The objective of this study was to examine the ability of arbuscular mycorrhizal (AM) fungi to take up nitrogen from soil and transport it to the host plant. Maize (Zea mays L.) associated with Glomus intraradices Schenck and Smith or left uninoculated was grown in containers which were divided by a nylon net into a root compartment and a hyphal compartment. A 40 μm pore size nylon net was used to exclude plant roots while allowing fungal hyphae to grow into soil confined by the net. 15N tracer was supplied either as inorganic N or as organic N to the hyphal compartment at a distance of 5 cm from the net.
Inoculation with the AM fungus increased the 15N content of maize compared to the non-mycorrhizal controls when N was applied as (15NH4)2SO4. However, there was no conclusive evidence that AM hyphae could derive N from organic 15N sources. Most of the increased N uptake of mycorrhizal plants occurred by hyphal translocation from the hyphal compartment to the root compartment. Higher N uptake by mycorrhizal plants with access to the hyphal compartment was indicated by depletion of total 15N in the soil of that compartment. Cutting the extraradical hyphae in the hyphal compartment in order to sever their connection with the host roots decreased the 15N uptake of the maize plants. A time-course study with inorganic 15N over 26 d showed that G. intraradices transported most of the 15N between 10 and 15 d after 15N application, whereas the non-mycorrhizal control plants had a consistently low concentration of 15N throughout the period of sampling.
Nitrogen transport by external hyphae of three AM fungi, G. intraradices, Acaulospora laevis Gerdemann and Trappe and Gigaspora margarita Becker and Hall associated with maize, was further investigated. The results indicated that different isolates of AM fungi differ in the efficiency of hyphal N transport as a consequence of the different patterns of hyphal spread in the soil or of the different capacity for uptake by unit length of hyphae.