NODULATION, N-2 FIXATION (N-15) AND N-NUTRITION RELATIONSHIPS IN MYCORRHIZAL OR PHOSPHATE-AMENDED ALFALFA PLANTS

The legume Medicago sativa L. (alfalfa) (+ Rhizobium meliloti) was grown under controlled conditions in two soils amended with increasing amounts of assimilable phosphate (P1, P2 and P3), or with a vesicular-arbuscular mycorrhizal (VAM) inoculum. A small quantity of N-15-labelled ammonium sulphate was added to each pot for a qualitative estimate of N2 fixation, allowing to compare the effects of VAM and P treatments on nodule activity. Plant response (dry matter) to VAM inoculation was equal to the response to P addition at the highest level (P3). The P and N concentrations were higher in the VAM than in the P3 treatment. However, the effects of VAM inoculation on the levels of nodulation and N2 fixation (N-15) were equivalent to those of the P2-treated plants, while the P3 treatment still increased both nodulation and N2 fixation rates over the P2 and VAM treatments. These observations suggest, for the tested plant cultivar/VAM fungus combination, a limitation for VAM plants to reach their full potential for nodulation and N2 fixation under the prevalent ecophysiological conditions of this experiment. In spite of this limitation, the results indicate that VAM fungi can enhance N acquisition by plants by mechanisms additional to P-mediated improvement of nodulation and N2 fixation. These mechanisms could be based on a capability of VAM fungi to use soil N which is less available to non VAM plants.