Nitrogen Fixation Rates of Alfalfa and Red Clover Grown in Mixture with Grasses 1

Nitrogen contributions of legumes to grass‐legume mixtures are estimated on the basis of legume monocultures and may need to be adjusted to reflect any effect of the companion grass. An experiment at two field locations was conducted to determine whether the N 2 ‐fixation rates of alfalfa ( Medicago sativa L.) and red clover ( trifolium pratense L.) were significantly affected by mixed culture with orchardgrass ( Dactylis glomerata L.) or timothy ( Phleum pratense L.). Legumes and grasses were grown in eight mono‐ or mixed cultures. Specific nodule activity (SNA) was estimated by acetylene reduction at first flower of legumes and 4 weeks after initial harvest. Grasses were analyzed for total N (TN). Legume dry weight per plant was reduced by association with grasses in only one determination at one location. Grass TN 4 weeks after initial harvest was in most instances increased significantly by the legume. Specific nodule activities of red clover grown alone and red clover grown with grasses were similar and did not change between determinations. The SNA of alfalfa grown with grass was significantly higher than that of alfalfa grown alone for the second determination at one location. The SNA of alfalfa in mixed culture increased between determinations; however, SNA of alfalfa monocultures did not change. It is concluded that grass plants are not detrimental to legume plants and may stimulate N 2 ‐fixation. Some transfer of N from legumes to grasses may occur when the two species are grown in mixed cultures.