IMPORTANCE OF DENITRIFICATION IN SOIL 151 



supply of available nitrogen. However, although nitrate may dis- 

 appear from the soil in numerous ways, its reduction to gaseous 

 substances occurs only under very specific conditions, and is not 

 apt to be of particular economic importance in most arable soils. 

 The conditions particularly conducive to denitrification are: 



(1) Presence of relatively large amounts of nitrate. 



(2) Presence of an abundance of decomposable organic 

 matter. 



(3) Lack of free oxygen (anaerobic conditions). 



These conditions may appear where soils become water-logged 

 through faulty drainage or where soils are purposely flooded, as 

 are rice fields. Conditions may also be favorable where concen- 

 trations of organic matter as composts of manures or plant residues 

 undergo transformation in the presence of nitrate. 



When nitrates are added to the soil, together with large quanti- 

 ties of stable manure or green manure, decomposition of the organic 

 substances may be accompanied by denitrification. The rapid 

 decomposition of the fresh organic substances leads to an exhaus- 

 tion of the available oxygen in the soil air and the formation of 

 equivalent amounts of carbon dioxide. Where carbon dioxide is 

 produced more rapidly than diffusion permits access of atmos- 

 pheric oxygen, anaerobic conditions are created and denitrification 

 may temporarily become a factor of importance. The application 

 of manures in amounts common to agricultural practice does not 

 cause such rapid consumption of the oxygen in the soil, and no 

 denitrification takes place. 



Denitrification is responsible for a large part of the loss of 

 nitrogen from animal manures stored in large quantities under 

 aerobic conditions. If the manures are kept anaerobic, little loss 

 of nitrogen in the elementary form occurs. As much as 40 to 

 50 per cent of the nitrogen may disappear as gaseous nitrogen 

 under conditions favorable for denitrification. The loss is asso- 

 ciated with the nitrification reaction. The ammonia produced 

 from the decomposition of urea becomes changed to nitrate in 

 aerobic regions of the manure heap. This nitrate in turn may 

 become washed into lower depths of the heap, where it is rapidly 

 reduced to gaseous nitrogen. This source of loss may be removed 

 by preventing the formation of nitrate through compacting and 

 moistening, thus creating anaerobic conditions. 



