New Phytol. (1992), 122, 281-288 Hyphal transport of i^N-labelled nitrogen by a vesicular-arbuscular mycorrhizal fungus and its effect on depletion of inorganic soil N BY A.J OH AN SEN, I. JAKOBSEN AND E.S.JENSEN Plant Biology Section, Environmental Science and Technology Department, Riso National Laboratory, DK-4000 Roskilde, Denmark (Received 27 February 1992; accepted 19 May 1992) SUMMARY Hyphal transport of nitrogen from a ^^^N-labelled ammonium source by a VA-mycorrhizal fungus was studied under controlled experimental conditions. Cucumis sativus L. cv. Aminex (Fl hybrid) was grown alone or together with Glomus intraradices Schenck and Smith in containers with a hyphal compartment separated from the rooting medium by a fine nylon mesh. Lateral movement of the applied ''N towards the roots was minimized by using a nitrification inhibitor (N-serve) and a hyphal buffer compartment. Recovery of ''N by mycorrhizal and non- mycorrhizal plants was 6 and 0"o, respectively, after a labelling period of 23 days. The corresponding figures, without N-serve added, were 4 and 7",,. A prolongation of the labelling period by 8 days (N-serve applied) resulted in an increase in the "^N recovery by mycorrhizal plants to 30 "o of the applied ''^N. Non-mycorrhizal plants contained only traces of ^^N. The external hyphae depleted the soil in the hyphal compartment efficiently for inorganic N. In contrast, hyphal compartments of control containers still contained considerable amounts of inorganic N. The ^'^N assimilated by the external hyphae in one hyphal compartment was not translocated in significant amounts to the external hyphae in another hyphal compartment. The possible implication of this for inter-plant N transfer by VA hyphal connections is discussed. Key words: Cucumis sativus, Glomus intraradices, hyphal N-transport, nitrogen-15, VA mycorrhiza. monium is adsorbed to negatively charged clay INTRODUCTION minerals and may also be fixed m the lattice of clay External hyphae of VA mycorrhizas are able to minerals (Nommik & Vahtras, 1982). Consequently, transport nutrients from the soil to plant roots, ammonium is less mobile in the soil than nitrate, and Mycorrhizas are of particular importance to the VA-mycorrhizal hyphae might contribute to the uptake of those nutrients which are transported transport of ammonium towards plant roots (see through soil mainly by diffusion (see Harley & Bowen & Smith, 1981). More information is needed Smith, 1983). Accordingly, mycorrhizal research has on the ability of VA-mycorrhizal hyphae to absorb, concentrated on phosphorus, which is strongly translocate and transfer soil N to the plant. This adsorbed in soil and also needed in large quantities information is also required to define those ecological by the plants. conditions where any hyphal N transport would be Hyphal transport of N has received much less of significance to plant growth. attention. Nitrate is the main N source in most VA-mycorrhizal hyphae appear to possess the agricultural soils. Fertilizer and mineral ammonium enzymes required for the assimilation of NH^^ are quickly nitrified and the nitrate readily trans- (Smith et al., 1985). Enzymes involved in the ported to the roots by mass fiow. VA mycorrhizas are assimilation of NO3- were not detected in VA- therefore expected to have only minor infiuence on mycorrhizal fungi by Oliver et a/. (1983) but spores the plant acquisition of N when nitrate is the from some species are able to reduce nitrate (Ho & dominant N source. At low pH, where rates of Trappe, 1975). nitrification are low, ammonium may constitute a Nitrogen-15 is useful for studying N transport in significant proportion of the inorganic N pool the soil-fungus-plant system. Mycorrhizas im- (Goring & Laskowski, 1982; Schmidt, 1982). Am- proyed the transport of applied ^"'N from a hyphal