We report on a spectral splitting effect of the cavity-less lasing emission of nitrogen ions at 391.4 nm pumped by the 800 nm femtosecond laser pulses. It was found that with the increase of the nitrogen gas pressure and pump pulse energy, both R and P branches experience spectral splitting. With an external injected seeding pulse, similar split spectral line is observed for the amplified emission. In contrast, for the fluorescence radiation, no such spectral...