Nitrogen fertilization and rhizobial inoculation effects on kura clover growth

Kura clover (Trifolium ambiguum M.B.) is a persistent, rhizomatous forage legume; however, its use is currently limited by slow establishment. We determined the effects of rhizobial inoculation and N fertilization on kura clover growth and N₂ fixation in the seeding year. Kura clover was seeded with or without a commercial rhizobial inoculant and with and without N fertilization in three environments. Fertilization treatments consisted of 100 kg N ha^-1 either applied at seeding or split in 10 kg N ha^-1 applications every other week after seeding. Nitrogen fertilization increased seeding-year herbage accumulation in all locations, but the response to fertilizer N was greater on a loamy sand with low organic matter and available N than on a silt loam with high soil organic matter. Rhizobial inoculation failed to consistently improve seeding-year herbage accumulation compared with no inoculation; a positive response was observed in only one of three environments. Dry matter accumulation responses of root and rhizome to N fertilization and rhizobial inoculation were similar to that of herbage. Dinitrogen fixation in the seeding year varied between 9 and 25 kg ha^-1 fixed N, depending on the environment. Seeding-year inoculation increased postseeding year herbage yield. Also, when a positive response to N fertilization occurred in the seeding year, the response was maintained in the postseeding year. A commercial rhizobial inoculant was ineffective in establishing adequate nodulation in the seeding year in a N-limited soil, indicating the need to identify more effective rhizobia for kura clover.