Page 124 - PC2019 Program & Proceedings

PLANT CANADA 2019

S33. Rhizosphere temperature, tree species and ectomycorrhizae affect nitrogen uptake
Hawkins, B.; S. Robbins
University of Victoria

Temperature is a key determinant of the rate of biological processes. Rising global temperatures will
profoundly affect ecosystems, potentially shifting existing species interactions. This study assessed the
effect of rhizosphere temperature on nitrogen uptake by roots of two contrasting tree species colonized
with two contrasting species of ectomycorrhizal fungi (EMF). Seedlings of Pinus contorta (from poor
sites) and Picea sitchensis (from rich sites) were colonized by Laccaria bicolor, a common EMF, and
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Thelephora terrestris, an EMF common on rich sites. Seedlings were grown at 16 C with root
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temperatures of 17 C or 25 C. Ammonium and nitrate uptake and proton net flux were measured with ion
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selective microelectrodes at 17 C or 25 C on seedling roots in a full factorial design. All EMF roots had
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higher rates of ammonium than nitrate uptake. Pine had higher rates of ammonium uptake than spruce,
while spruce had higher nitrate uptake. Roots associated with T. terrestris had high ammonium uptake
while roots associated with L. bicolor had high nitrate uptake. For roots associated with T. terrestris,
measurement temperature had a greater effect on nitrogen uptake than growth temperature. Ammonium
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uptake was highest in roots grown at 17 C and measured at 25 C, while nitrate uptake was highest in roots
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grown and measured at 25 C. Proton efflux was highest when roots were transferred from one
temperature to another.
Barbara Hawkins (bhawkins@uvic.ca)

S34. Soil moisture and nitrogen, but not phosphorus and light, limit nitrogen fixation in alders in
the south western boreal forest
Markham, J.; P. Anderson
University of Manitoba

Ironically, in areas of the globe where nitrogen limits productivity, nitrogen fixing plants are not
dominant. A number of models have used the energy cost of nitrogen fixation to explain the paucity of
nitrogen fixing plants, predicting both reduced light and increased nitrogen availability reduces the
abundance of nitrogen fixing plants via competitive exclusion. Other hypotheses suggest nitrogen fixing
plants have a greater need for phosphorus. Over 4 years we compared nitrogen fixation by Alnus
viridisspp. crispain a jack pine forest and adjacent open site, both amended with P and N fertilizer. Shrubs
consistently got most of their nitrogen form fixation, showing a peak in years when growing season
precipitation was 350mm. Contrary to expectation, shrubs in the forest derived more of their nitrogen
from fixation than shrubs in the open, likely because open grown shrubs were more water limited. The
addition of nitrogen caused a sharp decrease in nodule nitrogenase activity in both sites. In the forest site
this was accompanied by a reduction in the proportion of cells in the nodules containing nitrogen fixing
vesicles. Annually, nitrogenase activity remained low until mind June and peaked in late July.
Nitrogenase activity was suppressed below 12oC. The highest levels of nitrogenase activity occurred at a
soil water content of 10% suggesting plant water availability and soil aerobic conditions limit nitrogen
fixation.

John Markham (john.markham@umanitoba.ca)

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