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A question that I'm wondering about: where did the Earth's nitrogen come from? From what I understand of the planet formation process(admittedly an over-simplified version so it can be understood by Joe Public), the main material in which nitrogen is found would be ammonia, but wouldn't that be mainly located beyond the snowline? And where is the nitrogen on Venus?

The following answers should not be considered as authoritative.
These comments are based on a book called
Introductory Nuclear Physics by Hodgson, pages 570 to 575.

Stars make nitrogen by fusing carbon and a proton.
C12 + p => N13 + gamma
N13 => C13 + e + neutrino
C13 + p => N14 + gamma

That is how Nitrogen 14 is made in stars.
That star exploded, and Earth's nitrogen came from the remains of
an exploded star.



This is more probable in a star hotter than the Sun.
Hydrogen makes Helium 4
Helium 4 fuses to make the very common elements 12 16 20 24
Element 8, Beryllium decays fast. So it is not common like
Carbon12, oxygen16, sodium20, magnesium24

Carbon C12 is 3 Helium4 nuclei so it is common
The most common elements heavier than H have mass numbers that are multiples of 4.
The most common elements heavier than H are made of Helium4 nuclei
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Nitrogen is depleted by cosmic rays to form carbon 14.
Carbon 14 decays to make carbon 12.
(See ?€?radio carbon dating?€

Most of earths nitrogen was outgassed in volcanic eruptions over the course of the planet's history. Because nitrogen is a relatively inert gas, it just keeps on building up and up in the atmosphere.

http://www.soest.hawaii.edu/GG/ASK/atmo-nitrogen.html
http://en.wikipedia.org/wiki/Earth_atmosphere

Actually, Venus has MORE nitrogen than Earth, but its atmosphere is (a) so much denser and (b) contains so much more CO2 that it is a much smaller percentage of the total atmospheric content.

http://zebu.uoregon.edu/~soper/Venus/atmosphere.html

The nitrogen was just there when earth formed - while ammonia and ammonium compounts might have formed in the outer system, there was still enough ices flying around the inner solar system during planetary formation to allow the inner worlds to get some volatiles.

Thanks... so presumably any terrestrial planet with volcanism large enough to retain an atmosphere would end up with large quantities of nitrogen in its atmosphere.

Nitrogen is fairly unreactive and can be created easily in giant stars so it would presumable of any earth-sized planet to have plentiful nitrogen.
Just like of titan where there's no extreme heat to remove nitrogen, there's plenty of it.

[quote="GlobeMaker"]

Nitrogen is depleted by cosmic rays to form carbon 14.
Carbon 14 decays to make carbon 12.
(See ?€?radio carbon dating?€