Inoculation and nitrogen affect herbage and symbiotic properties of annual Medicago species

The interactive effects of Sinorhizobium inoculants and soil N status should affect the N contribution of annual medics (Medicago spp.) in cropping systems. We determined the effect of N and commercial medic inoculum on nodulation, dry matter, and N yield of annual medics and also determined Sinorhizobium strain occupancy in annual medic nodules. Field experiments were conducted on a sandy, mixed Udorthentic Haploboroll and on a fine-loamy, mixed, mesic Typic Hapludalf. More than 75% of annual medic plants (except Mo rugosa Desr.) were nodulated in the absence of commercial inoculum, and nodulation was due in part to a Sinorhizobium strain that frequently nodulates alfalfa (Medicago saliva L.). Among the five strains in the commercial medic inoculum, 102G3 and 102A13 had the greatest nodule occupancy. When no N was applied, inoculation improved the percentage of plants nodulated and nodule mass only in M. rugosa, compared with no inoculation, but inoculation increased 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 spring-seeded M. scutellata (L.) Mill. when inoculum was applied. This suggests that a more effective inoculum could be developed for M. scutellata so that 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 if summer-seeded and grown for 43 d.