  Microwave loss in niobium metallic structures used for superconducting
quantum circuits is limited by a native surface oxide layer formed over a
timescale of minutes when exposed to an ambient environment. In this work, we
show that nitrogen plasma treatment forms a niobium nitride layer at the
metal-air interface which prevents such oxidation. X-ray photoelectron
spectroscopy confirms the doping of nitrogen more than 5 nm into the surface
and a suppressed oxygen presence. This passivation remains stable after aging
for 15 days in an ambient environment. Cryogenic microwave characterization
shows an average filling factor adjusted two-level-system loss tangent
$\rm{F\delta_{TLS}}$ of $(2.9\pm0.5)\cdot10^{-7}$ for resonators with 3
$\rm{\mu}$m center strip and $(1.0\pm0.3)\cdot10^{-7}$ for 20 $\rm{\mu}$m
center strip, exceeding the performance of unpassivated samples by a factor of
four.
