INDICATIONS OF ASSOCIATIVE NITROGEN-FIXATION IN EASTERN GAMAGRASS

Associative nitrogen-fixation is one mechanism by which plants can mee t a portion of their nitrogen (N) needs in N deficient environments. E astern gamagrass [Tripsacum dactyloides (L.) L.] grows under condition s conducive for associative N-fixation. Our objective was to evaluate eastern gamagrass roots for nitrogenase activity and associated N-fixi ng bacteria, and compare its nitrogenase activity to switchgrass (Pani cum virgatum L.). Soil and root samples were collected from unfertiliz ed stands of ''PMK-24'' eastern gamagrass at 3 locations in northern M issouri and nitrogenase activity measured using the acetylene reductio n method. Eastern gamagrass roots supported greater nitrogenase activi ty when colonized by indigenous bacteria, compared to roots inoculated with Klebsiella pneumoniae or surface sterilized. Eastern gamagrass r oots colonized by indigenous bacteria had 17-46 fold greater nitrogena se activity than switchgrass roots. Bacterial composition on eastern g amagrass roots was dominated by fluorescent pseudomonads and Pseudomon as cepacia, with lower populations of other bacteria. Bacterial compos ition from incubation tubes showing nitrogenase activity was similar t o that found on the roots, except that the diazotrophic bacterium Azos pirillum braziliense was isolated from 1 of 3 sites. The potential for associative N-fixation with eastern gamagrass warrants further study and in situ measurement to quantify rates of N fixation.