CO2 fertilization of vegetation remains a critical offset to Anthropogenic emissions. Piñeiro et al, report the primary role of Phosphorus (P) in the growth responses of an understory plant community grown under experimentally elevated CO2 (eCO2). This study provides direct evidence that P constrains understory plant growth responses to eCO2. By carefully tracing whole pot respiration, and patterns of carbon allocation, this study indirectly highlights the role of microorganisms in plant responses to eCO2. In this commentary, we describe microbial processes that influence plant nutrition and growth in a CO2 enriched world but remain challenging to integrate into process-based models. We summarize the current patchwork understanding of the role of microbes in plant growth responses to eCO2, focusing primarily on plant-rhizosphere interactions, and mycorrhiza. Finally, we conclude by describing a series of research areas that stand to advance understanding of plant growth and nutrient acquisition under eCO2.