Mycorrhizal Effects on Interspecific Plant Competition and Nitrogen Transfer in Legume‐Grass Mixtures

Mycorrhizal fungi may play a role in the interactions between components of legume‐grass mixed swards by their enhancing effect on plant P uptake and on legume N 2 ‐fixation rate. The effects of mycorrhizal fungi on interspecific plant interactions and N transfer from legume to grass were studied in two legume‐grass forage mixtures grown under three P fertilization regimes. In two experiments, one involving an alfalfa‐bromegrass ( Medicago sativa L.‐ Bromus inermis Leyss.) mixture and the other, an alfalfa‐timothy ( Phleum pratense L.) mixture, plants were inoculated or not with Glomus intraradix and fertilized with 0, 14.2 or 28 kg P ha − 1. Phosphorus fertilization sometimes increased plant tissue P concentration, especially in timothy, but it never affected grass/legume biomass ratios. The effects of the mycorrhizal fungus were seasonal and were most evident in the August harvests, when mycorrhizal inoculation increased the yield of alfalfa at the expense of bromegrass or timothy, reducing the grass/legume dry mass ratio in both mixtures. Transfer of 15 N from legume to grass was demonstrated, but this transfer was not enhanced by mycorrhizal colonization of plants. Mycorrhizal colonization increased P accumulation in the alfalfa components of the mixtures (33% with bromegrass and 17% with timothy); however, P concentrations in the legume biomass were above the P sufficiency level in nonmycorrhizal plants and were not increased by mycorrhizal colonization. Therefore, the seasonal increase in alfalfa yield at the expense of the grass was apparently not caused by enhancement of P uptake by mycorrhizal colonization. Diagnosis and Recommendation Integrated System (DRIS) indices calculated from a complete nutrient analysis of the tissue revealed that the beneficial effect of mycorrhiza on alfalfa production was associated with a better nutrient balance (mainly Ca and Mg) of the plants.