  We present new and archival SOFIA FIFI-LS far-IR spectroscopic observations
of 25 local galaxies of either the [OIII]52um and/or the [NIII]57um lines.
Including other 31 galaxies from Herschel-PACS, we discuss a local sample of 47
galaxies, including HII region, luminous IR, low-metallicity dwarf and Seyfert
galaxies. Analyzing the mid- to far-IR fine-structure lines of this sample, we
assess the metallicity and compare with the optical spectroscopy estimates.
Using the IR, we find a similar O/H--N/O relation to that known in the optical.
As opposite, we find systematically lower N/O IR abundances when compared to
the optical determinations, especially at high values of N/O (log(N/O) > -0.8).
We explore various hypotheses to account for this difference: (i) difference in
ionization structure traced by optical (O+, N+ regions) versus IR lines (O++,
N++ regions); (ii) contamination of diffuse ionized gas affecting the optical
lines used to compute the N/O abundance; (iii) dust obscuration affecting the
optical-based determinations. However, we have not found any correlation of the
Delta(N/O)= (N/O)_OPT-(N/O)_IR with either ionization, or electron density, or
optical extinction. We speculatively suggest that accretion of metal-poor gas
from the circumgalactic medium could provide an explanation for this
difference, because the rapid decrease of total abundances during infall is
followed by a N/O ratio decrease due to primary production of young - possibly
embedded - massive stars, are preferentially traced by the IR diagnostics,
while optical diagnostics would better trace the secondary production, when
both N/O and O/H abundance ratios will increase.
