Abstract: A FACE system（free-air carbon dioxide enrichment）was used to investigate effects of elevated atmospheric CO₂ on soil ammonia-oxidizing bacterial population,dominant bacteria and their nitrification activities.It was found that the enrichment of atmospheric CO₂ reduced the population of ammonia-oxidizing bacteria in soil when N was in normal level,but had no difference when N was in high level.The elevation of atmospheric CO₂ also affected dominancy of ammonia-oxidizing bacteria with Nitrosococcus sp.and Nitrosovibrio sp. replacing Nitrosomonas sp. and Nitrosococcus sp.in the ambient soils.In addition,nitrification activities of the dominant genera were decreased to various extents under elevated CO₂.