Reciprocal N (15NH4+ or 15NO3−) transfer between nonN2‐fixing Eucalyptus maculata and N2‐fixing Casuarina cunninghamiana linked by the ectomycorrhizal fungus Pisolithus sp.

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• Two-way N transfers mediated by Pisolithus sp. were examined by excluding root contact and supplying 15NH4+ or 15NO3− to 6-month-old Eucalyptus maculata or Casuarina cunninghamiana grown in two-chambered-pots separated by 37 m screens. • Mycorrhizal colonization was 35% in Eucalyptus and 66% in Casuarina (c. 29% N2-fixation). Using an environmental scanning electron microscope, living hyphae were observed to interconnect Eucalyptus and Casuarina. Biomass and N accumulation was greatest in nodulated mycorrhizal Casuarina/mycorrhizal Eucalyptus pairs, less in nonnodulated mycorrhizal Casuarina/mycorrhizal Eucalyptus pairs, and least in nonnodulated nonmycorrhizal Casuarina/nonmycorrhizal Eucalyptus pairs. • In nonnodulated mycorrhizal pairs, N transfers to Eucalyptus or to Casuarina were similar (2.4–4.1 mg per plant in either direction) and were 2.6–4.0 times greater than in nonnodulated nonmycorrhizal pairs. In nodulated mycorrhizal pairs, N transfers were greater to Eucalyptus (5–7 times) and to Casuarina (12–18 times) than in nonnodulated mycorrhizal pairs. Net transfer to Eucalyptus or to Casuarina was low in both nonnodulated nonmycorrhizal (< 0.7 mg per plant) and nonnodulated mycorrhizal pairs (< 1.1 mg per plant). In nodulated mycorrhizal pairs, net transfer to Casuarina was 26.0 mg per plant. • The amount and direction of two-way mycorrhiza-mediated N transfer was increased by the presence of Pisolithus sp. and Frankia, resulting in a net N transfer from low-N-demanding Eucalyptus to high-N-demanding Casuarina.