Upper panel: time dependence of N/¹⁵N isotopic ratios in nitrogen hydrides. a) and b) correspond to the models defined in Table 4; The black heavy dotted line represents the elemental N/¹⁵N. Lower panel: time dependence of the fractional abundances relative to H₂ of NH, NH₂, NH₃, and NH₂D for models (a) and (b). The values reported for NH₃ (Ohishi et al. 1992) and NH₂D (Tiné et al. 2000) towards TMC1 are displayed as horizontal blue and red dashed lines, respectively, in the left panel.