  The nitrogen chemical evolution during star and planet formation is still not
fully understood. Ammonia (NH$_3$) is a key specie in the understanding of the
molecular evolution in star-forming clouds and nitrogen isotope fractionation.
In this paper, we present high spatial resolution observations of multiple
emission lines of NH$_3$ toward the protobinary system NGC1333 IRAS4A with Karl
G. Jansky Very Large Array (VLA). We spatially resolved the binary (hereafter
4A1 and 4A2) and detected compact emission of NH$_3$ transitions with high
excitation energies ($\gtrsim$100 K) from the vicinity of the protostars,
indicating the NH$_3$ ice has sublimated at the inner hot region. The NH$_3$
column density is estimated to be $\sim 10^{17}-10^{18}$ cm$^{-2}$. We also
detected two NH$_2$D transitions, allowing us to constrain the deuterium
fractionation of ammonia. The NH$_2$D/NH$_3$ ratios are as high as $\sim 0.3-1$
in both 4A1 and 4A2. From the comparisons with the astrochemical models in the
literature, the high NH$_2$D/NH$_3$ ratios suggest that the formation of NH$_3$
ices mainly started in the prestellar phase after the formation of bulk water
ice finished, and that the primary nitrogen reservoir in the star-forming cloud
could be atomic nitrogen (or N atoms) rather than nitrogen-bearing species such
as N$_2$ and NH$_3$. The implications on the physical properties of IRAS4A
cores are discussed as well.
