We tested if the microbial functional redundancy concept (theory relating changes in ecosystem functioning by species loss) apply for changes in the ammonium-oxidizing community (nitrifier bacteria) due to: a) grazing disturbance, b) seasonal variations (dry season vs. wet season), c) habitat type (soil vs. organic residues), and d) ecological conditions (two eco-regions), in an arid region of central-western of Argentina. We determined: a) abundance of culturable nitrifier bacteria, b) nitrification activity and efficiency, and c) nitrifier community genetic structure by PCR-DGGE (richness and similarity index). Grazing did not change the nitrifier abundance, activity and richness, while seasonally and eco-region effects were scarce. Contrarily, the habitat type affected nitrifier activity, which was higher in organic residues than in soil. All similarity indexes were low (average 0.50, range 0.76-0.18) which suggest high species diversity in these arid eco-regions. Our results indicate that the nitrification process in arid region of Argentina is redundant but it applies only for each habitat. To our knowledge, this is the first functional redundancy study on microbial ecological process in disturbed arid zones, in which we report an important species replacement but without modifications in microorganism abundance and activity.