The resource balance model predicts that under elevated atmospheric CO2, plants should preferentially allocate photosynthate to acquiring below-ground resources. Only short-term experiments are available to test this hypothesis, while long-term responses are really of interest in global change ecology. Arbuscular mycorrhizae represent one mode of below-ground nutrient acquisition available to the vast majority of plants. Percent root colonization by arbuscular mycorrhizal fungi (AMF), AMF soil hyphal length, and soil concentrations of the AMF protein glomalin increased linearly along a CO2 gradient provided in a grassland by a CO2 spring in Northland, New Zealand. These results are an important confirmation of numerous short-term studies, and present the first test of the resource balance model, applied to AMF, after long-term elevated CO2 exposure.