108 Decomposition of Plant and Animal Residues 



process, for the synthesis of their cell material. The decomposition 

 of 100 gm cellulose (containing 40 per cent carbon) in the form of 

 straw or other plant material, for example, gives rise to 20 or 30 gm 

 carbon as COo; the rest of the carbon may be tied up in the syn- 

 thesized bodies of the bacteria and fungi. Under anaerobic condi- 

 tions, not more than 10 gm of the carbon may be liberated as CO2, 

 whereas the larger part of it may be left in the form of organic acids 

 or methane. 



When proteins undergo decomposition, they are first hydrolyzed, 

 by proteolytic enzymes produced by microorganisms, to polypep- 

 tides, amino acids, and other nitrogen derivatives. These are further 

 acted upon by a variety of organisms. The nitrogen is finally con- 

 verted to ammonia; the amount thus liberated depends upon the 

 abundance of the proteins and also upon the other constituents of 

 the plant material, especially the carbohydrates. In a comparison 

 of the decomposition of rye plants harvested at different stages of 

 growth, young plants were found to decompose very rapidly, as 

 shown by the evolution of CO2; some of the nitrogen was liberated 

 as ammonia, as shown in Table 19. As the plants grew older, they 

 decomposed more slowly, and less nitrogen was liberated as am- 

 monia, until a point was reached at which additional nitrogen was 

 required to hasten decomposition of the plant materials. 



When plant residues contain more than 1.5 or 1.7 per cent of 

 nitrogen, some of it will be liberated as ammonia, the actual amount 

 depending upon the original concentration of the nitrogen in the 

 plant residues. When the nitrogen content is less than 1.5 per cent, 

 however, very little ammonia will be liberated, even after several 

 months of decomposition. The decomposition of cereal straw, which 

 contains only 0.2-0.5 per cent nitrogen, requires the addition of 

 available nitrogen to enable the microorganisms to decompose the 

 carbohydrates in the straw. This process is utilized in the prepara- 

 tion of artificial manures, as shown later. 



The ammonia which is produced in the soil in the decomposition 

 of plant and animal residues does not accumulate there except under 

 very special conditions, but is rapidly oxidized by the nitrifying 

 bacteria to nitrate. Some of the ammonia may also be consumed 

 by various microorganisms responsible for the decomposition of 

 the carbohydrates. 



