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Nodulation and nitrogen fixation by prairie legumes in roadside settings and restoration sites

Background

The Minnesota Department of Transportation (MNDOT) uses legume species including Amorpha canescens, Astragalus canadensis, Chamaechrista fasciculata, Dalaea purpurea and D. candida, Desmodium canadense and Lespedeza capitata in roadside restoration and wetland reconstruction activities, but until recently had paid only limited attention to the rhizobial requirements of these plants. We were funded to collect rhizobia for each of these legumes, and from these to select inoculant quality rhizobia for each host; to establish prairie areas and monitor the importance and contribution of inoculated legumes to the ecosystem, and to develop appropriate inoculation protocols for prairie legumes. More recently we have also received support through the Land Institute for studies on the inoculation of Desmanthus illinoensis.

Dalea candida Photo by K. Robertson

Planting inoculation experiments for selected prairie legumes, Becker, MN.

To obtain rhizobia for each of these legumes soil was collected from prairie areas in Minnesota and used to inoculate surface-sterilized legume seedlings, with rhizobia then trapped from the nodules which formed. Seventy five to 100 isolates were obtained for each legume, then these strains evaluated in growth chamber and field evaluation. Inoculant-quality rhizobia have now bean identified for each legumes, as follows:

• Amorpha UMR7520 and UMR7557
• Astragalus UMR6335 and UMR6355
• Chamaecrista UMR6404 and UMR6437
• Dalea UMR6808, UMR7205 and UMR7240
• Desmodium UMR6617 and UMR6437
• Lespedeza UMR6513 and UMR6564

Beginning in Spring 2003, and as a result of the difficulty seed producers faced in obtaining inoculants for these legumes, we began to market sterile-peat based inoculants for each species.

Three year old prairie area established at Becker, MN. The prairie is legume dominant, with legumes deriving 60-100% of their N needs from nitrogen fixation.

The prairie revegetation setting is an unusual one for inoculation. It contains a number of different legumes with distinct or overlapping Rhizobium requirements; each of these legumes is present at relatively low number in the prairie; seeding is often done at the end of the summer with inoculated seed expected to survive on the surface during the following winter; and seed germination is uneven and may not occur until the second or third year after sowing. Because of the problems this presents in the way of inoculation, we have begun to study alternate inoculation practices including the use of granular, clay-based inoculants broadcast at 7-10 kg ha^-1, and the inoculation of winter wheat and Canadian rye used as cover crops. In preliminary studies with winter wheat, we have recovered more than 10⁹ rhizobia per seed 30 days after inoculation. Further studies to determine how different strains are affected by inoculant formulation or application to cereal hosts, and to ensure survival over longer periods of time are planned.

From counts of Dalea rhizobia recovered from the rhizosphere of winter wheat, it appears that rhizobia survive well on this alternate host, and may even be endophytic.

Very striking responses to inoculation of Desmanthus illinoense have also been obtained with strains 30.8, 35.10 and 56.6 outstanding. At Becker in 2003 plant growth responses to inoculation exceeded 50% with clear host cultivar differences evident with some strains.

Response to inoculation in Desmanthus illinoense, Becker, MN, 2003.