Depletion of non‐exchangeable NH $ _4^+ $ at the root–soil and hyphae–soil interface of VA mycorrhizal maize ( Zea mays ) and parsley ( Petroselinum sativum )

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, 15 NH $ _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.