Mobilization of non‐exchangeable ammonium (NH$ _4^+ $) by hyphae of the vesicular‐arbuscular mycorrhizal (VAM) fungus Glumus mosseae was studied under controlled experimental conditions. Maize (Zea mays) and parsley (Petroselinum sativum) were grown either alone or in symbiosis with Glomus mosseae in containers with separated compartments for roots and hyphal growth. In one experiment, 15NH$ _4^+ $ was added to the soil to differentiate between the native non‐exchangeable NH$ _4^+ $ and the non‐exchangeable NH$ _4^+ $ derived from N fertilization. Non‐exchangeable NH$ _4^+ $ was mobilized by plant growth. Plant dry weight and N uptake, however, were not significantly influenced by mycorrhizal colonization of the roots. The influence of root infection with mycorrhizal fungus on the mobilization of non‐exchangeable NH$ _4^+ $ was negligible. In the hyphal compartment, hyphal uptake of N resulted in a decrease of NH$ _4^+ $ in the soil solution and of exchangeable NH$ _4^+ $. However, the NH$ _4^+ $ concentration was still too high to permit the release of non‐exchangeable NH$ _4^+ $. The results demonstrate that, in contrast to roots, hyphae of VAM fungi are not able to form a non‐exchangeable‐NH$ _4^+ $ depletion zone in the adjacent soil. However, under conditions of a more substantial depletion of the exchangeable NH$ _4^+ $ in the mycorrhizal sphere (e.g., with longer growth), an effect of mycorrhiza on the non‐exchangeable NH$ _4^+ $ might be found.

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