456 NITROGEN METABOLISM 



enous materials, (3) the organisms concerned, and (4) the acidity, aeration, 

 and moisture content of the soil. The ammonifying organisms utilize carbo- 

 hydrates as a source of energy more readily than proteins so the amount of 

 ammonia produced is markedly decreased when large amounts of carbohydrates 

 are available. 



3. Nitrification. — The ammonia produced in the decomposition of proteins 

 and other organic nitrogenous compounds may be acted upon by the so-called 

 nitrifying bacteria and transformed in two steps to nitrates. The first step is 

 the oxidation of ammonia to nitrites. This is accomplished by organisms be- 

 longing to the two genera: Nitrosomonas and Nitrosococcus. Neither of 

 these organisms can oxidize the nitrite which they produce but this compound 

 is oxidized to nitrates by a different organism, Nitrobacter. All of these 

 organisms differ from each other morphologically but they are similar physio- 

 logically in that they use the energy obtained from the oxidation of ammonia 

 or nitrites in the synthesis of carbohydrate compounds from carbon dioxide 

 and water. The carbohydrates synthesized apparently are not used as a 

 source of energy but are utilized only in assimilation (Chap. XXXI). The 

 respiration of the nitrifying bacteria is therefore essentially different from 

 that of the higher plants and from most other non-green plants in that they 

 obtain their energy by the oxidation of ammonia or nitrites rather than from 

 carbohydrates. The soil conditions favoring nitrification are : ( i ) pH values 

 on the alkaline side of neutrality, (2) the absence of large amounts of carbo- 

 hydrates in the soil and (3) good aeration. 



4. Denitrification. — A large number of organisms are capable of reducing 

 nitrates to nitrites and ammonia. This occurs commonly in the tissues of the 

 higher plants as we have seen earlier. Certain soil organisms, however, can 

 attack nitrates and reduce them to molecular nitrogen. These organisms are 

 known as denitrifying bacteria and they include a number of species of which 

 Bacterium dcnitrificans is probably the best known. Denitrification occu-" 

 only in the absence of atmospheric oxygen and most effectively when an 

 abundant supply of carbohydrates is present in the soil. It does not nor- 

 mally occur in well cultivated soils. 



Small amounts of inorganic nitrogen compounds reach the soil from the 

 atmosphere. Oxides are formed during electrical storms and these are brought 

 into the soil by the rain. Ammonia also escapes from various sources into 

 the atmosphere and may be returned to the soil in solution in raindrops. 

 Measurements made at the Rothamsted Experiment Station in England over 

 a five year period showed that rain water brought down 4.4 pounds of nitro- 

 gen per acre per year. The amount of nitrogen added to the soils at Rotham- 

 sted in this way approximately equalled losses of nitrogen suffered by leaching. 



