Tcrresliial E)wiri)ii})icnt 243 



Anaerobic forms, on the other hand, get their energy by partial de- 

 eomposition of organic matter vvithont free oxygen. Anaerobic or- 

 ganisms nevertheless depend upon the aerobic forms to produce the 

 organic matter upon which they live. As we shall see, the abundance 

 of oxygen in the environment may become critically low for aerobic 

 organisms but it never becomes harmfully high for these forms. 

 Natural concentrations of oxygen may, however, be seriously detri- 

 mental to some anaerobes. 



Oxygen is present in the air as one of the gases that are physically 

 mixed together, and it occurs in water in simple solution. Dissolved 

 oxygen does not combine chemically with water itself but it does react 

 with iron and other inorganic materials in the water. Points of con- 

 trast in this respect with carbon dioxide will be discussed later in the 

 chapter. Since oxygen is taken up and given off by life processes, 

 its concentration in the environment can sometimes be appreciably, or 

 even seriously, altered by the activities of the plant and animal in- 

 habitants. Under these circumstances oxygen is a modifiable factor 

 of the environment. 



Availability of Oxygen 



Terrestrial Environment. Oxygen constitutes 21 per cent of the 

 atmosphere, and this value varies by less than 1 per cent the world 

 over. Although plants and animals are continuously drawing upon 

 the oxygen supply in the air, and plants are periodically adding to 

 it, the concentration in the atmosphere is not changed appreciably 

 by these life activities because of the great volume and mobility of 

 the air medium. Most of the terrestrial environment is thus pro- 

 vided with a uniform and adequate supply of oxygen. 



In two types of situations of importance in the terrestrial environ- 

 ment a lack of oxygen exists— at high altitudes and in the subsurface 

 layers of the soil. At great elevations the concentration of oxygen is 

 low because all gases have become rarer. The amount of reduction 

 in oxygen with altitude is proportional to the reduction in total at- 

 mospheric pressure, discussed in Chapter 2. Thus at an altitude of 

 5500 m O2 is only half as abundant as it is at sea level. The oxygen 

 in soils drops from near the atmospheric value of 21 per cent at the 

 surface to about 10 per cent within well-drained loams and to lower 

 values in poorly aerated soils and in layers below the water table. 

 Lack of air circulation results from very fine texture or from the 

 flooding of the pore spaces. Under these circumstances the respira- 

 tion of roots and of soil organisms, particularly those involved in the 



