nitrified directly, NO 7 formation 

 should commence at once and at a maximal and linear rate. 

 A lag period implies that the substance can only be nitrified 

 after being converted into another compound and that time 

 is required for the formation of the appropriate adaptive 

 enzymes in the existing bacteria or for the growth of new 



