CARBON AND NITROGEN CYCLES IN THE SOIL 89 



tions the nitrate producer is somewhat sensitive even to ammonium 

 salts, indeed both Warington (296) and Omelianski (p. 87) suppressed 

 it by maintaining a sufficient concentration of ammonium sulphate ; 

 Lohnis has shown, however (185), that it is more tolerant in the soil. 

 Some substance toxic to them is produced when soil is heated to 98 C. 

 or more, and in such soils they cease to act. Neither nitrosomonas nor 

 nitrobacter has been observed to form spores, or to survive temperatures 

 above 45 C., or treatment with mild antiseptics like carbon disulphide 

 and toluene. But so widely distributed are they and so readily can they 

 spread in the soil, if the conditions are at all favourable, that they may 

 reappear unless special precautions are taken to prevent infection. 

 Thus, it is commonly stated that treatment of the soil with carbon 

 disulphide merely depresses without killing the organisms. Russell 

 and Hutchinson found, however, that the organisms did not reappear if 

 the soil was kept carefully free from re-infection (240). 



In pure cultures the organisms cannot tolerate absence of moisture, 

 but die at once. In soil, however, they are more resistant. Absence 

 of air puts an end to their activity. 



There is some evidence that nitrobacter is more sensitive to adverse 

 circumstances than nitrosomonas ; it is also more rapid in action. 

 Otherwise the two sets of organisms show very similar behaviour to 

 external influences, their main difference being the fundamental one 

 that nitrosomonas oxidises ammonia, but not nitrites, while nitrobacter 

 oxidises nitrites, but not ammonia. There are also certain morpho- 

 logical differences. Nitrosomonas, or coccus, occurs in several forms, 

 mostly oval in shape, -5 to I p wide and up to 2 jj, long, but whether these 

 are really distinct varieties is not known ; a zooglea stage is also found ; 

 nitrobacter is rod-shaped, I //, long and about 0*3 /JL thick, only one 

 variety has been recognised. No other organisms are known with cer- 

 tainty to produce nitrates in the soil, nor can any other compound 

 except ammonia be nitrified (220). 



The Evolution of Gaseous Nitrogen. 



A considerable loss of nitrogen occurs during the decay of plants, 

 of dung and of soil organic matter in presence of air. The loss has 

 been studied somewhat fully in the case of dung, because of its great 

 technical importance, and it is attributed to an evolution of gaseous 

 nitrogen during the processes of decay. 1 It only appears to go on so 

 long as a supply of oxygen is available. 



1 See papers by Pfeiffer and others in Landw. Versuchs-Stat., 1897, xlviii., 163-360, for 

 an account of the loss from dung. 



7 



