Ammonia-oxidizing bacteria (AOB) are key organisms in the N cycle, as they control the first, rate-limiting step of the nitrification process. The question whether current environmental disturbances, such as climate warming and plant diversity losses, select for a particular community structure of AOB and/or influence their activity remains open. The purpose of this research was to study the impact of a 3 degrees C warming and of plant species richness (S) on microbial activity and diversity in synthesized grasslands, with emphasis on the nitrification process and on the diversity (community structure and richness) of ammonia-oxidizing bacteria (AOB). We measured soil chemical characteristics, basal respiration, potential nitrification and AOB diversity in soils under increasing plant species richness (S = 1, S = 3, S = 9) at ambient and (ambient +3 degrees C) temperature. Species were drawn from a 9-species pool, belonging to three functional groups: forbs, legumes and grasses. Mixtures comprised species from each of the three functional groups. Warming did not affect AOB diversity and increased potential nitrification at S = 3 only. Under warmed conditions, higher plant species richness resulted in increased potential nitrification rates. AOB richness increased with plant species richness. AOB community structure of monocultures under legumes differed from those under forbs and grasses. Clustering analysis revealed that AOB community structure under legume monocultures and mixtures of three and nine species grouped together. These results indicate that functional group identity rather than plant species richness influenced AOB community structure, especially through the presence of legumes. No clear relationship emerged between AOB richness and potential nitrification whatever plant species richness and temperature treatment. Our findings show a link between aboveground and belowground diversity, namely plant species richness, AOB richness and community structure. AOB richness was not related to soil processes, supporting the idea that increased diversity does not necessarily lead to increased rates of ecosystem processes. (C) 2010 Elsevier Ltd. All rights reserved.