EFFECTS OF THE ECTO-MYCORRHIZAL AND VA-MYCORRHIZAL FUNGI HYDNAGIUM-CARNEUM AND GLOMUS-CLARUM ON THE DELTA-N-15 AND DELTA-C-13 VALUES OF EUCALYPTUS-GLOBULUS AND RICINUS-COMMUNIS

Glasshouse experiments with Ricinus communis showed that the presence/absence of a VA mycorrhizal fungus (Glomus clarum) changed the deltaN-15 value of the host by as much as 2 parts per thousand when the plants were given urea (released as NH4+) as their only N-source. This small change in deltaN-15 would create a large error in calculating sources of plant N. In particular, these results throw into doubt any models of N-cycling which assume that soil N can be treated as a single source. The correct N-source value for VAM-infected NH4- using plants may be the deltaN-15 of soil NH4+ + 2 parts per thousand. Treatment effects were also found in the distribution of deltaN-15 and % N among plant organs. Plants with VAM had a lower N:P atom ratio and were larger in total biomass. Carbon discrimination (deltaC-13) Was greater in the VA-infected plants. The measured effects of VAM infection suggest that for some plants the fungus may be the primary site of N assimilation. A parallel experiment with Eucalyptus globulus and the ectomycorrhizal fungus Hydnangium carneum resulted in no significant differences in any of the variables measured for this host-fungus pair when the sole N-sources were inorganic (NO3- and NH4+ released from urea). Ectomycorrhizal fungi are diverse in their physiological behaviour, and these data should not be taken as being representative of the whole group. More work is required with other types of mycorrhiza and more complex sources of N. Future work will include a water balance to partition the effects of water use and nutrient supply in determining deltaC-13. An on-line combustion-ANCA-MS method is described for fully automated measurement of natural abundance levels of N-15/14 and C-13/12 for plant materials. This method achieves the required precision while dramatically increasing sample throughout.