Background and aims Maize seed inoculation with the plant-growth promoting rhizobacterium Azospirillum is known to stimulate root growth and exudation, thereby enabling a better exploitation of the soil volume and indirectly enhancing N uptake, before it is leached or volatilised. We tested the hypothesis that Azospirillum induced-root stimulation could increase plant N uptake and thus enhance plant competition over N-cycling microorganisms for mineral N, leading to (a) a decrease of nitrification, and (b) a decrease of denitrification that can be counteracted by the stimulatory effect of root exudates-carbon on denitrifiers. Methods The extent of inoculation-induced changes in potential nitrification and denitrification activities, in the abundances of nitrifiers and denitrifiers and in root functional traits were assessed at 4 dates over two consecutive years in a multi-site field trial. Results Plant root traits, (de)nitrifier potential activities and abundances were affected by inoculation, with significant relationships between plant and microbial responses. However the observed effects varied among sites. Semi-potential denitrifying activity (i.e. when C or N sources were not made non-limiting) was measured in order to elucidate the differential response to inoculation observed on denitrifiers between sites. The site-specific level of C limitation for denitrifiers explained the differential inoculum effect observed across sites. Conclusion Our findings indicate that maize seed inoculation with Azospirillum can be a sustainable, though soil-specific, agricultural practice providing both agronomic and environmental benefits. Furthermore, soil organic C availability appears to be a good predictor of the potential for N losses through denitrification from the soil-plant system following inoculation.