Dry matter accumulation and dinitrogen fixation of annual Medicago species

Assessment of the N₂ fixation potential of annual medics (Medicago spp.) in the upper Midwest is important for predicting the net N contribution to cropping systems. Our objectives were to determine the dry matter (DM) accumulation pattern of herbage, roots, and nodules of several annual medic species used as summer annuals and to measure the percentage and amount of N derived from N₂ fixation by annual medics using isotope dilution and difference methods. Experiments were conducted on a Hubbard loamy sand (Udorthentic Haploboroll) at Becker, MN, and a Tallula silt loam (Typic Hapludoll) at Rosemount, MN. The experimental design at both locations was a randomized complete block with six replicates. Medicago truncatula Gaertn, M. polymorpha L., M. scutellata (L.) Mill., and M. rugosa Desr. were inoculated with commercial rhizobial inoculant and were seeded in late May, Herbage, root, and nodule DM of N₂-fixing annual medics continued to increase until about 72 d after planting (DAP). The highest maximum herbage, root, and nodule DM yields were 10 669, 648, and 169 kg ha^-1, respectively, from M. polymorpha in 1993. Based on the estimate from the isotope dilution (ID) method, herbage of annual medics at maximum DM accumulation contained 86% N derived from atmosphere (%Ndfa), using ryegrass (Lolium multiflorum Lam.) as a reference crop, or 79% Ndfa, using noninoculated M. rugosa as a reference crop. The amount of Ndfa in annual medic herbage ranged from 100 to 200 kg ha^-1 based on estimates using either the ID method or the difference method (D-method), with M. polymorpha producing the highest and M. rugosa producing the lowest amount. Estimates of Ndfa from the ID and D-methods were strongly correlated. We conclude that annual medics have the potential to contribute a significant amount of N to cropping systems when seeded in the spring and harvested or incorporated into the soil 2 to 3 mo. later.