• Eucalyptus maculata;
  • Casuarina cunninghamiana;
  • 15NH4+;
  • 15NO3;
  • Frankia;
  • Pisolithus sp.;
  • common ectomycorrhizal networks (CMNs);
  • two-way N transfer


  • • 
    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.