202 



Journal of Agriculiiiral Research 



Vol. XII, No. 4 



Table XVI. — Influence of Azoiobacter on nitrates in solution, giving the quantity of 



nitrogen fixed 



Table XIV showing the effect on the total nitrate content will be 

 discussed first. Strain A differed widely from strain B in its ability to 

 reduce nitrates. It will be noted that strain A reduces nitrate more 

 readily than strain B in the presence of both sodium and ammonium 

 nitrate. In order to determine the nature of the reduction of the nitrates, 

 qualitative and quantitative tests were made. The reduction of nitrates 

 by Azotobacter takes place with the formation of nitrites as shown in 

 Table XIV. Strain A effected a strong reduction of nitrate to nitrite 

 with both sodium and ammonium nitrate. Strain B also reduced nitrate 

 to nitrite, but to a lesser degree than did strain A. 



An inspection of the data in Table XV indicates that the reduction of 

 nitrates ceased with the formation of nitrite, since no appreciable amounts 

 of ammonia were produced by either strain of Azotobacter. 



In regard to the fixation of atmospheric nitrogen by these strains of 

 Azotobacter, it was found that nitrogen was assimilated both in the 

 presence and absence of nitrate. In the presence of nitrate there was a 

 large increase in the total organic nitrogen. Sodium nitrate stimulated 

 both strains, although strain B fixed the larger amount. Similar results 

 were obtained when the fixation of nitrogen on agar films was studied. 

 In the presence of ammonium nitrate the amount of nitrogen fixed was 

 considerably decreased, but the amount fixed was in excess of the control 

 cultures containing no nitrate. It seems evident that sodium and 

 ammonium nitrate in the amounts studied did not prevent the fixation 

 of atmospheric nitrogen. In fact, the presence of these salts seemed to 

 stimulate the process. 



Under aerobic conditions both strains of Azotobacter studied caused a 

 reduction in the total amount of nitrate present in the solution. This 

 reduction may be accounted for in two ways: (i) The reduction of 

 nitrate to nitrite and (2) the assimilation of nitrate by the organisms. 

 Practically no ammonia was formed under the conditions of these experi- 

 ments. These results agree with those of Stoklasa. However, in con- 



