A technique using 15N-labelled inorganic fertilizer was applied to estimate N2 fixation by the forage legume Hedysarum coronarium L. and to ascertain the role of vesicular-arbuscular (VA) mycorrhizas in plant N nutrition throughout a growing season under field conditions. The absence of the specific Rhizobium for the forage legume in the test soil allowed us the use of 15N methodology with the same legume as reference ‘non-fixing’ crop.
At the first harvest, mycorrhizal inoculation behaved similarly to the phosphate addition in improving the percentage (70 %) and the total amount of N derived from fixation. But thereafter, mycorrhizal inoculation not only enhanced dry matter yield, N concentration and total N yield but also the amount of N derived from soil and from fixation, as compared with either phosphate-added or control plants. This indicated that mycorrhizas acted both by a P-mediated mechanism to improve N2 fixation and by enhancing N uptake from soil. The latter agrees with recent findings by others that VA mycorrhizal hyphae can translocate and assimilate ammonium, a fact of physiological and ecological interest.