Mycorrhizae Effects on Plant Growth, Phosphorus Uptake, and N2(C2H4) Fixation in Two Alfalfa Populations1

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Abstract
Alkaline soils of the southwestern USA often affect phosphorus nutrition of crop plants, decreasing yield and N2‐fixation potential of alfalfa (Medicago sativa L.). The objective of this study was to determine the relationships among P fertilization, mycorrhizae inoculation, and alfalfa populations on plant growth, P concentrations in the tissue, and N2 (C2H4) fixation rate. The experiment was a pot study conducted in a shadehouse during the summer months. Soil was a Glendale clay fineloamy, mixed thermic Typic Torrifluvent with a pH of 8.5, a water extractable P level of 0.06 ppm, and sodium bicarbonate extractable P level of 5.9 ppm.Amounts of top growth and P concentration in the forage were influenced by the interactions of alfalfa populations with P fertilization levels, and the presence or absence of mycorrhizae inoculation. The unselected alfalfa population (N0) did not increase in top growth when fertilized with P but did increase in top growth when inoculated with mycorrhizae. Phosphorus concentration in the top growth of the unselected population was highest when both fertilized with P and inoculated with mycorrhizae. The selected alfalfa population (N3) increased in top growth and P concentration in the foliage when fertilized with P. Inoculation with mycorrhizae in this population produced no significant effects. None of the treatments affected N2 (C2H4 fixation rates. The results indicated some beneficial effects on alfalfa growth and P concentration in the top growth could be expected from both mycorrhizae inoculation and P fertilization in this soil, but the effect depended on genetic factors in the plants.

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