The self-diffusion of nitrogen is measured with secondary ion mass spectrometry in isotopically enriched polycrystalline and amorphous Si3N414∕Si3N415∕Si3N414 isotope heterostructures which were produced by reactive magnetron sputtering. The N diffusivities of polycrystalline films in the temperature range between 1130 and 1700°C follow an Arrhenius law over four orders of magnitude with a single activation enthalpy of ΔH=4.9eV and a pre-exponential factor of D0=1×10−6m2∕s. The calculated entropy of diffusion of ΔS≈0kB indicates a diffusion mechanism with localized point defects, in contrast to extended point defects usually found in semiconductors, like Si, Ge, and GaAs. The diffusivities in the amorphous state between 1130 and 1180°C do not differ significantly from those in the polycrystalline state.