RESPIRATION AND OXYGEN. • 37 



forested soil, the soil-air was much poorer in CO2 than that in neigh- 

 boring manured or humus fields. The C02-content of the soil-air 

 stands in a definite relation to the chemical activity of the soil, and 

 constitutes a definite measure of the latter. Under similar condi- 

 tions the C02-content of the soil-air in deep beech forests during the 

 growing-season is only about half that of a pine forest of equal age. 

 The C02-content of the soil-air is always less in closed forests than 

 in cultivated fields or in open bare soil. Soil-air is, as a rule, poorer 

 in oxygen as it is richer in CO2. Humus and calcareous soils in 

 bare situations are more active, richer in CO2, and poorer in oxygen 

 than all other bare soils. They even exceed dry moor-soil in this 

 respect. A living cover reduces decomposition and the production 

 of carbon dioxid. 



Wollny (1880 : 1373) studied the effect of plant cover and shade 

 upon the amount of carbon dioxid in the soil-air and reached the 

 following conclusions : Soil covered with living plants contained less 

 carbon dioxid than fallow soil or mulched soil during the warmer 

 half of the year, the respective amounts being 0.19, 0.88, and 0.67 

 per cent. During the colder half, the soil beneath the grass cover 

 contained more carbon dioxid in the ratio of about 2 to 1, the amount 

 being only about one-third as much as in the summer. The amount 

 of carbon dioxid increased more rapidly in fallow or mulched soil 

 with rising temperatures than in that with a plant cover. With the 

 increase of rainfall the amount of carbon dioxid increased several 

 times in cultivated soil and fell in bare soil or soil mulched with straw. 

 The soil under a cover of living plants was poorer in carbon dioxid in 

 proportion to the density of the plants, owing to the reduced water- 

 content and the lower temperature resulting from the shade. He 

 also found that the CO2 content under similar conditions rises and 

 falls in general with the amount of organic material in the soil. 

 The atmospheric air is materially concerned in the formation of CO2 

 in the soil. This is not completely prevented by the removal of 

 the air through one of the gases not concerned in the decomposition 

 of organic material. Carbon dioxid is also formed in the soil through 

 the agency of lower organisms. 



Wollny (1886 : 165) investigated the influence of the physical 

 properties of the soil on the amount of CO2 in it. He found the 

 latter to be greatest at a certain slope (20°), while it decreased both 

 with less slope (10°), or a steeper one (30°). As to exposure, on an 

 average the south slopes were the richest and the north the poorest 

 in CO 2, while the latter was intermediate on east and west slopes. 

 With reference to the color of the soil, the dark-colored soils were 

 poorer in CO2 than the lighter ones. This was true, however, only 

 under dry conditions, for, when the water-content of the two soils 

 was the same, the darker contained more CO2. The C02-content 

 of the soil under like amounts of organic material was the greater 



