In the context of a changing climate, there has been an increased research focus on predicting the response of plants to elevated atmospheric CO₂ concentrations. Simultaneously, the availability of phosphate fertiliser is becoming increasingly limited. Although the effects of climate change on arbuscular mycorrhizal fungi (AMF) have been given some attention, it has not been possible to generalise effects of AMF confidently across plant functional groups such as C₃ and C₄ plants. We grew the C₄ cereal Zea mays cv. B73 with or without inoculation with the AMF Funneliformis mosseae, at three different soil P concentrations ranging from 0 to 90 mg P kg⁻¹ soil, and at ambient (400 ppm: aCO₂) or elevated (900 ppm: eCO₂) CO₂ concentrations. Plant biomass, shoot P nutrition, and colonisation by AMF were analysed. With limiting soil P, plant growth responded positively to colonisation by AMF, but not eCO₂. At the medium soil P treatment, shoot P concentration but not growth was increased by AMF colonisation, while growth was instead increased by eCO₂. The positive growth response to eCO₂ persisted at the highest soil P treatment, where there were no effects of AMF colonisation on plant growth or P nutrition. The effects of AMF colonisation and CO₂ on maize growth and P nutrition in a future climate scenario will likely be highly dependent on the availability of soil P. Furthermore, differential effects on plants of contrasting functional groups (eg., C₃ vs C₄) should be considered when investigating the role of AMF under eCO₂ conditions.