CARBON AND NITROGEN CYCLES IN THE SOIL 91 



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 //, wide and up to 2 /JL 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*3/1- 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. 



The oxidising bacteria are usually credited with the change, but 

 no organism has yet been isolated capable of bringing it about. 

 Nothing whatever is known about the mechanism of the process. No 

 experiments appear to have been made with pure substances, or pure 

 cultures of organisms, but only with the highly complex mixtures 

 present in soil or dung. Further work is very desirable. 



A similar loss goes on in sewage beds and has been studied by 

 Letts and Adeney (i), and by Miintz and Laine (209). 



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

 an account of the loss from dung, 



7* 



