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As I read a book on meteorology, 4 billion years ago, atmosphere was made by steam, carbon dioxide, carbon dioxide and a few nitrogen. Then photosynthesis came, run by underwater organisms, which led to a rise in the oxygen concentration in the atmosphere. But, where does the nitrogen comes from in the current atmosphere? Is there a reaction involved here?

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From what we know, the Earth's original atmosphere was not made of nitrogen (or oxygen, or carbon dioxide), but of gases such as hydrogen and helium. These would have been lost to space early on, because the Earth's gravity was not strong enough to hold on to them. The present-day oxygen came from organic sources, as you said. Here's what Wikipedia has to say about this newer atmosphere (emphasis mine):

The next atmosphere, consisting largely of nitrogen plus carbon dioxide and inert gases, was produced by outgassing from volcanism, supplemented by gases produced during the late heavy bombardment of Earth by huge asteroids.

The page attributes that passage to this, which concludes that volcanoes are one potential source of Earth's atmospheric nitrogen (see "Volcanic Atmospheres").

Also, I found another page that supports the outgassing theory:

The original atmosphere may have been similar to the composition of the solar nebula and close to the present composition of the Gas Giant planets, though this depends on the details of how the planets condensed from the solar nebula. That atmosphere was lost to space, and replaced by compounds outgassed from the crust or (in some more recent theories) much of the atmosphere may have come instead from the impacts of comets and other planetesimals rich in volatile materials.

So it appears that the primary sources were volcanoes and extraterrestrial bodies.

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  • $\begingroup$ I want to thank you for this, this helps me a lot. $\endgroup$
    – Chirac
    Commented Nov 6, 2014 at 22:52
  • $\begingroup$ @Chirac No problem. $\endgroup$
    – HDE 226868
    Commented Nov 6, 2014 at 22:52
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The air around us is made of several different gases. Mostly nitrogen (78%) and some oxygen (21%) and traces other gases like argon, water, carbon dioxide, ozone, and more.

These gases weren't always in our atmosphere, and they came from all sorts of different places. Scientists believe that most of the nitrogen in the air was carried out from deep inside the earth by volcanoes.

The nitrogen molecule is heavier than most other molecules in the atmosphere, so it tends to settle towards the bottom. Other lighter gases, like hydrogen or helium, would tend to the top of the atmosphere, where it is more likely to be lost to space. So the heavier gases like nitrogen and oxygen tend to stick around, since they are at the bottom of the atmosphere.

Source: math dan's answer to why is there so much nitrogen in the air

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it likely comes from nitrogen dissolved in the molten iron and nickel in the earth. A large comet would be roughly capable of delivering 1/10000 of the earths atmosphere if it where all nitrogen. They are not all nitrogen, but they have at least a fair amount. The interested reader will note that Mars has an atmosphere very rich in N2. Most likely originating in a similar way to that of earths. Things like magnesium, titanium, and lithium will all burn in nitrogen. It might have came with the water that was delivered to earth.

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    $\begingroup$ -1. This is completely wrong. Our nitrogen does not come from "nitrogen dissolved in the molten iron and nickel in the earth", comets typically have less than 1% nitrogen by weight, and Mars has a very thin atmosphere that is depleted of nitrogen compared to the Earth. $\endgroup$ Commented Nov 7, 2014 at 12:03
  • $\begingroup$ i don't understand your point. I've covered all possible sources of N2. oxygen in the earth is so plentiful. Its likely that nitrogen is associated with that oxygen. Another way to look at is from the point of view of entropy. Nitrogen was widely distributed in the early solar system so it should have worked its way into the material forming the earth. The mystery of earths water might be more of a problem of where earths hydrogen came from. There is plenty of oxygen combined with silicon. So there should also be a lot of nitrogen combined with other elements. $\endgroup$ Commented Nov 7, 2014 at 14:25
  • $\begingroup$ You have not covered all possible sources of nitrogen. You missed the Earth's mantle and crust. The "molten iron and nickel in the earth" -- that's the Earth's core, not the mantle. $\endgroup$ Commented Nov 7, 2014 at 14:33
  • $\begingroup$ what? isn't the core a solid ball in the center of the planet? It could ,obviously, be coming from the crust and mantle. With entropy again, there might be a lot of diffusion from the molten iron to the upper layers. So most likely from the crust and mantle. $\endgroup$ Commented Nov 7, 2014 at 14:51
  • $\begingroup$ Mars does not have plentiful nitrogen gas, it only has less than 3% N2 $\endgroup$
    – user60063
    Commented Nov 23, 2014 at 22:00

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