Improved Methodologies for the Inoculation of Prairie Legumes in Roadside/Revegetation Settings

Project summary:

In previous studies, we isolated inoculant rhizobia for the prairie legumes used in prairie restoration, and showed response to inoculation and biological N2-fixation in Amorpha canescens, Dalea purpurea, D. candida, Astragalus canadensis, Desmodium canadense, Lespedeza capitata, and Chamaecrista fasciculata. We established legume-dependent prairies, and using d15 N methods, showed that these species were deriving 60-100% of their N from symbiosis. However, we still needed to develop better inoculant delivery systems, and to ensure that the different strains being used were able to persist and function under re-vegetation conditions, which is a unique setting for legume inoculation. It involves seed mixes in which each legume is only a minor component, rates of inoculation that must be considered low, the possibility of strain transfer between hosts, and very significant delays in legume establishment and nodulation. Normal methods of inoculation cannot be guaranteed to work, and higher than normal inoculation rates are probably needed for good nodulation and subsequent N2 fixation. This project evaluated different inoculation options, including inoculation of cover crops (for example, winter wheat and rye) sown with the prairie seed. It also sought to quantify rhizobial persistence and strain contribution to nodulation.

Responses to five different inoculation treatments were determined in a three-year-old prairie area established at the Becker Sandplain Experiment Station in Fall 2004. Seed inoculation was generally ineffective, but overall legume numbers and biomass in the prairie restorations were enhanced by both soil-applied granular and cover-crop applied inoculants. Soils collected from the prairie areas in 2007 also showed marked improvement in soil quality as a result of inoculation.