  We study the relation between nitrogen and oxygen abundances as a function of
metallicity for a sample of emission-line objects for which a direct
measurement of the metallicity has been possible. This sample is representative
of the very different conditions in ionization and chemical enrichement that we
can find in the Universe. We first construct the N/O vs. O/H diagram and we
discuss its large dispersion at all metallicity regimes. Using the same sample
and a large grid of photoionization models covering very different values of
the N/O ratio, we then study the most widely used strong-line calibrators of
metallicity based on [NII] emission lines, such as N2 and O3N2. We demonstrate
that these parameters underestimate the metallicity at low N/O ratios and
viceversa. We investigate also the effect of the N/O ratio on different
diagnostic diagrams used to discriminate narrow-line AGNs from star forming
regions, such as the [OIII]/Hbeta vs. [NII}]/Halpha, and we show that a large
fraction of the galaxies catalogued as composite in this diagram can be, in
fact, star forming galaxies with a high value of the N/O ratio. Finally, using
strong-line methods sensitive to the N/O abundance ratio, like N2O2 and N2S2,
we investigate the relation between this ratio and the stellar mass for the
galaxies of the SDSS. We find, as in the case of the mass-metallicity relation,
a correlation between these two quantities and a flattening of the relation for
the most massive galaxies, which could be a consequence of the enhancement of
the dispersion of N/O in the high metallicity regime.
