Summary The goal of this project is to investigate how plant carbohydrates control arbuscular mycorrhizal fungi-mediated nitrogen transfer. Using isotopic labeling (13C and 15N) technology, we have shown that plant C limitation does not reduce N transfer from arbuscular mycorrhizal fungi (AMF) to Plantago lanceolata. Instead, we have revealed greater N-for-C exchange efficiency between plant and AMF under plant C limitation (low CO2 concentration). C-limited host plants with AMF optimized the use of resources by increasing both C and N allocation to shoots to alleviate C limitation. This project was funded by Max Planck Institute and DAAD.