Left panel: temporal evolution of the abundances of N, N₂, HCN, and NH₃ in a constant density model with our fiducial parameters (see Sect. 2.3) and using the R15 rates. Middle panel: same for the isotopic ratios. The full curves are for models with adsorption only, while dashed curves are for models with both adsorption and desorption. Right panel: temporal evolution of the freeze-out timescales τ_fo of N and N₂ and of the dust grain radius a_gr.