Nitrogen ions emit narrowband coherent emission under the pump of intense femtosecond laser pulses of different wavelengths (mid-infrared, near-infrared, or ultraviolet). The 428 and 423 nm radiation of the nitrogen ions obtained by excitation with 400 nm femtosecond laser pulses has received less attention and their properties are unknown. In this study, the polarization of the 428 nm emission and its dependence on the nitrogen gas pressure and the pump laser energy are systematically measured. It is found that the polarization of the 428 nm emission is the same as the linearly polarized pump pulses. Moreover, the radiation signal presents nonlinear dependence on the gas pressure and the pump pulse energy. Based on the modeling of the strong-field ionization and the coupling of different energy levels of nitrogen ions in presence of the laser field, the population distribution of the nitrogen ions is simulated numerically. It is revealed that population inversion between the relevant energy levels of the upper level state $ {\mathrm{B}}^{2}{\mathrm{\Sigma }}_{\mathrm{u}}^{+} $and the lower $ {\mathrm{X}}^{2}{\mathrm{\Sigma }}_{\mathrm{g}}^{+} $level can be robustly established for a relatively large range of pump laser intensity, which agrees with the experimental observation.