Major formation paths of nitrogen hydrides at t = 10⁶ years for translucent gas conditions (upper panel) and diffuse gas conditions (lower panel). Gas phase species appear in white and grain surface species in grey. Solid lines indicate gas phase reactions, dashed lines photo-dissociation, dotted lines dissociative recombination, electronic radiative recombination and attachment processes, and dot-dashed lines non-thermal desorption from the grain surfaces.