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?

  • 1
    $\begingroup$ Welcome to SE Biology. This is a question and answer site, not a personal proof-reading site as you will see if you finish reading the Tour. Please do so. We work on the basis of one question per post. What is your question? Initially it appears to be about the percentage of atmospheric nitrogen fixed each year — a consideration that is hardly at the forefront of modern biology, but must be of interest to somebody. However then it seems to morph into astrophysics, in relation to the conservation of nitrogen on earth, together with the speed of the cycle — whatever that may mean. $\endgroup$
    – David
    Nov 19, 2022 at 16:26
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    $\begingroup$ I thought my question is perfectly clear: "what percentage of atmospheric nitrogen is involved in the cycle per year?" The rest is merely stating the reason why I'm asking the question. I don't see what proofreading has to do with it, nor astrophysics. $\endgroup$
    – Pete
    Nov 19, 2022 at 16:35
  • $\begingroup$ Proof-reading: "Am I correct so far". Astrophysics: "all the nitrogen molecules from Caesar's time still exist unchanged" suggested loss from the earth's atmosphere. $\endgroup$
    – David
    Nov 19, 2022 at 16:43
  • $\begingroup$ Not proofreading. It's just establishing if I've understood the basic premise correctly. You can ignore that line if you wish. As for astrophysics, it's not about losing from Earth's atmosphere, it's about chemical reactions destroying molecules, and creating different molecules. I thought I made that clear. $\endgroup$
    – Pete
    Nov 19, 2022 at 16:53
  • $\begingroup$ OK, OK. You could have found the answer yourself — simple queries, first page of answers — but I've done it for you. $\endgroup$
    – David
    Nov 19, 2022 at 18:32

1 Answer 1


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 %


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

  • $\begingroup$ I wasn't expecting us to "run out" of nitrogen, since any nitrogen fixed by bacteria will eventually be released back. Aside from that, thanks for the answer. $\endgroup$
    – Pete
    Nov 19, 2022 at 18:37
  • $\begingroup$ @Pete — Seemed a bit bald without a comment. Perhaps I’ll try a different tack. The actual turnover time is a more difficult question. It may have been 50 years or for the average Roman emperor to return to the earth, but much less for the rats in the streets. $\endgroup$
    – David
    Nov 19, 2022 at 19:39
  • $\begingroup$ +1 Nice article (second link). I did not know that, like, no clue whatsoever. So Thanks for that. $\endgroup$ Nov 19, 2022 at 19:58
  • $\begingroup$ Suggestion: The best practice is to write numbers in scientific notation in terms of the base unit kg, not the unprefixed unit g. This helps when doing calculations, e.g. kg × m/s^2 = N (newton). So, 4×10^21 g = 4×10^18 kg. $\endgroup$
    – Nayuki
    Nov 20, 2022 at 16:56
  • $\begingroup$ @Nayuki — You may well be right, but I am unaware of that practice. Can you provide a link documenting it? If you are correct I will change it. $\endgroup$
    – David
    Nov 20, 2022 at 17:01

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