CHAPTER III 



THE CYCLES 



I. THE CARBON CYCLE 



During photosynthesis, COg is removed from the atmosphere and from 

 the hydrosphere. A part of this COg is returned almost immediately during 

 the respiration of plants. The remainder is, for the most part, returned 

 indirectly by the respiration of animals and during the putrefaction of 

 dead plants and animals. This is known as the biological carbon cycle. 



The CO2 content of the atmosphere hardly varies and this is due to the 

 buffering of drastic fluctuations by the CaC03-Ca(HC03)2-COa 

 system of the ocean. The main path of the carbon cycle is through the 

 biosphere and the ocean. Carbonates are being continually diverted in the 

 form of sediments, and organic carbon is being held up during its series of 

 transformations in the form of fossilized carbon. 



It is possible to calculate that since the start of the laying down of these 

 two types of sediment, twelve times the total amount of COg in the atmo- 

 sphere has been trapped in the form of sediments. This removal of carbon 

 has taken place gradually and has been compensated by CO2 of volcanic 

 origin. 



The autoregulation of the carbon cycle, which maintains the COo of the 

 atmosphere at a constant level, depends on the one hand on the COg— 

 bicarbonate-carbonate system of the air, seas and sediments, and on the 

 other hand it depends on photosynthesis, the intensity of which is regulated 

 by the concentration of available COo. 



The carbon cycle is not at all dependent on the presence of animals or 

 plants. It could continue in the presence of micro-organisms alone and 

 with special intensity in the oceans where the photosynthesis is eight 

 times more intense than that due to land plants. This marine photo- 

 synthesis is due to the presence of Diatoms and the Dinoflagellates of 

 phytoplankton. Figure 100 shows a carbon cycle which is entirely microbial 

 in character. 



II. THE NITROGEN CYCLE 



The main stages of this cycle are the fixation of nitrogen or nitrates by 

 plants and certain bacteria, the mineralization of proteins to form ammonia 

 and the conversion of this ammonia into nitrates. Fig. 101 shows the various 

 stages of the cycle. 



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