EVOLUTION OF CARBON DIOXIDE 185 



According to the results given in Table 44, it seems likely that as 

 much as one-third of the carbon dioxide may originate from roots 

 at certain stages of plant development. The gas which arises 

 from the plant roots may come from two sources: (1) from actual 

 elimination of the gas from the root cells, as a product of cell 

 respiration; (2) from microbial decomposition of organic exuda- 

 tion products of the roots. Which of these two sources may sup- 

 ply the greatest amounts of carbon dioxide is determined by the 

 age of the plant and its environmental conditions. In general, 

 the larger portion of the gas comes from root respiration, but at 

 times as much as 45 per cent of the gas may be the product of 

 microbial activities, as shown by Lundegardh: 



Root respiration in unsterilized sand 5.57 mgm. per hour 



Root respiration in sterilized sand 3.05 mgm. per hour 



Difference due to microorganisms 2.52 mgm. per hour 



This difference due to microorganisms was equal to 45 per cent 

 of the total amount produced as a result of the plant growth. 

 Consequently, although gas arising from roots as a product of plant 

 respiration may be considerable, the amounts proceeding from 

 microorganisms acting upon root materials and other soil sub- 

 stances are far greater. 



It may be assumed that 30 mgm. of carbon dioxide are evolved 

 in 24 hours per kilogram of soil to a depth of 16 inches. Assuming 

 that there are 2,024,000 kgm. of soil per acre to a depth of 16 

 inches, there would be evolved 60.72 kgm. of carbon dioxide per 

 day. Since more of the gas is produced during the warm than the 

 cold months, it may be expected that most of the gas would be 

 evolved during 200 days of the year in the temperate zone. Thus 

 there would be produced about 12,144 kgm. of carbon dioxide 

 from each acre of soil during the year. 



It has also been determined that the daily evolution of carbon 

 dioxide by a cereal plant is about 30 mgm. Assuming a growth 

 period of 100 days and that there are 810,000 plants per acre, there 

 would be about 2,430 kgm. of carbon dioxide produced by plant 

 roots. From these calculations it appears that plants may be an 

 important source of carbon dioxide which is formed within the 

 soil. During the active growing period there may be times when 

 more of the gas comes from the roots than from the microbial 

 activities in certain regions of the soil, but over the entire year, 



