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Measuring abundances through IR fine-structure lines has the major advantages, wrt to optical lines, of not suffering extinction, having a negligible dependence on gas temperature and a weak dependence on gas density. It is crucial to explore not only the total metallicities, expressed by the O/H ratio, but also the N/O ratio. In young stellar populations N/O is dominated by primary O and N production, while, when the population evolves, the N abundance increases by secondary production of intermediate-mass stars. Recently we studied a sample of local galaxies, including Starburst/HII region galaxies, Seyfert and Dwarf galaxies with new and archival SOFIA FIFI-LS spectra of 25 objects plus other 31 galaxies observed with Herschel-PACS on the [OIII] and [NIII] far-IR fine structure lines. We found that the IR measure of N/O is significantly lower than its optical measure. We explored, as possible reasons of such a discrepancy, the different regimes traced by IR and optical lines in terms of ionization, density and extinction, however no significant correlation has been found. We suggest that the origin of this discrepancy might be due to the metallicity evolution of the different stellar populations after the accretion of metal-poor gas.