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 plant s were given urea (released as NH4+) as their only N-source. This smal l change in deltaN-15 would create a large error in calculating source s 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 sour ce. The correct N-source value for VAM-infected NH4- using plants may be the deltaN-15 of soil NH4+ + 2 parts per thousand. Treatment effect s 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 V A-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 a ny of the variables measured for this host-fungus pair when the sole N -sources were inorganic (NO3- and NH4+ released from urea). Ectomycorr hizal fungi are diverse in their physiological behaviour, and these da ta should not be taken as being representative of the whole group. Mor e work is required with other types of mycorrhiza and more complex sou rces of N. Future work will include a water balance to partition the e ffects of water use and nutrient supply in determining deltaC-13. An o n-line combustion-ANCA-MS method is described for fully automated meas urement of natural abundance levels of N-15/14 and C-13/12 for plant m aterials. This method achieves the required precision while dramatical ly increasing sample throughout.