NITRATE REDUCTION 



147 



Nitrate may disappear from soils through several channels : 



(1) It may be assimilated by plants. 



(2) It may pass into deeper regions of the soil in the move- 

 ments of the water, or it may disappear from the soil dissolved 

 in drainage waters. 



(3) It may be assimilated by microorganisms in the pres- 

 ence of an available source of energy. In this case the nitrate 

 is not lost but is temporarily taken from circulation and stored 

 in the soil in complex organic forms. The reactions involved 

 in the synthesis of cell constituents from nitrate involve reduc- 



HNO3 

 Nitric Acid 



l + H, 



H2O +HNO2 

 Nitrous Acid 



H2O + HNO^ 



Hyponitrous Acid 



+H, 



-HoO 

 H2N2O2 > N2O 



Nitrous Oxide 



-H, 



+H; 



H,NOH 



Hydroxylamine 



+H2 



H2O+H3N 

 Ammonia 



-2HoO 



H4N2O2 — ^N2 



i Nitrogen 



-H2 +H2 



\ 



HeNsOs 



Fig. 64. — Reduction products of nitric acid. 



tion processes but need not lead to the accumulation of reduced 

 inorganic compounds of nitrogen in the medium. The general 

 transformation may be labeled nitrogen assimilation to distin- 

 guish it from other changes to be considered below. 



(4) It may be reduced to many compounds, such as nitrous 

 acid, hyponitrous acid, hydroxylamine, ammonia, the gaseous 

 nitrous oxide, and free nitrogen. The hyponitrous acid and 



