THE CARBON AND NITROGEN CYCLES 99 



Carbon dioxfd is being added to the air from several sources: 

 the combustion of fuel, the respiration of animals, and the decay 

 of organic matter. It is also being evolved in enormous quanti- 

 ties from mineral springs and volcanoes. Krogh estimates that 

 the annual consumption of coal adds yearly to the atmosphere 

 about one-thousandth of its present content in carbon dioxid. 

 Were there no factors offsetting this increase in atmospheric car- 

 bon dioxid animal life would soon become extinct. 



On the other hand, there are two large factors at work remov- 

 ing carbon from the atmosphere: (i) the decomposition of car- 

 bon dioxid by plants with the liberation of oxygen, and (2) the 

 consumption of carbon dioxid in the weathering of rocks. No 

 precise valuation can be given to either of these factors, although 

 various writers have attempted to estimate their magnitude. Cook 

 computes that leaf action alone more than compensates for the 

 production of carbon dioxid. Chamberlain estimates that the 

 amount of carbon dioxid annually withdrawn from the atmos- 

 phere is 1,620,000,000 tons, and that the greater part of this is 

 taken up by the weathering of minerals. This is continually 

 being returned to the atmosphere by bacterial action. There are 

 then two compensating sets of factors: (l) decay, respiration, 

 and combustion liberating carbon 5 and (2) plant growth and 

 rock-weathering fixing it. These balance each other, thereby 

 completing the carbon cycle and rendering the carbon-dioxid con- 

 tent of the atmosphere nearly constant. 



The Nitrogen Cycle. — Since nitrogen occurs as an essential 

 part of the structure of every plant and animal, it is found in all 

 crops and crop residues. It occurs in the top soil in proteins, 

 protein decomposition products, ammonia, nitrites, and nitrates. 

 It is not found in the mineral matter of the earth except in shales 

 and other deposits containing the residues of plant and animal 

 bodies. Hence, the quantity in the combined form is not great 

 when compared with other essential elements. Yet it is required 

 by all living organisms in large quantities. Many of these are 

 returning it to its inert atmospheric form. This fact led Sir 

 William Crooks, in his famous address before the British Asso- 



