184 PRINCIPLES OF SOIL MICROBIOLOGY 



of organic matter. The process is checked by heating the soil or treat- 

 ing it with chloroform, which results in the destruction of the bacteria 

 responsible for the reduction of the nitrates. 28 In the decomposition of 

 organic nitrogenous compounds, free from nitrates, both in the presence 

 and absence of oxygen, nitrogen gas is not produced; when nitrates are 

 present, an active reduction takes place in the absence of oxygen, with 

 the formation of gaseous nitrogen and various oxides of nitrogen. 29 

 This reduction diminishes with an increase in the amount of oxygen 

 present but does not stop entirely. Even those investigators who 

 believed at first that denitrification is a purely chemical process, carried 

 out by means of the soil colloids, were convinced by later studies that 

 nitrate reduction is not of a chemical nature. 30 



Bacteria may bring about the formation of nitrogen gas from nitrates 

 in two different ways: (a) indirectly and (b) directly. The nitrite 

 which is formed in the process of reduction of nitrate by Bad. coli, 

 Bad. vulgare, Bad. prodigioswn, Bac. vulgatus, may interact chemically 

 with the amino nitrogen of the peptone molecule or the various amino 

 acids formed from the decomposition of peptone, liberating gaseous 

 nitrogen. The various oxides of nitrogen formed from the reduction of 

 nitrate may also interact with the ammonia nitrogen formed from the 

 peptone and result in free nitrogen gas: 



NH 4 N0 2 = 2H 2 + N 2 



These indirect processes play only a questionable role in the soil. How- 

 ever, in addition to these bacteria, which in themselves are unable to 

 produce nitrogen gas directly from nitrates, the soil harbors various 

 specific bacteria capable of reducing the nitrate molecule directly to 

 atmospheric nitrogen. Breal 31 found that a nitrate solution to which 

 straw is added liberates a great deal of gaseous nitrogen. Similar 

 results have been obtained on inoculating a nitrate solution with horse 



28 Ehrenberg, A. Experimentaluntersuchungen iiber die Frage nach dem Frei- 

 werden von gasformigen Stickstoff bei Fiiulnissprocessen. Ztschr. physiol. 

 Chem. 11: 145-178, 438-471. 1886. 



2D Tacke, Br. tJber die Entwicklung von Stickstoff bei Fiiulniss. Landw. 

 Jahrb. 16: 917-939. 1888. 



30 Vogel, J. tiber das Verhalten von Nitrat im Ackerboden. Centrbl. Bakt. 

 II, 34: 540. 1912; Landw. Vers. Sta. 78: 265-301. 1912; 82: 159-160. 1913. 



31 Breal, E. De la presence dans la paille d'un ferment a^robie r<5ducteur de 

 l'acide nitrique. Ann. Agron. 18: 181-195. 1892. Compt. Rend. Acad. Sci. 

 114: 681-684. 1892. 



