The nitrogen cycle As I understand it, bacteria absorb nitrogen from the air, and deposit nitrates in the soil. These are absorbed by plants which use them to build complex molecules such as proteins. This enters the food chain. Eventually an organism dies and decays, and nitrogen is released into the atmosphere again. This is called the nitrogen cycle. Am I correct so far? My question: what percentage of atmospheric nitrogen is involved in the cycle per year? Background to the question: On another forum I was involved in discussing the old statement that every breath we take contains a molecule of Caesar's dying breath. We raised the point about molecules being ripped apart, and created by biological processes. One person said that the nitrogen cycle is so slow that virtually all the nitrogen molecules from Caesar's time still exist unchanged. That statement is one I found surprising. Is it correct?

**Mass of Nitrogen in Atmosphere**

ca. 4 x 1018kg (Wikipedia page on “Atmosphere of Earth”)

**Nitrogen fixed by natural, nonagricultural organisms per annum**

ca. 2 x 1011kg (“Global Nitrogen: Cycling out of Control”)

**Atmospheric nitrogen fixed per annum**

**ca. 0.5 x 10 –5 %**

**Conclusion**

If we are still breathing molecules of Caesar’s dying breath, they will be quite diluted.