240 DENITRIFICATION 



The splitting-off of free nitrogen, theoretically, could be due to the 

 reduction of nitrates or the action of nitrous acid on ammonia or 

 amins: 



2HNO 3 = 2H 2 O + 2O 2 + N 2 

 NH 3 + HNO 2 = N 2 + 2H 2 O 



CH 3 NH 2 + HNO 2 = N 2 + CH 3 OH + H 2 O 



Lawes, Gilbert, and Pugh showed that losses of nitrogen often 

 take place when nitrogenous organic matter was made into an 

 "agglutinated condition" with water and allowed to decompose in 

 the presence of air. Practically no ammonia could be detected. 

 Three possible reactions were suggested by Lawes and Gilbert: (1) 

 an oxidation analogous to that of the action of chlorin on ammonia 

 by which free nitrogen is evolved; (2) a reduction similar to that of a 

 great number of substances upon the oxygen compounds of nitrogen, 

 by which the oxygen is appropriated and the nitrogen set free; (3) 

 the two actions may operate in succession, the one to the other. 



Organisms Concerned. Gay on and Depetit, how r ever, were the 

 first to announce that the nitrogen originated from the nitrates. 

 They found that the ferments which possess this power need organic 

 matter for their development and that part of the organic nitrogen 

 is transformed into ammonia and perhaps also into amido-deriva- 

 tives of organic substance. 



In 1886 they isolated two organisms B. denitrificans, a. and /3 



-capable of reducing nitrates with the evolution of gaseous nitrogen. 



They also encountered a number of bacteria that could reduce 



nitrates to nitrites, and since that time the denitrifiers have been 



found very widely distributed. 



The discovery by Breal that many substances of organic origin, 

 and especially straw, are the carriers of denitrifying organisms was 

 of far-reaching importance. These organisms are, therefore, carried 

 with the litter to the manure and later with the manure to the soil. 

 It was found by Kunnemann that horse manure as a rule contains 

 denitrifying organisms and these are usually of two species, one of 

 them also being found on straw. The organism found only in 

 manure reduces nitrates in symbiosis with B. coli and is B. denitrifi- 

 cans I of Burri and Stutzer; the organism occurring in both manure 

 and straw is the B. denitrificans II of the same authors. The denitri- 

 fying organisms are less frequently present in cultivated soil and 

 are usually a different kind. Yet they are abundant in the upper 

 layers of the soil. Bazarewski found them irregularly distributed 

 in the deeper layers of the soil, but frequently they occurred abun- 

 dantly at a depth of one meter. They have also been found to a 

 great depth in the Nebraska soils. Putnam examined 201 species 

 and 139 were found to reduce nitrates to nitrites, while the other 

 species did not effect this reduction. Burri and Stutzer called atten- 

 tion to the fact that while there are many bacteria which will reduce 



