  The discontinuities of $T_{c}$ in Niobium under pressure are examined by
means of the pseudopotential plane-wave implementation of the electron-phonon
coupling calculated from density-functional perturbation theory. Both low- and
high-pressure discontinuities of $T_{c}$ have their origin in the Kohn
anomalies and are caused by the low-frequency phonons, but the mechanism
leading to the discontinuities is different in the two cases. The low-pressure
anomaly is associated with a global decrease of the nesting factor in the whole
Brillouin Zone and not to a visible change in the band structure. The
high-pressure anomaly is instead connected with a well-pronounced change in the
band structure.
