THE NITROGEN CYCLE 455 



such conditions, there is a return of nitrates to the upper 

 soil as capillary water moves upward to replace evapo- 

 rated water. In fact, wherever evaporation takes place 

 to any considerable extent there is some movement of 

 this kind. The need for catch crops to take up and pre- 

 serve nitrogen is therefore greater in a humid region than 

 in an arid or a semiarid one. A system of cropping that 

 allows the land to stand idle for some time, or a crop that 

 requires intertillage, as does maize, fails to utilize all the 

 nitrates produced, and promotes the loss of nitrogen in 

 drainage water. 



374. Nitrate reduction. — The nitrogen-transforming 

 bacteria thus far studied have been those that cause 

 the oxidation of nitrogen as the result of their activi- 

 ties. A number of forms of bacteria that accomplish a 

 reverse action may now be considered. The several 

 processes involved are commonly designated by the 

 general term denitrification, and comprise the follow- 

 ing : 1, reduction of nitrates to nitrites and ammonia; 

 2, reduction of nitrates to nitrites, and of these to ele- 

 mentary nitrogen. 



The number of organisms that possess the ability to 

 accomplish one or more of these processes is very large — 

 in fact, greater than the number involved in the oxida- 

 tion processes ; but, in spite of their numbers, permanent 

 loss of nitrogen in ordinary arable soils is unimportant 

 in amount, although in heaps of barnyard manure it 

 may be a very serious cause of loss. *~ 



Some of the specific bacteria reported as bringing about 

 nitrate reduction are : B. ramosus and B. pestifer, which 

 reduce nitrates; B. mycoides, B. subtilis, B. mesentericus 

 vulgatus, and many other ammonification bacteria which 

 are capable of converting nitrates into ammonia. 



