Soil Organisms concerned in Nitrogen Cycle. 533 



auximone fractions definitely inhibit the process to a marked degree. Since 

 the auximones are water-soluble, it might be expected that the water extract 

 would also depress the rate of denitrification. However, the presence of 

 some readily oxidisable organic matter is necessary in order that denitrification 

 may proceed, and the medium used in these experiments contains such in the 

 form of tartrate. In addition to this, the water extract supplies extra 

 organic matter, and although the humus of the soil had been found by 

 Stoklasa and Ernest* to be not very serviceable for denitrification, it is quite 

 probable that ^ueh an addition might increase the rate of denitrification to a 

 degree sufficient to counterbalance the depressing effect of the auximones 

 contained. 



From all the evidence collected in the present work it is apparent tbat 

 soluble humus, and especially that produced by bacterial decomposition, is a 

 very important factor from the point of view of the activities of soil bacteria. 

 Its effect upon the organisms appears to be largely independent of any 

 inorganic matter which it may contain, or any physical action brought about 

 by its colloidal nature, and is shown to be due to the presence in the humus 

 of growth-promoting substances or auximones. The influence of these 

 auximones upon the organisms concerned in the nitrogen cycle may be 

 briefly summed up in the general statement that they increase the rate of 

 nitrogen fixation and nitrification, depress the rate of denitrification, and do 

 not appreciably affect the rate of ammonification. These results are 

 interesting from the indication they give of the specific role of the auximones. 

 If these substances merely act as stimulants to the bacterial protoplasm, it is 

 to be expected that similar effects would be produced by them upon all classes 

 of bacteria. If, on the other hand, they play some definite part in the 

 building up of the complex nitrogenous molecule, it follows that a directly 

 opposite effect might be anticipated from the addition of these substances 

 to two classes of bacteria whose activities are directed upon such widely 

 divergent lines as those concerned in the constructive processes resulting in 

 the oxidation and the fixation of nitrogen in an organic form, on the one 

 hand, and those destructive organisms which bring about its decomposition 

 and liberation in the form of the free element on the other. 



In conclusion, I wish to express my sincere thanks to Prof. W. B. Bottomley 

 for the valuable advice and help which he has so kindly given me during the 

 progress of this work. 



* Stoklasa and Ernest, 'Centr. Bakt. Par.,' Abt. II, voL 17, pp. 27-33 (1907)l 



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