Association of nitrogen-fixing bacteria with ectomycorrhizae in Douglas-fir Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/m039k7198

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  • Many higher plants have mycorrhizae associated with their roots. These structures are often essential for survival and growth (Harley & Smith 1983). Mycorrhizae are known for the abilities to enhance nutrient absorption. While nitrogen-fixing organisms are found as components of the mycorrhizosphere, mycorrhizal fungi are not presently known to fix nitrogen (Trappe & Fogel 1977). The role of mycorrhizal fungi in nitrogen fixation in the rhizosphere was explored in this study. The objectives were (1) to determine if nitrogen fixation is occurring in the rhizosphere of the Douglas-fir ectomycorrhizae formed with Hysterangiufli setchelli Fischer and Gautieria monticola Harkness in the Woods Creek area of Mary's Peak, Oregon; (2) to determine seasonal changes in nitrogen fixation and in populations of nitrogenfixing bacteria; (3) to isolate and identify nitrogen-fixing bacteria associated with ectomycorrhizae. The samples were collected each month from October 1985 to September 1986. Nitrogen fixation (acetylene reduction activity) was significantly higher each month in the mycorrhizosphere of the Douglas-fir than in the non-mat soil controls. Change in acetylene reduction rate with log-transformed incubation time was adequately expressed by a straight-line relationship. The average nitrogenase activities associated with H. setchelli and G. monticola were 1.36 and 1.44 nmoles/g/day, whereas the activity recorded for the non-mat soil controls was 0.38 nmoles/g/day. Acetylene reduction activity was significantly higher in spring, fall, and winter than in sumer, but the activity during spring, fall, and winter did not differ from each other. Nitrogen-fixing bacteria associated with mycorrhizae were also estimated by the most probable number methods. Nitrogenfixing populations showed considerable variability from month to month. However, some of them (those in March, August, September) showed a similar trend to acetylene reduction activities. Three different nitrogen-fixing bacteria were isolated from the fungal sheaths of H. setchelli, G. monticola, and within the sporocarps of H. setchelli. The three different strains showed a characteristic spiral movement when observed in H20 using light microscopy. Cells are straight to curved, plump, and slightly pointed with phase-dense granules. They grew well on nutrient agar and could reduce C2H2 on Dobereiner's N-free medium under micro-aerophilic conditions. On the basis of the findings of Tarrand et al. (1978), these strains probably are strains of Azospiril lum brasilense. The potential of nitrogen-fixing bacteria associated with coniferous ectomycorrhizal roots and their associated fungal sporocarps is of considerable importance to foresters. The N2- fixer could be selected for inoculation so that it utilized the protective and nutritional advantage of the mycorrhizae. These discoveries open exciting possibilities for understanding the role of mycorrhizae in nitrogen cycling and developing information on how management practices might be modified to optimize biological nitrogen fixation in the rhizosphere of young-growth Douglas-fir.
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