Temperature-and pressure-dependent vibrational dephasing results for the alpha, gamma, and epsilon phases of crystalline nitrogen are obtained by time-resolved CARS measurements at high pressure (0-36 kbar) and low temperature (5-35 K). The very small homogeneous contribution to the vibrational line width in crystalline nitrogen at low temperature leads to vibronic dephasing that is dominated by weak inhomogeneous effects. Specifically, crystalline nitrogen cannot be cooled to low temperature without experiencing polymorphic solid-state phase transitions, For example, the beta-N-2 --> alpha-N-2 phase transition at ambient pressure, or the beta-N-2 --> gamma-N-2 transition between 4 and 21 kbar, may generate defects leading to an inhomogeneous distribution of vibronic site energies. In each case, crystalline beta-N-2 is formed starting from the supercritical fluid, by either cooling or compression. The negligible homogeneous contribution to the vibron line width for nitrogen at low temperature (<10 K) allows one to probe details of the extremely small inhomogeneous contributions to the vibron line shape. At higher temperature the vibronic dephasing rate increases and is analyzed in terms of a dominant quartic dephasing process involving thermally populated optical phonons.