A survey of asymbiotic nitrogen fixation in the rhizosphere of weeds.
The occurrence of asymbiotic nitrogen fixing bacteria in the rhizosphere of 20 weed species was investigated. Using the roll tube method, root-rhizosphere soil samples were diluted and added to test tubes containing low N media. Tubes were flushed with nitrogen, stoppered and rolled to form a thin film of media on the inner wall. After 7 days incubation the nitrogen fixation rates were estimated using the acetylene reduction technique. Rates ranged from 38 to 783 nmoles acetylene reduced/hour. Of 20 species tested, 3 showed high rhizosphere populations of asymbiotic nitrogen fixing bacteria. These were Cenchrus incertus, Richardia scabra and Sinapis arvensis. Species showing intermediate activity were Helenium amarum, Plantago lanceolata, 2 Aster spp. and Panicum repens. From summary.