Context : Bar-induced gas inflows towards galaxy centres are recognised as a key agent for the secular evolution of galaxies .
One immediate consequence of this inflow is the accumulation of gas in the centre of galaxies where it can form stars and alter the chemical and physical properties .

Aims : Our aim is to study whether the properties of the ionised gas in the central parts of barred galaxies are altered by the presence of a bar and whether the change in central properties is related to bar and/or parent galaxy properties .

Methods : We use a sample of nearby face-on disc galaxies with available SDSS spectra , morphological decomposition , and information on the stellar population of their bulges , to measure the internal Balmer extinction from the H \alpha to H \beta line ratio , star formation rate , and relevant line ratios to diagnose chemical abundances and gas density .

Results : The distributions of all the parameters analysed ( internal Balmer extinction at H \beta ( c ( H \beta ) ) , star formation rate per unit area , electron density , [ N ii ] \lambda 6583/H \alpha emission-line ratio , ionisation parameter , and nitrogen-to-oxygen ( N/O ) abundance ratio ) are different for barred and unbarred galaxies , except for the R _ { 23 } metallicity tracer and the oxygen abundance obtained from photoionisation models .
The median values of the distributions of these parameters point towards ( marginally ) larger dust content , star formation rate per unit area , electron density , and ionisation parameter in the centres of barred galaxies than in their unbarred counterparts .
The most remarkable difference between barred and unbarred galaxies appears in the [ N ii ] \lambda 6583/H \alpha line ratio that is , on average , \sim 25 % higher in barred galaxies , due to an increased N/O abundance ratio in the centres of these galaxies compared to the unbarred ones .
We analyse these differences as a function of galaxy morphological type ( as traced by bulge-to-disc light ratios and bulge mass ) , total stellar mass , and bulge Sérsic index .
We observe an enhancement of the differences between central gas properties in barred and unbarred galaxies in later-type galaxies or galaxies with less massive bulges .
However , the bar seems to have a lower impact on the central gas properties for galaxies with bulges above \sim 10 ^ { 10 } M _ { \odot } or total mass M _ { \star } \gtrsim 10 ^ { 10.8 } ~ { } M _ { \odot } .

Conclusions : We find observational evidence that the presence of a galactic bar affects the properties of the ionised gas in the central parts of disc galaxies ( radii \lesssim 0.6–2.1 kpc ) .
The most striking effect is an enhancement in the N/O abundance ratio .
This can be interpreted qualitatively in terms of our current knowledge of bar formation and evolution , and of chemical evolution models , as being the result of a different star formation history in the centres of barred galaxies caused by the gas inflow induced by the bar .
Our results lend support to the scenario in which less massive and more massive bulges have different origins or evolutionary processes , with the gaseous phase of the former currently having a closer relation to the bars .