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. ramosm 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. 



