Nodulated N2-fixing Casuarina cunninghamiana is the sink for net N transfer from non-N2-fixing Eucalyptus maculata via an ectomycorrhizal fungus Pisolithus sp. using 15NH4+ or 15NO3- supplied as ammonium nitrate

To determine the effects of nitrogen source on rates of net N transfer between plants connected by a common mycorrhizal network, we measured transfer of N supplied as 15NH4 14NO3 or 14NH4 15NO3 in three Casuarina/Eucalyptus treatments interconnected by a Pisolithus sp. The treatments were nonnodulated nonmycorrhizal/nonmycorrhizal; nonnodulated mycorrhizal/mycorrhizal; and nodulated mycorrhizal/mycorrhizal. Mycorrhization was 67% in Eucalyptus and 36% in Casuarina. N2 fixation supplied 38% of the N in Casuarina. Biomass, N and 15N contents were lowest in nonmycorrhizal plants and greatest in plants in the nodulated/mycorrhizal treatment. Nitrogen transfer was enhanced by mycorrhization and by nodulation, and was greater when N was supplied as 15NH4+ than 15NO3-. Nitrogen transfer rates were lowest in the nonmycorrhizal treatment for either 15N source, and greatest in the nodulated, mycorrhizal treatment. Transfer was greater to Casuarina than to Eucalyptus and where ammonium rather than nitrate was the N source. Irrespective of 15N source and of whether Casuarina or Eucalyptus was the N sink, net N transfer was low and was similar in both nonnodulated treatments. However, when Casuarina was the N sink in the nodulated, mycorrhizal treatment, net N transfer was much greater with 15NH4+ than with 15NO3-. High N demand by Casuarina resulted in greater net N transfer from the less N-demanding Eucalyptus. Net transfer of N from a non-N2-fixing to an N2-fixing plant may reflect the very high N demand of N2-fixing species.