Nitrogen 15 studies with nonleguminous nitrogen-fixing plants Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/j6731640g

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  • Snowbrush (Ceanothus velutinus), red alder (Alnus rubra), and bitterbrush (Purshia tridentata) are three important nitrogen- fixing nonleguminous species. They commonly grow in association with commercially important conifers in western United States and conceivably could be utilized to add nitrogen to forest ecosystems. Nitrogen 15 gas was used to study nitrogen fixation by these three non - legumes. A method of exposing excised or attached nodules to excess nitrogen 15 was developed. Sealed atmospheres surrounding the nodules were enriched in nitrogen 15 using gas -tight syringes to remove a given volume of air from the atmosphere and to inject a given volume of nitrogen 15. This method was convenient, allowed nodules to be exposed within five minutes after excision, resulted in no waste of nitrogen 15 gas, and appeared accurate. Excised nodules were found to be very convenient and suitable. When exposed to excess gaseous nitrogen 15, nodules of red alder and bitterbrush became enriched in nitrogen 15, confirming that nodules of these two species are capable of nitrogen fixation. Nodules were taken from snowbrush seedlings grown from east- and west- Cascade seed sources and were exposed to excess nitrogen 15. Different nodule samples were not shown to have significantly different nitrogen- fixing rates; however, the results were not conclusive because the nodules had low nitrogen- fixing activities. Based on the accumulation of excess nitrogen 15 in nodules in a short period following excision and immediate exposure to excess nitrogen 15, snowbrush nodules were tentatively estimated to fix 21. 6 mg of nitrogen /g dry wt of nodules /day. Based on the accumulation of excess nitrogen 15 in nodules during the entire exposure, red alder, bitterbrush, and snowbrush nodules were found to increase their total nitrogen content by 1.49, 2. 72, and 2.46%, respectively. Excess nitrogen 15 accumulations indicated that at the time of determination nitrogen -fixing activities were the same for nodules of plants grown for 4. 5 months on soils amended at the time of seeding with 0, 5, or 50 ppm ammonium nitrogen. However, at the time of the measurements the soil ammonium levels were very low; consequently, the effect of sustained high ammonium levels on nodule nitrogen -fixing activity was not conclusively determined. Excised snowbrush nodules exposed to excess nitrogen 15 accumulated more nitrogen 15 when placed at 23 ° C than nodules placed at 15, 30, or 35° C. Optimum temperature for fixation appeared to be near 23°C. Nodules from 9- month -old inactive plants grown from May to February had very low nitrogen- fixing activity, whereas nodules from 4.5- month -old active plants grown from June to October had very high activity. Evidently, nodule nitrogen- fixing activity is low when the plant is not actively growing. Nodulated nonleguminous plants are an important part of the nitrogen cycle. More knowledge of the extent and process of non - leguminous nitrogen fixation is needed in order to better understand the world -wide distribution of soil nitrogen and to improve methods of managing forest and range lands.
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