Ammonia oxidising bacteria (AOB) are important soil microorganisms that carry out the first step in nitrification, the oxidation of ammonia to nitrite. In this paper we investigated the impact of long-term elevated CO₂ on soil nitrification and soil AOB community composition. Soil samples were taken from Hakanoa natural CO₂ springs, Kamo, Northland, New Zealand. This site has been exposed to elevated CO₂ for several decades. Soils were collected from different points near to CO₂-emitting vents where the CO₂ concentration at canopy height had been characterised. Nitrification activity was measured using a short-term nitrification assay, and AOB community composition was characterised using polymerase chain reaction and denaturing gradient gel electrophoresis (DGGE). A principal component analysis of the DGGE banding pattern was carried out to identify the effect of CO₂ on AOB community composition. Soil nitrification activity was markedly decreased with increasing CO₂. The variation in DGGE banding patterns revealed differences in the composition of the soil AOB community that were related to CO₂ concentration. Principal component analysis showed that the changes in community composition and nitrifying activity were linked and that these changes were related to atmospheric CO₂ concentration.