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Publication Date

1981

Description

Forage legumes such as alfalfa (Medicago sativa L.), birdsfoot trefoil (Lotus comiculatus L.), and red clover (Trifolium pratense L.), grown in crop rotations, can provide an inexpensive source of nitrogen (N) to succeeding crops. Few quantitative measurements of seasonal patterns of N fixation in managed communities of perennial forage legumes have been undertaken, chiefly because of the lack of satisfactory methods. Therefore, there is little knowledge of how N fixation varies with crop development, yielding ability, and weather patterns. Quantitative measurements of N fixation are important in choosing the appropriate legume for use in a crop rotation and for understanding the constraints to N fixation in normally managed crop communities. Our objectives were to compare the N-fixation patterns of alfalfa, birdsfoot trefoil, and red clover under field con­ditions in 2 successive years and to relate these seasonal changes in symbiotic activity to dry-matter yield, leaf area, and precipitation. Nitrogen fixation was measured by the 15N isotope-dilution technique, in which the isotope composition of the nitrogen of the legume is co􀃚pared by mass spectrometry with that of a perennial grass used as the non-fixing control. In the seeding year the proportion of plant N derived from symbiosis ranged from 27% to 50% among species at the first harvest and from 66% to 80% among species at the second harvest. Seasonal N fixation was greater for alfalfa (15.9 g N/m2) than for either red clover (11.9 g N/m2) or birdsfoot trefoil (10.6 g N/m2). N fixation varied significantly among harvests and among the species at individual harvests. In the second year the proportion of N from symbiosis ranged from O % to 62 % at the first harvest and 32 % to 60 % at second harvest. Alfalfa was the only species harvested a third time, and it contained 35% N from symbiosis. Seasonal N fixation of alfalfa (17.1 g N/m2) was again greater than that of red clover (9.8 g N/m2) or birdsfoot trefoil (7.6 g N/m2). N fixation of alfalfa was more sensitive to heavy midsummer rainfall than was that of birdsfoot trefoil or red clover. On a seasonal basis, alfalfa, the highest-yielding species, fixed more N/unit of land area than birdsfoot trefoil, the lowest-yielding species. Within an individual harvest, N fixation was not closely associated with yield because the high yields common early in the growing season contained less N from symbiosis than did lower yields obtained later in the season. The results clearly show that N-fixation capability varies significantly with species of forage legume. N fixation also varies significantly with management and environmental factors that affect plant development and total N yield.

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Symbiotic Nitrogen Fixation of Alfalfa, Birdsfoot Trefoil, and Red Clover

Forage legumes such as alfalfa (Medicago sativa L.), birdsfoot trefoil (Lotus comiculatus L.), and red clover (Trifolium pratense L.), grown in crop rotations, can provide an inexpensive source of nitrogen (N) to succeeding crops. Few quantitative measurements of seasonal patterns of N fixation in managed communities of perennial forage legumes have been undertaken, chiefly because of the lack of satisfactory methods. Therefore, there is little knowledge of how N fixation varies with crop development, yielding ability, and weather patterns. Quantitative measurements of N fixation are important in choosing the appropriate legume for use in a crop rotation and for understanding the constraints to N fixation in normally managed crop communities. Our objectives were to compare the N-fixation patterns of alfalfa, birdsfoot trefoil, and red clover under field con­ditions in 2 successive years and to relate these seasonal changes in symbiotic activity to dry-matter yield, leaf area, and precipitation. Nitrogen fixation was measured by the 15N isotope-dilution technique, in which the isotope composition of the nitrogen of the legume is co􀃚pared by mass spectrometry with that of a perennial grass used as the non-fixing control. In the seeding year the proportion of plant N derived from symbiosis ranged from 27% to 50% among species at the first harvest and from 66% to 80% among species at the second harvest. Seasonal N fixation was greater for alfalfa (15.9 g N/m2) than for either red clover (11.9 g N/m2) or birdsfoot trefoil (10.6 g N/m2). N fixation varied significantly among harvests and among the species at individual harvests. In the second year the proportion of N from symbiosis ranged from O % to 62 % at the first harvest and 32 % to 60 % at second harvest. Alfalfa was the only species harvested a third time, and it contained 35% N from symbiosis. Seasonal N fixation of alfalfa (17.1 g N/m2) was again greater than that of red clover (9.8 g N/m2) or birdsfoot trefoil (7.6 g N/m2). N fixation of alfalfa was more sensitive to heavy midsummer rainfall than was that of birdsfoot trefoil or red clover. On a seasonal basis, alfalfa, the highest-yielding species, fixed more N/unit of land area than birdsfoot trefoil, the lowest-yielding species. Within an individual harvest, N fixation was not closely associated with yield because the high yields common early in the growing season contained less N from symbiosis than did lower yields obtained later in the season. The results clearly show that N-fixation capability varies significantly with species of forage legume. N fixation also varies significantly with management and environmental factors that affect plant development and total N yield.