200 Journal of Agricultural Research voi. xii. no. 4 



evident that in pure cultures both sodium and calcium nitrates in the 

 amounts studied produced an increase in the amount of nitrogen fixed. 

 The sodium salt stimulated this process to a slightly greater extent than 

 did the calcium salt. In unsterilized soil nitrates exerted the same 

 action but to a more marked extent. The amount of nitrogen fixed 

 under these conditions was generally in excess of that fixed under similar 

 conditions in sterilized soil inoculated with a pure culture of Azotobacter. 



Such large relative increases in total nitrogen in the soil in the presence 

 of nitrates would not normally take place under field conditions for here 

 no accumulations of nitrate occur in quantities sufficiently large enough to 

 influence this process. 



Summing up all the experiments performed in relation to the influence 

 of nitrates on the fixation of atmospheric nitrogen by Azotobacter, it 

 appears that the increase in total nitrogen in the presence of these salts 

 is by no means comparable to the increase in the number of organisms 

 in sterilized soil under the same conditions. An increase in the number of 

 Azotobacter does not mean a parallel increase in the amount of nitrogen 

 fixed. 



INFLUENCE OF AZOTOBACTER ON NITRATES IN SOLUTION 



Attention has been thus far directed toward the influence exerted 

 by nitrates on the growth and nitrogen-assimilating power of Azotobacter. 

 The following points are now to be considered: Do the nitrogen-fixing 

 bacteria reduce nitrates to nitrites and ammonia ? Is there an increase 

 or decrease in the amount of organic nitrogen as a result of the presence 

 of nitrate in the medium? 



Beijerinck and Van Delden (5) found that Azotobacter ckroococcum 

 reduced nitrate directly to ammonia. Stoklasa (44, p. 4^2-503) 

 studied the changes in a nutrient solution containing 0.2 per cent 

 of sodium nitrate inoculated with Azotobacter. He found under an- 

 aerobic conditions that the nitrate was largely reduced to nitrite and 

 ammonia and that a very small amount of organic nitrogen was formed. 

 Under aerobic conditions there v/as more nitrite formed than under 

 anaerobic conditions and very little ammonia or oganic nitrogen. He 

 concluded, therefore, that Azotobacter did not fix atmospheric nitrogen 

 in the presence of nitrates. 



The following experiments were performed in an endeavor to answer 

 the questions raised in the initial paragraph of this section. To Erlen- 

 meyer flasks of 500-c. c. capacity, containing loo-c. c. portions of mannit 

 solution, sodium and ammonium nitrates were added in amounts equiva- 

 lent to 150 mgm. of nitrate in 100 c. c. of the solution. Nine flasks were 

 prepared for each nitrate and the same number for the controls containing 

 no nitrate. The flasks and contents were sterilized at 10 pounds pres- 

 sure for 30 minutes. After cooling, six of each set were inoculated, three 



