Electron affinity of nitrogen and exchange energy
$ \rm N^{-}$ has the outer electron structure of $ \rm 2s^2 2p^4$. This means we have coulombic repulsion in one of the p orbitals which means adding a single electron will be endothermic. This accounts for the decrease in ionization energy between N and O.

When you get to $\rm N^{3-} $ we have 6K units of exchange energy which is enough to offset the coulombic repulsion. 

You see this effect when considering the electron configurations of the transition elements.