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We analysed the optical spectra of HII regions extracted from a sample of 350galaxies of the CALIFA survey. We calculated total O/H abundances and, for thefirst time, N/O ratios using the semi-empirical routine HII-CHI-mistry, which,according to P\'erez-Montero (2014), is consistent with the direct method andreduces the uncertainty in the O/H derivation using [NII] lines owing to thedispersion in the O/H-N/O relation. Then we performed linear fittings to theabundances as a function of the de-projected galactocentric distances. Theanalysis of the radial distribution both for O/H and N/O in the non-interactinggalaxies reveals that both average slopes are negative, but a non-negligiblefraction of objects have a flat or even a positive gradient (at least 10\% forO/H and 4\% for N/O). The slopes normalised to the effective radius appear tohave a slight dependence on the total stellar mass and the morphological type,as late low-mass objects tend to have flatter slopes. No clear relation isfound, however, to explain the presence of inverted gradients in this sample,and there is no dependence between the average slopes and the presence of abar. The relation between the resulting O/H and N/O linear fittings at theeffective radius is much tighter (correlation coefficient $\rho_s$ = 0.80) thanbetween O/H and N/O slopes ($\rho_s$ = 0.39) or for O/H and N/O in theindividual \hii\ regions ($\rho_s$ = 0.37). These O/H and N/O values at theeffective radius also correlate very tightly (less than 0.03 dex of dispersion)with total luminosity and stellar mass. The relation with other integratedproperties, such as star formation rate, colour, or morphology, can beunderstood only in light of the found relation with mass.

The dependence of oxygen and nitrogen abundances on stellar mass from the CALIFA survey