Where did all that nitrogen come from?
Made in the core of stars. Our own air is 78% nitrogen. So there is a lot of it around. Its abundance Universe-wide is a paltry 0.1%. That’s not a paradox. It obviously differentiates very nicely into planetary crusts and atmospheres, as you’d expect of such a light element. But that’s just my lay opinion. I’m sure someone more knowledgable than me can clarify things for us!
From Uranus. Ha ha ha ha! Ho ho ho ho! He he he he!
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,