Abstract
We examined plant community responses to interactions between arbuscular mycorrhizal (AM) fungi and availability of atmospheric CO 2 and soil N. Communities of 14 plant species were grown in mesocosms containing living or killed AM fungal inoculum, ambient or elevated atmospheric CO 2 and low or enriched soil N. After one growing season, significantly different plant communities existed in the different treatments. Plant species richness was lowest in +N mesocosms and highest in +AM + CO 2 mesocosms. At ambient CO 2 , AM fungi reduced richness but at elevated CO 2 they increased it. This was caused by changes in mortality rates of several C 3 forbs and may suggest that CO 2 enrichment ameliorates the carbon cost of some AM symbioses. Soil moisture was higher in +CO 2 mesocosms but +AM counteracted this effect. These results suggest that AM symbioses may be important mediators of plant community responses to anthropogenic CO 2 and N enrichment.