  Partitioning of elemental nitrogen in star-forming regions is not well
constrained. Most nitrogen is expected to be partitioned among atomic nitrogen,
molecular nitrogen (N2), and icy N-bearing molecules, such as ammonia (NH3) and
N2. Atomic nitrogen is not directly observable in the cold gas. In this paper,
we propose an indirect way to constrain the amount of atomic nitrogen in the
cold gas of star-forming clouds, via deuteration in ammonia ice, the
[ND2H/NH2D]/[NH2D/NH3] ratio. Using gas-ice astrochemical simulations, we show
that if atomic nitrogen remains as the primary reservoir of nitrogen during
cold ice formation stages, the [ND2H/NH2D]/[NH2D/NH3] ratio is close to the
statistical value of 1/3 and lower than unity, whereas if atomic nitrogen is
largely converted into N-bearing molecules, the ratio should be larger than
unity. Observability of ammonia isotopologues in the inner hot regions around
low-mass protostars, where ammonia ice has sublimated, is also discussed. We
conclude that the [ND2H/NH2D]/[NH2D/NH3] ratio can be quantified using a
combination of VLA and ALMA observations with reasonable integration times, at
least toward IRAS 16293-2422 where high molecular column densities are
expected.
