CONCERNED WITH PUTREFACTION 123 



1. Decomposition and Denitrifieation 



In order that this complex material should be of service in the 

 economy of nature, and its constituents not lost, it is necessary that 

 it should be broken down again into simpler conditions. This 

 prodigious task is accomplished by the agency of two groups of 

 organisms, the decomposition and denitrifying * bacteria. The organ- 

 isms associated with decomposition processes are numerous ; some 

 denitrify as well as break down organic compounds. This group 

 will be referred to under " Saprophytic Bacteria." The reduction by 

 the denitrifying bacteria may be simply from nitrate to nitrite, or 

 from nitrate to nitric or nitrous oxide gas, or indeed to nitrogen 

 itself. In all these processes of reduction the rule is that a loss of 

 nitrogen is involved. How that free nitrogen is brought back again 

 and made subservient to plants and animals we shall understand at 

 a later stage. 



Professor Warington has set forth the chief facts known of 

 this decomposition process.f That the action in question only 

 occurs in the presence of living organisms was first established 

 by Mensel in 1875 in natural waters, and by Macquenne in 1882 

 in soils. If all living organisms are destroyed by sterilisation 

 of the soil, denitrification cannot take place, nor can vegetable 

 life exist. "Bacteria reduce nitrates," says Professor Warington, 

 " by bringing about the combustion of organic matter by the oxygen 

 of the nitrate, the temperature distinctly rising during the operation." 

 The reduction to a nitrite is a common property of bacteria. But 

 only a few species have the power of reducing a nitrate to gas. 

 These few species are, however, widely distributed. In 1886 Gay on 

 and Dupetit first isolated the bacteria capable of reducing nitrates 

 to the simplest element, nitrogen. They obtained their species from 

 sewage, but ten years later denitrifying bacteria were isolated from 

 manure. That soil contains a number of these reducing organisms 

 is proved by introducing a particle of surface soil into some broth, 

 to which has been added 1 per cent, of nitre. During incubation of 

 such a tube gas is produced, and the nitrate entirely disappears. 



Whenever decomposition occurs in organic substances there is a 

 reduction of compound bodies, and in such cases the putrefying 

 substances obtain their decomposing and denitrifying bacteria from 

 the air. The chief conditions requisite for bringing about a loss of 

 nitrogen by denitrification are enumerated by Professor Warington 

 as follows: (1) the specific micro-organism; (2) the presence of a 

 nitrate and suitable organic matter; (3) such a condition as to 



* " Denitrifying" means reducing nitrates. 



t R. Warington, M.A., F.R.S., Jour. Roy. Agric. Soc. Eng., series iii., vol. viii. , 

 part, iv., p. 577 el seq. See also Trans. Chem. Soc., 1884, 1888, etc. 



