• 6-. 



Decomposition of Soil Organic Matter 

 and Evolution of Carbon Dioxide 



Evolution of COo in Decomposition of Plant and Animal 



Residues 



Carbon makes up an average of about 50 per cent of all the 

 elements in plant and animal tissues. In certain carbohydrates and 

 organic acids, it may be somewhat less than 40 per cent, and in fats 

 and waxes it is above 60 per cent. In the decomposition of these 

 residues by microorganisms, most of the carbon is liberated as CO2; 

 the e\olution of this gas can, therefore, be taken as a measure of the 

 rate and extent of the decomposition process. Some of the carbon 

 is assimilated by microorganisms for cell synthesis, whereas an- 

 other part is left in the form of intermediary products, in both aerobic 

 and anaerobic decomposition. The total amount of CO2 liberated 

 depends on the nature of the material, the microorganisms con- 

 cerned, and the conditions of decomposition. 



When cellulose, hemicelluloses, sugars, and starches are decom- 

 posed by fungi and by aerobic bacteria, as much as 50-80 per cent 

 of the carbon is liberated as COo. In a comparative study of the de- 

 composition of rye straw by pure and mixed cultures of microorgan- 

 isms, the mixed soil population decomposed only about one-third as 

 much of the total material in absence of added nitrogen as when 

 ammonium salt was added. The corresponding amount of carbon 

 liberated as CO^ was about one-fourth in absence of added nitrogen. 

 Of the total carbon in the material, 72 and 83 per cent were liberated 

 as COo. Less COo was liberated by pure cultures of fungi because 

 of greater consumption of the carbon and greater quantities of inter- 

 mediary products left. 



Plant materials in a young green state decompose much more 

 rapidly than those in a mature state, and much more COo is produced 



149 



