Decomposition of organic 

 matter 



CO2 in air 



jf'^ " IMPLANTS: 



CO2 in water and Carbohydrates, fats, proteins 



bicarbonates 



Limestone, coal, oil 

 carbonates, etc. 



FIG. I 1-2 The carbon eyeh 



nitrites (NO;) and nitrates (NO3) through the ac- 

 tion of autotrophic bacteria: the process is called 

 nitrification. Other types of bacteria act on ammonia 

 in the process of denitrification. by which nitrogen 

 (No) is liberated into the atmosphere. Nitrogen is 

 removed from the air by the nitrogen-fixing bacteria 

 which live either freely in the soil or as symbionts in 

 the root nodules of legumes and some non-legumes ; 

 Ceanothus, Elaeagnus. Alnus, and Myrica. among 

 others. Some blue-green algae, fungi, and yeasts also 

 fix nitrogen. 



Nitrates and perhaps also the simpler nitrogen 

 compounds are absorbed and used by plants for the 

 synthesis of amino acids and proteins. Ammonia 

 compounds, nitrates, and other substances are added 

 to the soil in small amounts with rainfall : sources of 

 these nitrogen compounds are volcanic eruptions, 

 terrestrial decomposition, and atmosphere nitrogen 

 fixed by lightning. An attempt to estimate the quan- 

 tities of nitrogen involved in the different parts of the 

 cycle has been made by Hutchinson (1944). 



Carbon cycle 



Animals obtain much of their carbon, as well as 

 nitrogen, from plants, although some forms are also 

 able to fix carbon directly from salts dissolved in 

 water (Hammen and Osborne 1959). In photosyn- 

 thesis, carbon dioxide obtained from the air and from 

 dissolved bicarbonates in the substratum is combined 

 with water to form carbohydrates, a portion of which 

 may be converted to fats. Plants combine carbon 

 with oxygen, nitrogen, hydrogen, and sulphur to 

 form proteins. Carbon dioxide in the air comes 

 chiefly from the respiration of animals, but small 

 amounts arise from the respiration of plants, the decay 

 and fermentation of organic matter, springs, volcanic 

 action, and solution of sedimentary rock. Volcanoes 



were probably the original providers of carbon diox- 

 ide to the biosphere. Organisms tie up carbon diox- 

 ide as carbonates in skeletons and shells. Carbon is 

 also tied up in the formation of peat, oil, shale, and 

 coal. When limestone and other carbonaceous sedi- 

 ments are exposed to water erosion, the carbonates 

 may be hydrolized to bicarbonates and thus become 

 a source of CO2. The concentration of carbon dioxide 

 in the air is stabilized at 0.03 per cent by the buffering 

 action of bicarbonates and carbonates in the oceans 

 and fresh-water bodies (Hutchinson 1948) : 



CO2 + H2O ^ H2CO3 ^ HCO3- ^ CO3-2 



On the other hand, oxygen in the air (20 per cent) 

 is derived almost entirely from the photosynthesis of 

 plants. 



Other elements 



In addition to oxygen, carbon, hydrogen, and 

 nitrogen, animals require at least 13 other elements 

 that are all derived from the soil : calcium, phos- 

 phorus (Hutchinson 1948), potassium, sodium, chlo- 

 rine, sulfur, magnesium, iron, copper, manganese, 

 iodine, cobalt, and zinc. Only traces of some of these 

 elements are required, but calcium is required in large 

 amounts for skeletons, shells, antlers, and other or- 

 gans, and in the metabolism generally. Phosphorus 

 is a constituent of nucleoproteins, phospholipids, and 

 skeleton. Goiter occurs in mankind and some animals 

 in regions deficient in iodine. These elements are ob- 

 tained from food, drinking water, salt licks, and grit 

 taken into the stomach. A salt lick is a local, usually 

 clayey, area characterized by a high concentration of 

 salts where deer and other animals foregather to lick 

 the soil for the salt. Soils deficient in or lacking these 

 various elements support sparse animal populations ; 

 individuals are in more or less poor health ; reproduc- 

 tion rates are low (Albrecht 1944, Crawford 1950). 



Reactions, soil formation, and cycles 



67 



