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The chemical evolution of galaxies on a cosmological timescale is still amatter of debate despite the increasing number of available data provided byspectroscopic surveys of star-forming galaxies at different redshifts. Thefundamental relations involving metallicity, such as the mass-metallicityrelation (MZR) or the fundamental-metallicity relation, give controversialresults about the reality of evolution of the chemical content of galaxies at agiven stellar mass. In this work we shed some light on this issue using thecompleteness reached by the 20k bright sample of the zCOSMOS survey and usingfor the first time the nitrogen-to-oxygen ratio (N/O) as a tracer of the gasphase chemical evolution of galaxies that is independent of the star formationrate. Emission-line galaxies both in the SDSS and 20k zCOSMOS bright surveywere used to study the evolution from the local Universe of the $MZR up to aredshift of 1.32 and the relation between stellar mass and nitrogen-to-oxygenratio (MNOR) up to a redshift of 0.42 using the N2S2 parameter. All thephysical properties derived from stellar continuum and gas emission-lines,including stellar mass, star formation rates, metallicity and N/O, werecalculated in a self-consistent way over the full redshift range. We confirmthe trend to find lower metallicities in galaxies of a given stellar mass in ayounger Universe. This trend is even observed when taking possible selectioneffects into account that are due to the observed larger median star formationrates for galaxies at higher redshifts. We also find a significant evolution ofthe MNOR up to z = 0.4. Taking the slope of the O/H vs. N/O relation intoaccount for the secondary-nitrogen production regime, the observed evolution ofthe MNOR is consistent with the trends found for both the MZR and itsequivalent relation using new expressions to reduce its dependence on SFR.

The cosmic evolution of oxygen and nitrogen abundances in star-forming galaxies over the past 10 Gyrs