|Abstract or Summary
- 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 -
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.