RESPIRATION AND OXYGEN. 39 



air diffused into the soil and changed by loss of oxygen and consider- 

 able increase of CO2. The sum of these two, however, is not very 

 different from the original proportion of oxygen. More CO 2 is 

 found at lower than at higher soil-levels. The amount increases 

 during spring and summer, diminishes after August, and remains 

 more or less stationary during the winter months, the variations 

 being due to temperature changes. The amount of CO 2 in the soil- 

 air, depends upon its porosity and its organic matter, the relation 

 being inverse in the first and direct in the second. Rainfall has a 

 marked influence on the CO 2 of the soil-air. 



Stoklasa and Ernest (1905 : 723) have shown that the C02-content 

 of the soil-air is derived either from the respiration processes of 

 micro-organisms, especially bacteria, molds, and algae, or the respira- 

 tion of the root-systems of plants. With greatly limited access of 

 air, decomposition begins, as a consequence of which the mineraUza- 

 tion of the organic materials takes place much more slowly. Proof 

 that the source of the CO 2 is not in the chemical processes, but in 

 the activity of the micro-organisms, is afforded by the fact that no 

 development of CO2 is noticed when the soil is sterilized or the 

 organisms destroyed by antiseptics. 



Lau (1906 : 33) reached the following conclusions as a conse- 

 quence of his study of the air in the soil: Soil-air is richest in CO 2 

 in summer, next in autumn, then in spring, and least in winter. 

 The maximum occurs in the months of July and August, the mini- 

 mum in February. The C02-content of soil-air increases with the 

 depth. Soil-air is poorest in CO2 in sand, intermediate in clay, and 

 richest in moor-soil. It is poorer in oxygen as it is richer in CO2. 

 At the same depth the soil-air is richer in CO2 at the root-level in a 

 cultivated field than in a bare one. It is richer in CO 2 in the root- 

 layer than beneath it. The amount of carbon dioxid increases with 

 the greater development of plants and increasing soil-temperature. 

 Soil-air is richer in CO2 in soil containing potatoes or lupines, which 

 respire intensively, than in soil with oats or barley, which respire 

 weakly. Soil-air is poorer in CO2 about 2 a. m. than it is at 2. p. m. 

 It is richer in sandy soils manured with organic material than in 

 those unfertilized. The soil-air in unfertilized lupine beds is but 

 little poorer in CO2 than in fertilized ones. 



Vageler (1907 : 19) has made a comprehensive study of the soil- 

 air of various moor communities. A large number of analyses of 

 the soil-air of moors shows that the greatest amount of CO 2 and the 

 least oxygen occurs in the Molinietum, namely, 2.68 per cent and 16.68 

 per cent, while the Arundinetum, for example, has 0.13 per cent of 

 CO2 and 20.23 per cent oxygen. The carbon dioxid in the moor-soil 

 is an ecological factor in so far as its amount furnishes a measure of 

 the decomposition processes in the soil, modified through the higher 

 or lower air-content of the rhizosphere and the nature of the soil 



