Inoculation and nitrogen affect herbage and symbiotic properties of annualMedicago species

The interactive effects of Sinorhizobium inoculants and soil N status shoul d affect the N contribution of annual medics (Medicago spp.) in cropping sy stems. We determined the effect of N and commercial medic inoculum on nodul ation, dry matter, and N yield of annual medics and also determined Sinorhi zobium strain occupancy in annual medic nodules. Field experiments were con ducted on a sandy, mixed Udorthentic Haploboroll and on a fine-loamy, mixed , mesic Typic Hapludalf, More than 75% of annual medic plants (except M. ru gosa Desr.) were nodulated in the absence of commercial inoculum, and nodul ation was due in part to a Sinorhizobium strain that frequently nodulates a lfalfa (Medicago sativa L.). Among the five strains in the commercial medic inoculum, 102G3 and 102A13 had the greatest nodule occupancy. When no N wa s applied, inoculation improved the percentage of plants nodulated and nodu le mass only in nl. rugosa, compared with no inoculation, but inoculation i ncreased herbage yields of spring-seeded M. truncatula Gaertn. cv, Sephi, M . polymorpha L., and M. rugosa by about 60%, compared with no inoculation, Nitrogen addition reduced the nodule mass of all species when inoculum was applied, and N addition increased only the herbage dry matter yield of spri ng-seeded M. scutellata (L.) Mill. when inoculum was applied. This suggests that a more effective inoculum could be developed for M. scutellata so tha t N would not limit herbage growth. Annual medics fixed from 40 to 80 kg N ha(-1) if spring-seeded and grown for 60 d, and from 20 to 50 kg N ha(-1) i f summer-seeded and grown for 43 d.