Elevated atmospheric CO concentration (eCO) effects on plants depend on several factors including plant photosynthetic physiology (e.g. C, C), soil nutrient availability and plants' co-evolved soil-dwelling fungal symbionts, namely arbuscular mycorrhizal (AM) fungi. Complicated interactions among these components will determine the outcomes for plants. Therefore, clearer understanding is needed of how plant growth and nutrient uptake, along with root-colonising AM fungal communities, are simultaneously impacted by eCO. We conducted a factorial growth chamber experiment with a C and a C grass species (± AM fungi and ± eCO). We found that eCO increased plant biomass allocation towards the roots, but only in plants without AM fungi, potentially associated with an eCO-driven increase in plant nutrient requirements. Furthermore, our data suggest a difference in the identities of root-colonising fungal taxa between ambient CO and eCO treatments, particularly in the C grass species, although this was not statistically significant. As AM fungi are ubiquitous partners of grasses, their response to increasing atmospheric CO is likely to have important consequences for how grassland ecosystems respond to global change.