THe CARBON DIOXIDE OF THE SorL AIR 323 
imilar results with timothy, but they were unable to decide whether or 
ot this increase of carbon dioxide was due to the plant-root excretions 
r to the decay of root particles that had died during the growth of the 
rop. The work of Stoklasa and Ernest (1905) showed that the younger 
he plant is, the greater is the amount of carbon dioxide formed. Kosso- 
itch (1904) noted that mustard grown in quartz sand and nutrient 
olutions produced an increased amount of carbon dioxide up to the 
ime of blooming. This was observed also by Barakov (1910) in the 
ase of plants growing in lysimeters. 
That different kinds of plants produce different amounts of carbon 
ioxide has been shown by Lau (1906), who found that potatoes and 
legumes give off more carbon dioxide than do other crops. Red clover, 
beets (Beta vulgaris), and oats were found by Stoklasa and Ernest (1905) 
to produce more carbon dioxide than other plants, and in the order named. 
Russell and Appleyard (1915), however, could find no difference in the 
carbon-dioxide content of soils on which different species of plants were 
growing. 
Chemical factors 
From the brief survey given, it would seem correct to say that most 
of the carbon dioxide found in the soil is the result of biological activity. 
There is some evidence, however, showing that chemical action may play 
asmall part. E. Wollny (1880b) noted a very slight production of carbon 
dioxide in soil treated with chloroform. The same investigator demon- 
strated later (EK. Wollny, 1897) that organic matter in the absence of 
oxygen reduces manganese and iron oxides and forms carbon dioxide. 
Very little carbon-dioxide production in sterilized soil kept at a 
temperature of 22° C. was observed by Dehérain and Demoussy (1896). 
They found, however, a very considerable production of carbon dioxide 
in soil heated to 90° C. and above. An oxidizing enzyme in the excretions 
of the root hairs was considered by Molisch (1888) to be capable of 
producing carbon dioxide from organic substances. It is probable that 
carbon dioxide produced by chemical means forms an extremely small 
part of the total carbon dioxide found in the soil. 
Summary 
. In this review of the literature of the subject, certain facts stand out. 
Authorities are agreed that bacteria play an important part, probably 
