Using ~SN as a tracer, interspecific N-transfer was studied during the course of plant development. The use of barriers of differing permeabilities between donor and receiver plants allowed separation of the effect of mycorrhizal colonization, root or hyphal contact and interplant hyphal bridging, on ~SN-transfer from soybean (Glycine max (L.) Merrill) to maize (Zea mays L.). More transfer was measured between mycorrhizal plants, but transport of 15N from the labelled host plant to Glomus versiforme (Karsten) Berch did not seem to occur at the symbiotic interface, suggesting that the fungus is independent of its host for its N-nutrition, and that the role of hyphal bridges in N-transfer between plants, is not significant. Uptake by the receiver plant of the N excreted by the donor plant root system appears to be the mechanism of N-transfer between plants. The factor most affecting 15N-transfer between plants was found to be the extent of the contact between plant root systems. The presence of the endomycorrhizal fungus in plant roots reduced 15N-loss from soybean, but at the same time, its extensive hyphal network improved the efficiency of the maize root system for the recovery of the tSN excreted by soybeans. The net result was a better conservation of the N resource within the plant system. The transfer of N between mycorrhizal plants was particularly enhanced by the death of the soybean.

The effect of mycorrhizal inoculation on 15N transfer from soybean to maize was studied in fumigated and non-fumigated soil. Three Glomus species and a non-inoculated control were compared.In spite of higher levels of root colonization and more abundant hyphae associated with plants growing in fumigated soil, mycorrhizae-enhanced ~SN transfer to maize was significant only in non-fumigated plots. High 15N transfer was not only associated with high mycelium density in soil but also with low soil microbial carbon, suggesting that the effect of mycorrhizal fungi on soil microbial populations may be an important factor affecting N transfer between mycorrhizal plants.