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Subarctic nitrogen fixation in monoculture alfalfa and mixed alfalfa/grass forage swards

Ball, M.T. 2008. M.Sc Thesis. Dept. of Soil Science, University of British Columbia, Vancouver, Canada

Abstract

Forage growth in the subarctic is sub-optimal due to low soil nutrient levels. Forage crops in the Yukon Territory consistently require nitrogen (N) and phosphorus fertilization to meet plant requirements. Fertilization is expensive due to transportation costs and potentially harmful to the environment so alternative, more sustainable, sources of nutrients are being sought. Alfalfa is an alternative, but there is limited knowledge in the Yukon of the benefits and management of this crop as a replacement for fertilizer N. Experiments were carried out in south central Yukon during the 2005 and 2006 field seasons to examine the potential of co-inoculation of alfalfa with N-fixing Ensifer meliloti and phosphate-solubilizing Penicillium bilaii to increase the dry matter yield and N fixation of monoculture alfalfa (Medicago sativa) cv Peace and binary mixed alfalfa with smooth bromegrass (Bromus inermis) cv Carlton or timothy (Phleum pratense) cv Climax forage swards. Interactions between alfalfa inoculation and N fertilization and late season harvest treatments were assessed. The TagTeam inoculant from Philom Bios was used as the rhizobium source which contains both Ensifer meliloti isolate NRG-34 and Penicillium bilaii isolate PB-50. Nitrogen fixation was determined using the total plant N difference method. Alfalfa growth and nodulation was successful in the trials. Inoculation had a positive impact on N fixation, whereas urea fertilizer at 25 kg N/ha had a negative impact in most cases. In the mixed alfalfa and smooth bromegrass stand there was a positive contribution from the alfalfa in both the establishment and second year with N fixation rates of up to 14 kg/ha. In the mixed timothy and alfalfa stand the N fixation reached 35 kg/ha in the establishment year and 102 kg/ha in the second year. In the establishment year the dry matter yield and N fixation of the TagTeam inoculated, monoculture alfalfa plots were 3.1 t/ha and 77 kg N/ha. In the second year, the unharvested inoculated alfalfa treatment yielded 3.4 t/ha with N fixation of 66 kg/ha compared to the late harvest treatment which yielded only 1.5 t/ha and an N fixation rate of 20 kg/ha. The effects of the late season harvest are startling and reflect the importance of removing grazing animals during the fall to allow plant energy reserves to accumulate in the roots. Fertilizer N replacement is possible with the seeding of alfalfa into existing hay stands or in monoculture.