Tur CarRsoON DIOXIDE OF THE Sort AIR 347 
carbon dioxide in the cropped soil is due to the decomposition by bac- 
teria of root particles thrown off from the growing crop, then one would 
expect to find that those factors which produce fluctuations in the carbon 
dioxide in the bare soil would produce corresponding, but more magni- 
fied, fluctuations in the cropped soil. But, as is pointed out above, a 
factor such as temperature did not produce corresponding changes in the 
two soils. | 
Again, if the decomposition of root particles from the growing crop 
gave rise to the increase of carbon dioxide in the cropped soil, it is reason- 
able to suppose that there would be a much larger increase in carbon 
dioxide at a time when the roots were beginning to die off rapidly, that is, 
toward the ripening period. Such, however, was not the case. 
Summary of experiment 8 
It is probable, therefore, that the larger part of the excess carbon dioxide 
produced in a cropped soil is due to respiratory activities of the plant 
roots, and. that the amount resulting from the decay of root particles 
from the growing crop is small—altho after the crop has matured, any 
excess of carbon dioxide found is undoubtedly due to the decay of the 
mass of roots left in the soil. This excess, however, is very small when 
compared with the very large amounts of carbon dioxide found in the 
cropped soil at the time of heading, for example. ~ 
In support of the conclusion that the larger production of carbon dioxide 
in the cropped soil is due to respiratory activities of the plant roots, the 
data presented in experiment 1 show that there seems to be a correlation 
between the water requirements of the plant and the amount of carbon 
dioxide produced. 
GENERAL SUMMARY 
The results of the work reported in this paper with regard to the effect 
of crop and other factors on the production of carbon dioxide in a Dunkirk 
clay loam maintained at a constant moisture content of 30 per cent (oven- 
dry basis), may be summed up as follows: 
1. An oat crop increased the production of carbon dioxide in the soil. 
This increase became marked after the first month from the time of 
seeding, and increased to a maximum Just previous to or after the plants 
headed, after which there was a gradual decline. 
