From the same swirling primordial nebula from which the planetesimals formed which eventually developed into the planets (or, failing that, into dwarf planets, asteroids, comets, etc.) of our Solar System.
Why are you interested in where in particular the nitrogen came from? I mean, if the article had mentioned mountains of, e.g., carbon dioxide ice (instead of frozen nitrogen): Would you have wondered where that carbon dioxide came from?
Regards,
I just think it’s interesting that there is so much nitrogen concentrated in one place. Pluto seems to be somewhat similar to Saturn’s moon Titan, only smaller. I read an explaination of how scientists think that Titan formed, but it didn’t make much sense to me.
Also - if something has evaporated, how does one tell exactly what it was that has evaporated when it is gone?