Submitted to: Alfalfa Improvement Conference Proceedings
Publication Type: Abstract only
Publication Acceptance Date: 7/16/1996
Publication Date: N/A
Citation: Interpretive Summary:
Technical Abstract: Annual medics improve soil fertility and improve yields of subsequent crops. Information on medic N2 fixation potential is important in predicting their N credit and developing farm management practices. The objective of this study was to measure the amount of N2 derived from the atmosphere by spring seeded annual Medicago species. Four annual medic species, M. truncatula, cv. Mogul, M. polymorpha, cv. Santiago, M. scutellata, cv. Sava, and M. rugosa, cv. Sapo were seeded at rates of 484 live seeds/square meter. Noninoculated Sapo was used as reference crop. All other annual medics were treated with commercial Rhizobium inoculum. Ammonium sulphate enriched with 99.4 atom % 15N excess was applied at 10 days after seeding. Herbage were sampled by manually cutting 1 square meter area to a 2-cm stubble height in late July or early August. Dry matter yield, total N, and 15N concentration were measured to calculate the eamount of N derived from atmosphere. The herbage production of annual medics ranged from 4 to 8 Mg/ha with Santiago and Sava being the highest yielding and Sapo being the lowest yielding. Total N production of annual medics ranged from 120 to 250 kg/ha. The proportion of N derived from the atmosphere (%Ndfa) ranged from 69 to 83%. Medics did not differ in %Ndfa at Becker, however, at Rosemount, the %Ndfa was higher for Santiago than for Sava and Sapo. Medics had higher %Ndfa at Becker than at Rosemount because the Becker soil had lower organic matter and N contents. The amount of Ndfa ranged from 100 to 200 kg/ha and Santiago had the highest amount of Ndfa. We concluded that annual medics have the potential to contribute a significant amount of N to systems when seeded in the spring and harvested or incorporated 2 to 3 months later.