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

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.

Key Words: N₂-fixation • Top growth • Interaction • Genotype • Medico sativa L.

² Former graduate student, professor, associate professor, and research associate, respectively, Dep. of Crop and Soil Sciences, New Mexico State Univ., Las Cruces.

Received for publication April 19, 1982.