What evidence is there that is was a nitrogen volcano, and not just a normal molten lave type?

Spectra.

Exactly!

http://solarsystem.nasa.gov/planets/...?Object=Triton

Remote temperature readings place it as the coldest surface known anywhere in the Solar System.

__________________
"The bread's hollowed out --- the veggies go on forever --- and --- oh my God! --- it's full of meat!" - Maksutov