Competition between different isogenic mutants of Pseudomonas fluorescens unable to carry out the first steps of the denitrification pathway was compared in soil micro‐columns non‐planted or planted with maize. A new isogenic mutant of P. fluorescens YT101 affected in both nitrate and nitrite respirations was constructed and used as a model of non‐denitrifying strain (FM69MS strain). The outcome of the selection exerted by the plant after co‐inoculation of FM69MS at the same ratio either with an isogenic denitrifier unable to reduce nitrate (Nar− mutant) or with an isogenic NO2− accumulator (Nir− mutant) was investigated in non‐limiting NO3− conditions. Regardless of the inoculated mixture, both strains were able to grow in both rhizosphere and non‐planted soil. The proportion of Nar− or Nir− strain in the Nar−+FM69MS or Nir−+FM69MS total introduced population remained stable in non‐planted soil. In the rhizosphere, we observed a higher competitiveness of the Nir− mutant compared with FM69MS, whereas the latter showed the same competitiveness as the Nar− mutant. These results provide the first demonstration that NO3− reduction is the main nitrogen‐dissimilating step controlling the competitiveness of P. fluorescens in the rhizosphere.