Temporal evolution of the nitrogen hydride abundances. Upper left: dense gas; upper right: translucent gas; lower left: diffuse gas; lower right: Sgr B2 (M) envelope model (see Table 3). The dot-dashed and dotted lines for NH represent the estimated X(NH for the T_K = 30 K and 50 K (translucent) and T_K = 30 K and 10 K (dense) models. The observed abundances and upper limits corresponding to the different models are indicated with solid and dashed horizontal lines, respectively, following the respective species colour code.